Formation and Structure of Wood - American Chemical Society

1 Formation and Structure of Wood RUSSELL A. PARHAM and RICHARD L. GRAY

Downloaded by 50.156.141.211 on February 12, 2016 | http://pubs.acs.org Publication Date: May 5, 1984 | doi: 10.1021/ba-1984-0207.ch001

ITT Rayonier, Inc., Shelton, W A 98584

Wood—a natural, cellular, composite material of botanical origin—possesses unique structural and chemical characteristics that render it desirable for a broad variety of end uses. The level of suitability for a given end use (i.e., wood quality) is frequently determined by the wood's response to imposed physical and chemical treatments. However, in addition to these criteria, wood quality is also often based on the behavior of wood when subjected to the natural forces of the environment (e.g., weather, fire, and decay). All of these performance criteria are related either directly or indirectly to wood chemistry together with the wood's organizational architecture at the macroscopic and microscopic levels. This chapter reviews both of these structural domains, the development of wood characteristics in a growing tree, and why these characteristics can govern the behavior of wood under various treatment regimes. Emphasis is also given to the potential sources of wood variability, including wood type (softwood or hardwood), tree genus or species, and the variability present even within a single tree.

Wood Sources Botanical Origin. W o o d is a n a t u r a l m a t e r i a l f a m i l i a r i n at l e a s t s o m e w a y t o e v e r y o n e . W o o d is o b t a i n e d f r o m t w o b r o a d c a t e g o r i e s o f p l a n t s k n o w n c o m m e r c i a l l y as softwoods a n d hardwoods. These g e n e r a l n a m e s c a n n o t b e u s e d u n i v e r s a l l y to refer to t h e a c t u a l p h y s ical hardness or d e n s i t y of all w o o d s because some softwoods are q u i t e hard (e.g., Douglas-fir a n d southern y e l l o w pines) a n d some h a r d w o o d s a r e soft ( e . g . , y e l l o w b u c k e y e , a s p e n , a n d c o t t o n w o o d ) . N e v e r t h e l e s s , t h e n a m e s d o a c c u r a t e l y a p p l y to m a n y w o o d s w i t h i n t h e s e t w o c a t e g o r i e s a n d t h u s c a n b e u s e d as p r a c t i c a l d e s i g n a t i o n s f o r t h e t w o g e n e r a l classes o f c o m m e r c i a l t i m b e r s . F r o m a m o r e scientific perspective, softwoods are tree 0065-2393/84/0207-0003/$ 15.50/0 © 1984 A m e r i c a n C h e m i c a l Society In The Chemistry of Solid Wood; Rowell, Roger; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.

species

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T H E CHEMISTRY O F SOLID W O O D


o f a class o f p l a n t s c a l l e d gymnosperms (seeds are b o r n e n a k e d ) , a n d h a r d w o o d s are w o o d y , d i c o t y l e d e n o u s (two seed leaves) angiosperme (seeds a r e b o r n e i n a f r u i t s t r u c t u r e ) (see S c h e m e I). T h e s o f t w o o d s a r e a l s o r e f e r r e d t o as conifers because many produce seed cones, p o l l e n cones, or b o t h . T h e conifers have n e e d l e l i k e (e.g., pine) or scalelike (e.g., cedar) leaves a n d a p p e a r to b e e v e r g r e e n i n that t h e y r e t a i n n e w leaves for u p to s e v e r a l years ( w i t h t h e e x c e p t i o n of l a r c h e s and baldcypress). H a r d w o o d s have leaves that are generally b r o a d or b l a d e l i k e , a n d most c o m m e r c i a l s p e c i e s — a t least i n t e m p e r a t e c l i m a t e s — a r e d e c i d u o u s , w h i c h m e a n s t h e y c o m m o n l y s h e d t h e i r leaves each fall at t h e e n d o f t h e t r e e ' s g r o w i n g s e a s o n . T h e s e l e c t i o n of a p a r t i c u l a r tree species for various e n d uses is, i n m o s t cases, b a s e d o n t h e p h y s i c a l a n d c h e m i c a l characteristics o f t h e w o o d . H o w e v e r , i n m a n y s i t u a t i o n s p r o x i m i t y to t h e w o o d s o u r c e a n d t o t a l w o o d p r o c u r e m e n t c o s t c a n s i g n i f i c a n t l y affect s p e c i e s s e lection. A c o m p r e h e n s i v e listing of c o m m e r c i a l softwoods a n d hardwoods i n c l u d i n g d o m e s t i c a n d f o r e i g n s p e c i e s , a n d t h e i r s c i e n t i f i c (or L a t i n ) n a m e s , is p r e s e n t e d i n C h a p t e r 2. T e c h n i c a l N a t u r e . W o o d is a c o m p l e x p l a n t t i s s u e c o m p o s e d of several distinct types of cells. I n the trees discussed i n this text, w o o d is e a s i l y r e c o g n i z e d as t h a t t i s s u e l o c a t e d t o t h e i n s i d e o f t h e tree bark a n d f o r m i n g the interior b u l k of all major stems, branches, a n d roots. T e c h n i c a l l y , w o o d is t h e m a i n c o n d u c t i v e a n d m e c h a n i c a l (or s u p p o r t i v e ) t i s s u e o f t h e t r e e , a n d is l a r g e l y r e s p o n s i b l e f o r t h e u p w a r d translocation of water a n d dissolved minerals from the root s y s t e m t o t h e a c t i v e t r e e c r o w n ( b u d s a n d f u n c t i o n i n g l e a v e s ) (J). T h e histological a n d c e l l u l a r structure of w o o d a n d h o w it serves c o n d u c tive, m e c h a n i c a l , a n d storage functions i n the tree w i l l be dealt w i t h later i n this chapter. I n t h e i r fully m a t u r e state, the vast majority of w o o d cells are d e a d a n d h o l l o w , a n d t h e r e s u l t i n g t i s s u e — k n o w n t e c h n i c a l l y as s e c ondary x y l e m — i s c o m p o s e d essentially of only cell walls a n d voids, t h e v o i d s b e i n g t h e h o l l o w i n t e r i o r s o f t h e c e l l s (or l u m e n s ) ( F i g u r e 1). I n s o f t w o o d s , t h e c e l l s m a k i n g u p 9 0 - 9 5 % o f t h e w o o d v o l u m e a r e fibrous i n f o r m ( m o r p h o l o g y ) a n d a r e t h u s t e r m e d fibers. Hard w o o d s , o n t h e o t h e r h a n d , a r e c o m p o s e d l a r g e l y o f fibers a n d m u c h w i d e r c e l l s c a l l e d vessel elements. T h e vessel elements are j o i n e d e n d - w i s e to f o r m t u b e s o r v e s s e l s a l o n g t h e s t e m , b r a n c h , o r r o o t axis a n d a r e s e e n as pores o n t h e w o o d c r o s s s e c t i o n ( F i g u r e 1). A m o n g v a r i o u s h a r d w o o d s , fiber v o l u m e w i l l v a r y o v e r a w i d e r a n g e b u t a v e r a g e s a b o u t 5 0 % f o r c o m m e r c i a l s p e c i e s (2). B o t h s o f t w o o d s

In The Chemistry of Solid Wood; Rowell, Roger; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.

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Formation and Structure of Wood

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Plant Kingdom

No Embryos Produced

Embryos Produced

Algae, Fungi, Bacteria No Vascular Tissue

Vascular Plants Downloaded by 50.156.141.211 on February 12, 2016 | http://pubs.acs.org Publication Date: May 5, 1984 | doi: 10.1021/ba-1984-0207.ch001

Contain Xylem & Phloem

With Seeds

Liverworts Hornworts Mosses

Without Seeds

Ferns, Whisk Ferns, Clubmosses, Horsetails

Monocots Angiosperme

One Seed Leaf

Seeds Borne in a Fruit Nonwoody Dicots Two Seed Leaves Woody

Gymnosperms Seeds Borne Naked

Softwood Trees

Others

Others

Hardwood Trees

Scheme I. Botanical origin of commercial woods.




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Figure 1. Commercial timber is obtained from angiosperme (hardwoods) or from gumnosperms (softwoods). The wood tissue is made up hrgely of dead, hollow plant cells that are arranged to form a composite material of substantial void volume.

and hardwoods contain various other nonflber cells w h i c h w i l l given appropriate attention i n the " W o o d A n a t o m y " section.

be

F r o m a c h e m i c a l perspective, w o o d tissue (including cells a n d i n t e r c e l l u l a r s u b s t a n c e ) is a c o m p o s i t e m a t e r i a l c o n s t r u c t e d f r o m a variety of organic p o l y m e r s — m o l e c u l e s that are m a d e of m a n y r e peating subunits or m o n o m e r s . T h e basic structural or skeletal m a -


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t e r i a l o f a l l w o o d c e l l w a l l s is c e l l u l o s e , a l o n g - c h a i n , l i n e a r s u g a r m o l e c u l e or polysaccharide (and carbohydrate) c o m p o s e d of glucose m o n o m e r s . G l u c o s e is a h e x o s e o r s i x - c a r b o n r i n g s u g a r , a n d as a c e l l u l o s e p o l y m e r it accounts for about 4 0 - 4 5 % of t h e d r y w e i g h t of n o r m a l w o o d t i s s u e (3). S e r v i n g as a m a t r i x s u b s t a n c e f o r t h e c e l l u l o s e s u p e r s t r u c t u r e are other, l o w e r m o l e c u l a r w e i g h t p o l y s a c c h a r i d e s that c o n t a i n short side chains. T h e s e carbohydrates represent mostly combinations of various five-carbon sugars (xylose a n d arabinose) a n d six-carbon sugars (glucose, m a n n o s e , a n d galactose). T h e y are different i n sev eral respects from cellulose (mainly i n conformation and molecular w e i g h t ) , b u t t h e y are s i m i l a r e n o u g h i n f o r m to w a r r a n t the n a m e hemicelluloses. T h e t h i r d m a j o r c o n s t i t u e n t o f w o o d c h e m i s t r y is lignin, a t h r e e dimensional, highly branched, and polyphenolic molecule of complex s t r u c t u r e a n d h i g h m o l e c u l a r w e i g h t . L i g n i n is f r e q u e n t l y c o m p a r e d to a n i n c r u s t a n t s u b s t a n c e b e c a u s e i t e n j o y s a n e s s e n t i a l l y u b i q u i t o u s d i s t r i b u t i o n i n f u l l y m a t u r e w o o d t i s s u e . It p e r m e a t e s b o t h c e l l w a l l s a n d i n t e r c e l l u l a r r e g i o n s , o r middle lamella, a n d r e n d e r s w o o d a h a r d , r i g i d m a t e r i a l a b l e to w i t h s t a n d c o n s i d e r a b l e m e c h a n i c a l stress. T h e m i d d l e l a m e l l a r e g i o n ( F i g u r e 2 , A a n d B ) is a b o u t 7 0 - 8 0 % (or m o r e ) l i g n i n b y w e i g h t a n d is t h e c e m e n t i n g m a t e r i a l t h a t h e l p s b i n d a l l w o o d c e l l s t o g e t h e r . A l t h o u g h t h e m i d d l e l a m e l l a r e g i o n has a v e r y h i g h l i g n i n c o n t e n t , b e c a u s e t h e c e l l w a l l s a r e also l i g n i f i e d a n d o c cupy most of the w o o d s solid v o l u m e , about 7 0 % or more of the total w o o d l i g n i n is l o c a t e d i n t h e c e l l w a l l s t h e m s e l v e s (4, 5) ( F i g u r e 2 C ) . T h e w o o d c e l l wall polysaccharides—cellulose a n d the h e m i c e l l u l o s e s — h a v e a strong affinity for w a t e r molecules i n e i t h e r t h e i r l i q u i d o r v a p o r s t a t e . L i g n i n , o n t h e o t h e r h a n d , is a l m o s t w a t e r r e p e l l i n g . I n fact, w i t h i n t h e w o o d ' s i n t e r n a l a r c h i t e c t u r e o r u l t r a s t r u c t u r e (see l a t e r s e c t i o n s ) l i g n i n acts t o b l o c k p o t e n t i a l sites f o r water absorption. It f o l l o w s t h a t t h e g a i n o r loss o f w a t e r o r o t h e r l i q u i d s o r v a p o r s into or out of w o o d tissue can be i n f l u e n c e d greatly b y the nature, amount, and distribution of w o o d polysaccharides. As will be dis c u s s e d i n C h a p t e r 3, t h e s e s a m e s o r p t i o n p h e n o m e n a ( i . e . , a d s o r p tion and desorption), together w i t h the architectural arrangement of w o o d cells i n the tree, are r e s p o n s i b l e for p a r t i c u l a r patterns i n w o o d s w e l l i n g a n d s h r i n k a g e w h e n w o o d is s u b j e c t e d t o v a r i o u s e n v i r o n ments. T h e arrangement of polysaccharide molecules within the cell w a l l i t s e l f , e s p e c i a l l y t h a t o f t h e c e l l u l o s e , a l s o h a s a p r o f o u n d effect on the physical a n d m e c h a n i c a l properties of i n d i v i d u a l cells a n d w o o d as a w h o l e .



T H E CHEMISTRY O F SOLID WOOD


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Wood Formation


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in Trees

Meristems. T h e f o r m a t i o n o f w o o d i n t r e e s is a n i n t e g r a l a s p e c t of total tree g r o w t h a n d includes not only increases i n t h e diameters of t h e s t e m , b r a n c h e s , a n d roots, b u t also t h e e l o n g a t i o n o f these m a j o r t r e e p a r t s . A l l o f t h i s v i s i b l e o r m a c r o s c o p i c g r o w t h is t h e r e s u l t o f a c t i v i t y b y s p e c i a l c e l l z o n e s c a l l e d meristems, derived from the G r e e k , meristos, m e a n i n g d i v i s i b l e (i). T h u s , meristematic cells are those i n t h e tree that r e t a i n a c o n t i n u e d ability to d i v i d e , e v e n t u a l l y p r o d u c i n g m a n y cells (derivatives o r d a u g h t e r cells) o f a g i v e n t y p e ( e . g . , w o o d a n d b a r k ) f r o m o n l y o n e c e l l . T h e l a t t e r c e l l is c a l l e d a n initial a n d r e m a i n s i n t h e m e r i s t e m . D e v e l o p i n g t r e e s c o n t a i n t w o m a j o r t y p e s o f m e r i s t e m s : (1) t e r m i n a l o r a p i c a l m e r i s t e m s a n d (2) l a t e r a l m e r i s t e m s . Apical meristems are l o c a t e d at t h e tips o f a l l stems a n d b r a n c h e s (both t e r m e d shoots) w h e r e t h e y a r e c o n t a i n e d w i t h i n t e r m i n a l b u d s ; t h e y a r e also l o c a t e d w i t h i n t h e t i p regions o f all roots. I n t h e t i p regions, t h e m e r i s t e m a t i c z o n e i s u s u a l l y p r o t e c t e d b y a n o t h e r z o n e o f c e l l s c a l l e d t h e root cap. Root hairs, o r microscopic roots, have n o apical meristems, b u t these m i n u t e structures a r e lateral projections o f roots that d o have apical meristems. In v e r y y o u n g trees o r seedlings, apical meristems are r e s p o n sible for essentially a l l shoot a n d root g r o w t h — c a l l e d p r i m a r y g r o w t h . D u r i n g t h e first year o f s e e d l i n g g r o w t h , a lateral m e r i s t e m is i n i t i a t e d t h r o u g h o u t m o s t o f t h e m a i n s h o o t s a n d r o o t s . T h i s l a t e r a l m e r i s t e m — t h e vascular cambium—is responsible for the manufac t u r e o f a l l i n i t i a l a n d s u b s e q u e n t w o o d t i s s u e . T h e c a m b i u m is a t h i n , c i r c u m f e r e n t i a l sheath o f cells that produces w o o d o r secondary x y l e m t o t h e i n s i d e ( i . e . , t o w a r d t h e t r e e c e n t e r ) a n d phloem o r i n n e r bark tissue to t h e outside ( F i g u r e 3 A ) . E v e n i n its first f e w m o n t h s o f g r o w t h , a s e e d l i n g already has w e l l - d e v e l o p e d tissues that function i n translocation a n d p h y s i c a l s u p port. T h e s e early b u t critically i m p o r t a n t tissues a r e c a l l e d primary xylem a n d primary phloem a n d a r e p r o d u c e d b y d e r i v a t i v e s o f a p i c a l meristems; they are l o n g i t u d i n a l l y arranged into tissue zones called vascular bundles ( J , 2). T h e s e s t r u c t u r e s a r e t h e o n l y c i r c u l a t o r y Figure 2. The technical nature of wood tissue. (A) Scanning electron micrograph (SEM) of the surface of a softwood resulting from a cut directed perpendicular to the tree axis. This view is known technically as the transverse or cross-sectional plane. (B) Schematic showing the Location of the major constituents of wood chemistry. (C) Transmission electron micrograph (TEM) of the transverse section of a white spruce wood lignin skeleton that was prepared by removing cellulose and nemicelluloses with hydrofluoric acid. Key: L, lumen; W, wall; and ML, middle lamella. (Reproduced with permission from Ref. 38. Copyright 1974, Forest Products Research Society.)



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Figure 3A. The location and manufacture of wood cells in an oak tree cross section illustrating various tissue systems. (Reproduced from Ref. 39. Copyright 1982, American Chemical Society.)

Figure 3B. Light micrograph of the location and manufacture of wood cells in the cross section of a young pine (softwood) stem. The cambial zone (CZ) produces phloem or inner bark cells to the outside and wood cells to the inside.


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PARHAM A N D GRAY


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tissues i n t h e tree u n t i l t h e y are c o m p l e m e n t e d i n o l d e r tree parts b y secondary xylem a n d secondary phloem f r o m t h e v a s c u l a r c a m b i u m . W h e t h e r p r i m a r y or secondary i n origin, xylem functions largely i n u p w a r d conduction a n d mechanical support, a n d p h l o e m acts as a c o n d u i t f o r d o w n w a r d m o v e m e n t o f p h o t o s y n t h a t e s ( m a n u f a c t u r e d foodstuffs) a n d h o r m o n e s f r o m t h e leaves a n d b u d s . B o t h x y l e m a n d p h l o e m also f u n c t i o n i n a storage capacity, w h i c h takes p l a c e l a r g e l y i n parenchyma c e l l s (see l a t e r s e c t i o n s ) . Wood Cell Production. T h e site o f w o o d c e l l p r o d u c t i o n , t h e v a s c u l a r c a m b i u m , i s i l l u s t r a t e d i n F i g u r e 3 B . T e c h n i c a l l y , i t is a m i c r o s c o p i c sheath o f m e r i s t e m a t i c cells. H o w e v e r , t h e exact c i r c u m f e r e n t i a l l i n e o f c a m b i a l cells is v e r y difficult to locate p r e c i s e l y , p a r ticularly d u r i n g t h e tree's g r o w i n g season, because o f t h e presence of r e c e n t x y l e m a n d p h l o e m d e r i v a t i v e s . T h e r e f o r e , i t is m o r e c o m m o n t o r e f e r e n c e t h i s l a t e r a l m e r i s t e m as t h e cambial zone (2). A l l c e l l s i n t h e c a m b i a l z o n e a r e l i v i n g . H o w e v e r , as x y l e m d e rivatives (i.e., d e v e l o p i n g w o o d cells) b e g i n a s e q u e n c e o f transfor mations that w i l l convert t h e m into m a t u r e w o o d elements, they e m b a r k o n a p a t h o f c e l l s p e c i a l i z a t i o n o r differentiation that w i l l l e a d e v e n t u a l l y (for f i b e r s , v e s s e l e l e m e n t s , a n d c e r t a i n o t h e r c e l l s ) t o c e l l death. R e c e n t x y l e m d e r i v a t i v e s m a y f u n c t i o n f o r a p e r i o d o f t i m e as mother cells, d i v i d i n g t o f o r m s t i l l o t h e r d e r i v a t i v e s . N e v e r t h e l e s s , t h e u l t i m a t e fate o f m o s t x y l e m d e r i v a t i v e s is s e l f d e s t r u c t i o n , autolysis, o f t h e i r l i v i n g c o n t e n t s , protoplast, a n d the eventual products are fully d i f f e r e n t i a t e d , o r s p e c i a l i z e d , w o o d cells possessing r a t h e r e l a b o r a t e w a l l s a n d h o l l o w c e n t e r s , lumens. O n l y a relatively small n u m b e r o f c e l l s i n w o o d — c a l l e d parenchyma—retain a viable pro toplast after e x i t i n g t h e c a m b i a l a n d differentiation zones. P a r e n c h y m a are s m a l l , nonfibrous cells that have special storage o r secre tory functions. In h a r d w o o d s , p a r e n c h y m a cells m a y b e o r g a n i z e d into n u merous v e r t i c a l strands a n d c a n o c c u p y u p to 2 5 - 5 0 % o f t h e w o o d v o l u m e (2). H o w e v e r , s u c h s t r a n d s u s u a l l y o c c u p y a r e l a t i v e l y s m a l l percentage of t h e total w o o d v o l u m e i n conifers ( 1 - 2 % ) . I n m a n y softwoods a n d h a r d w o o d s t h e v e r t i c a l strand p a r e n c h y m a are essen t i a l l y a b s e n t (2). P a r e n c h y m a a l s o c o m p o s e m o s t (or a l l i n h a r d w o o d s ) o f t h e w o o d t i s s u e r e g i o n s r e f e r r e d t o c o l l e c t i v e l y as w o o d rays. T h e s e s t r u c tures are n a r r o w ribbons of cells that are o r i e n t e d lengthwise along t h e t r e e r a d i u s a n d p e r p e n d i c u l a r t o t h e s t e m axis ( F i g u r e 4). R a y h e i g h t is m e a s u r e d a l o n g t h e s t e m axis, a n d r a y w i d t h is m e a s u r e d p a r a l l e l (tangent) to t h e t r e e c i r c u m f e r e n c e . I n a d d i t i o n to t h e i r s t o r a g e o r s e c r e t o r y f u n c t i o n s (see l a t e r s e c t i o n s ) , r a y p a r e n c h y m a



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Figure 4. The location of wood rays. (A) Portion of an oak tree cross section showing that rays are oriented normal to tree growth rings and along the tree radius. (Photo by W. J. McCleary.) (B) SEM of wood rays (R), vessels or pores (V), and wood fibers (F) as seen in a two-plane view of soft maple. cells also f u n c t i o n i n r a d i a l t r a n s p o r t o f tree p h o t o s y n t h a t e s a n d b i o chemicals inward from the p h l o e m a n d cambial zone. D u r i n g t h e division a n d enlargement phases of w o o d cell d e v e l o p m e n t , t h e c e l l w a l l is a t h i n , d e f o r m a b l e , a n d e x t e n s i b l e e n v e l o p e o f m a t e r i a l r e f e r r e d t o as t h e primary wall. N e a r t h e c e s s a t i o n o f c e l l e n l a r g e m e n t , h o w e v e r , a secondary wall m a y b e g i n t o b e manufactured to the l u m e n side of the p r i m a r y wall. W o o d fibers, vessel e l e m e n t s , a n d c e r t a i n o t h e r x y l e m o r p h l o e m e l e m e n t s that function i n passive conduction and/or support normally develop a s e c o n d a r y w a l l ( F i g u r e 5). P e r h a p s as e a r l y as t h e m a n u f a c t u r e o f t h e s e c o n d a r y w a l l , d i f ferentiating w o o d fibers ( a n d most o t h e r w o o d cells) a n d t h e regions



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Figure 5. Cross-sectional view of fully differentiated wood fibers. (A) SEM of a Doughs-fir. Note the thick secondary walls (S) and the fiber lumen (L). (Reproduced from Ref. 39. Copyright 1982, American Chem ical Society.) (Β) TEM of two adjacent fibers in white spruce. Key: P, primary wau, S, secondary wall; L, lumen; and ML, middle lamella. (Re produced with permission from Ref. 38. Copyright 1974, Forest Products Research Society.) b e t w e e n t h e c e l l s ( m i d d l e l a m e l l a ) s t a r t t o b e c o m e lignified. That is, l i g n i n m a n u f a c t u r e d b y t h e c e l l s starts to infiltrate a n d i n c r u s t t h e entire w o o d tissue, a n d most of the process comes near the e n d of secondary w a l l deposition. I n fully differentiated w o o d tissue, lignin composes about 2 5 - 3 5 % of the total w o o d weight (moisture-free basis) a n d p l a y s a m a j o r r o l e i n i m p a r t i n g r i g i d i t y t o t h e p o l y s a c c h a r i d e w a l l s u b s t a n c e s (3).


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T h e o v e r a l l p r o c e s s o f w o o d c e l l p r o d u c t i o n is i n f l u e n c e d b y n u m e r o u s factors, i n c l u d i n g genetics, c l i m a t e , p h o t o p e r i o d or day l e n g t h , a n d forest soil c o n d i t i o n s . H o w e v e r , these factors are o n l y indirectly i n v o l v e d i n the control of w o o d cell development, but they d i r e c t l y affect t h e t r e e c r o w n — t h e l e a v e s a n d b u d s — a n d t h e c r o w n t h e n exerts a d i r e c t i n g i n f l u e n c e o n c a m b i a l zone activities, i n c l u d i n g the form and n u m b e r of derivatives produced. W o o d Tissues and G r o w t h Increments. W o o d cell production occurs d u r i n g o n l y part of each year (in the s p r i n g a n d s u m m e r months) i n temperate zones. W o o d production occurs on a more i r r e g u l a r t i m e s c h e d u l e i n t r o p i c a l r e g i o n s o f t h e w o r l d a n d , to s o m e extent, i n s u b t r o p i c a l areas. I n t r o p i c a l a n d s u b t r o p i c a l regions, the c a m b i u m h a s n o t r u e s e a s o n a l d o r m a n c y , a n d t r e e g r o w t h is r e l a t e d m o r e to v a r i a t i o n s i n l o c a l c l i m a t e , p a r t i c u l a r l y t o t h e s u p p l y o f a v a i l a b l e w a t e r (6). I n t r o p i c a l t r e e s , z o n e s o f w o o d t i s s u e t h a t c a n b e a t t r i b u t e d to s p e c i f i e d p e r i o d s o f g r o w t h are not u s u a l l y d e c i p h e r a b l e because the trees can g r o w essentially o n a continuous basis. I n the s u b t r o p i c s , o r i n h i g h l y e l e v a t e d t r o p i c a l areas, s o m e trees can lose t h e i r leaves d u r i n g part of the year. T h i s situation can t h e n be reflected i n a visible change i n the type of w o o d p r o d u c e d . T h e g e n e r a l r e s u l t is a d e t e c t a b l e growth increment t h a t a p p e a r s as a growth ring o n a c r o s s - s e c t i o n o f t h e t r e e s t e m . G r o w t h increments reach their most advanced form in t e m p e r a t e - z o n e t r e e s w h e r e t h e n o r m is a s i n g l e , g e n e r a l l y d i s t i n c t , a n d c i r c u m f e r e n t i a l b a n d o f w o o d p r o d u c t i o n e a c h y e a r ( F i g u r e 6). T h e appearance of these so-called annual rings varies b e t w e e n hardwoods a n d softwoods w i t h species, tree age, a n d g r o w i n g c o n d i t i o n s . T h e s e f a c t o r s , t o g e t h e r w i t h c e r t a i n o t h e r e n v i r o n m e n t a l effects, c a n also


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l e a d t o a b e r r a t i o n s i n g r o w t h k n o w n as multiple rings o r discontinuous rings (2). G r o w t h rings a n d t h e types a n d d i s t r i b u t i o n of cells therein give rise to t h e situation w h e r e b y t h e g e n e r a l s t r u c t u r e o f w o o d tissue c a n be r e f e r e n c e d b y t h r e e major planes o r surfaces. E a c h surface has a u n i q u e appearance that stems f r o m t h e m a n n e r i n w h i c h cells o f specific orientations are c u t b y planes d i r e c t e d either parallel o r p e r p e n d i c u l a r to t h e tree's l o n g i t u d i n a l axis. T h e s e t h r e e planes a r e t h e cross-sectional o r transverse plane, the radial plane, a n d the tangen t i a l p l a n e . T h e y a r e i l l u s t r a t e d a n d d e f i n e d i n F i g u r e 7. Because trees g r o w f r o m t h e tips o f stems a n d roots, g r o w t h rings are laid onto t h e tree i n t h e f o r m o f i n v e r t e d h o l l o w cones, a n d each r i n g (except t h e most recent) extends o n l y part w a y u p the tree a n d t a p e r s t o a p o i n t i n t h e d i r e c t i o n o f t h e s t e m t i p ( F i g u r e 8). T h e r e f o r e , c o u n t i n g g r o w t h i n c r e m e n t s o n a tree cross section w i l l almost always n o t r e v e a l t h e total age o f a tree b u t o n l y t h e age o f that p a r t i c u l a r v e r t i c a l l e v e l i n t h e s t e m . O t h e r aspects of tree growth, some b e i n g species-dependent, p r e c l u d e the accurate esti-

Figure 7. Representation of a tree cross section cut to reveal the three major structural planes of wood. This particuhr stem was cut in the spring of its 10th growing season. Key: X , cross-sectional or transverse plane! surface; R, radial surface; T, tangential surface. (Reproduced with permission from Ref. 40. Copyright 1982, Technical Association of the Pulp and Paper Industry Press.)




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Figure 8. Representation of growth increments in a 6-year-old tree stem. Note that the increments are deposited as inverted hollow cones. (Reproduced with permission from Ref. 40. Copyright 1982, Technical Association of the Pulp and Paper Industry Press.) mate of tree age v i a r i n g c o u n t i n g . T h e s e topics are dealt w i t h i n o t h e r t e x t s (2, 7). T h e initiation of a growth increment and the production of n e w w o o d cells each year i n a g i v e n tree are g o v e r n e d b y a complex system of tree physiology i n v o l v i n g interaction of the tree c r o w n w i t h


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climate, soil, a n d o t h e r e n v i r o n m e n t a l factors, i n c l u d i n g i m p o s e d f o r e s t m a n a g e m e n t (8). N e v e r t h e l e s s , i n t e m p e r a t e t r e e s i t is n o r mally possible to separate visibly each " a n n u a l " ring into t w o distinct t i s s u e s . T h e s e t w o t i s s u e z o n e s a r e k n o w n t e c h n i c a l l y as early wood (also s p r i n g w o o d o r i n i t i a l w o o d ) a n d latewood (also s u m m e r w o o d o r terminal wood). T h e y are manufactured d u r i n g approximately the spring a n d s u m m e r months, respectively, of a given geographical region. G r o w t h rings are n o r m a l l y r e a d i l y d e t e c t e d i n b o t h softwoods and most hardwoods because o f differences i n t h e d i a m e t e r and wall thickness o f cells c o m p o s i n g t h e e a r l y w o o d a n d latewood zones. SOFTWOODS. E a r l y w o o d fibers i n softwoods are t h i n - w a l l e d a n d h a v e w i d e r a d i a l d i a m e t e r s . A s s p r i n g passes i n t o s u m m e r , t h e f i b e r s develop thicker walls, either abruptly or gradually, depending on the s p e c i e s . A t t h e s a m e t i m e fiber r a d i a l d i a m e t e r i s r e d u c e d ( F i g u r e 9). T h e s e c h a n g e s i n fiber s t r u c t u r e a n d m o r p h o l o g y p r o v i d e a l a t e w o o d tissue that is n o r m a l l y s e v e r a l t i m e s d e n s e r (weight/volume) and considerably h a r d e r than earlywood of the same g r o w t h i n c r e m e n t (2). T h e l a t e w o o d z o n e a p p e a r s d a r k e r t h a n e a r l y w o o d o n t h e t r e e c r o s s s e c t i o n as w e l l as o n t h e r a d i a l a n d t a n g e n t i a l s u r f a c e s , a n d this difference i n a p p e a r a n c e b e t w e e n e a r l y w o o d a n d l a t e w o o d , t o g e t h e r w i t h t h e a n g l e at w h i c h t h e t r e e o r w o o d is c u t o r m a c h i n e d , h e l p s g i v e r i s e t o figure o r grain i n s o f t w o o d t i m b e r . HARDWOODS. F o r most h a r d w o o d s g r o w t h rings are d i s t i n g u i s h a b l e b u t n o t b e c a u s e o f a s i g n i f i c a n t c h a n g e i n t h e fiber c e l l s f r o m e a r l y w o o d to latewood. F i b e r m o r p h o l o g y a n d w a l l thickness are r e l a t i v e l y s i m i l a r w i t h i n a g i v e n ring. I n h a r d w o o d s , g r o w t h i n c r e m e n t s are most often d i s t i n g u i s h e d b y changes i n vessel o r p o r e d i a m e t e r s — c h a n g e s that range from subtle to v e r y distinct. F i g u r e 10 s h o w s t h a t c o m m e r c i a l t e m p e r a t e h a r d w o o d s c a n b e c l a s s i f i e d as h a v i n g a b r u p t t r a n s i t i o n , g r a d u a l t r a n s i t i o n , o r l i t t l e i f any t r a n s i t i o n i n p o r e size f r o m e a r l y w o o d to l a t e w o o d . T h e s e t h r e e t y p e s o f w o o d s a r e c o m m o n l y k n o w n as ring-porous, semi-ring-porous ( o r semi-diffuse-porous), a n d diffuse-porous, respectively. H e r e , as i n t h e s o f t w o o d s , t h e s t r u c t u r a l d i f f e r e n c e s , o r l a c k t h e r e o f , b e t w e e n e a r l y w o o d a n d l a t e w o o d t i s s u e g i v e rise t o t h e a e s t h e t i c p r o p erty of w o o d grain. Because o f the more complex o r varied anatomy of hardwoods, grains here show m u c h more diversity than i n the softwoods. Wood Quality. T h e relative n u m b e r of thin-walled and thickw a l l e d fibers i n s o f t w o o d s a n d h a r d w o o d s , a n d t h e v e s s e l s i z e a n d n u m b e r i n hardwoods, have a major impact o n w o o d characteristics that often d i c t a t e t h e w o o d s e n d use. S u c h c h a r a c t e r i s t i c s i n c l u d e the m e c h a n i c a l p r o p e r t i e s o f w o o d , t h e type o f surface resulting f r o m w o o d m a c h i n i n g , t h e p e r m e a b i l i t y o f w o o d t o l i q u i d s a n d gases, w o o d weatherability, a n d perhaps others. I n essence, the types a n d p r o -



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Figure 9. Light micrograph of the earlywood-to-latewood transition within softwood growth increments as viewed in cross section. Key: A , abrupt transition in eastern larch, with thick-walled latewood fibers (D = resin duct); and B, gradual transition in eastern white pine, with relatively thin-walled latewood fibers. portions of various c e l l types i n a g i v e n p i e c e of w o o d , tree, or species a r e r e s p o n s i b l e f o r t h e g e n e r a l c o n c e p t o f w h a t is w i d e l y r e f e r r e d to as w o o d q u a l i t y . H o w e v e r , w o o d q u a l i t y is s u c h a n a r b i t r a r y t e r m b e c a u s e i t is e m p l o y e d t o d e s c r i b e t h e g e n e r a l s u i t a b i l i t y o f a p a r t i c u l a r w o o d s o u r c e f o r a v e r y s p e c i f i c e n d u s e a p p l i c a t i o n (8). T h a t specific use m i g h t be f u r n i t u r e parts, baseball bats, p l y w o o d , p a n eling, railroad ties, toothpicks, firewood, p e n c i l s , fence posts, etc. T h e p o i n t is t h a t a w o o d s o u r c e r a t e d as e x c e l l e n t f o r o n e a p p l i c a t i o n m a y not b e suitable for c e r t a i n o t h e r uses, a n d v i c e versa. T h i s d i s c u s s i o n m a y s e e m to i m p l y that o n e of t h e m o s t i n f l u e n t i a l factors c o n t r o l l i n g w o o d q u a l i t y , i . e . , w o o d v a r i a b i l i t y , is n o t d e s i r a b l e a n d t h a t a m a j o r g o a l o f f o r e s t m a n a g e m e n t s h o u l d b e to e l i m i n a t e o r at l e a s t m i n i m i z e t h i s v a r i a b i l i t y . T h i s is o f t e n i n d e e d


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the o b j e c t i v e i n s o m e c a s e s , p a r t i c u l a r l y f o r t r e e s o f a g i v e n f o r e s t stand or for the same species b e t w e e n stands. H o w e v e r , a m o n g spe cies o r t r e e t y p e s , i t is this s a m e n a t u r a l v a r i a b i l i t y that gives r i s e to the d i v e r s i t y o f a v a i l a b l e t i m b e r r e s o u r c e s a n d t h e s p e c i f i c o r e v e n unique w o o d properties u p o n w h i c h many e n d products are based.

Wood Anatomy


Softwoods. T h e w o o d o r s e c o n d a r y x y l e m o f g y m n o s p e r m s is c o m p o s e d o f r e l a t i v e l y f e w c e l l t y p e s (see b o x ) . The p r e d o m i n a n t c e l l t y p e i n softwoods is t h e v e r t i c a l l y o r i e n t e d ( a l o n g t h e s t e m axis) longitudinal tracheid. More commonly known as fibers, t h e s e t r a c h e i d s a r e h o l l o w , s q u a r e t o r e c t a n g u l a r i n c r o s s s e c t i o n , h a v e c l o s e d a n d t a p e r i n g e n d s , a n d a r e a r r a n g e d so t h a t t h e i r

Figure 10. Light micrograph of the three types of pore patterns of growth increments in hardwoods as seen in cross section. Key: A, ring-porous (red oak); B, semi-ring-porous (aspen); and C, diffuse-porous (yellow birch). (Reproduced with permission from Ref. 40. Copyright 1982, Tech nical Association of the Pulp and Paper Industry Press. ) M a j o r T y p e s o f Cells i n Softwoods Vertically

Oriented

A. . Longitudinal tracheids B. A x i a l p a r e n c h y m a C . E p i t h e l i a l cells* 7

Horizontally

Oriented

A. Ray tracheids B. R a y p a r e n c h y m a C. E p i t h e l i a l cells* H o m o c e l l u l a r rays: A or Β H e t e r o c e l l u l a r rays: A + Β (Fusiform ray: A + Β + C ) 0

P r e s e n c e is s p e c i e s - d e p e n d e n t . S u r r o u n d n o r m a l resin ducts i n p i n e s , spruces, larches, a n d Douglas-fir, a n d trau matic resin ducts (formed as a result o f tree injury) i n these a n d other softwoods (2). a

b


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e n d s o v e r l a p a d j a c e n t fibers. T h e y a r e a l s o a r r a n g e d i n t o w e l l - a l i g n e d r a d i a l r o w s (see F i g u r e 9).


W i d t h s o f l o n g i t u d i n a l t r a c h e i d s g e n e r a l l y r a n g e f r o m 3 5 to 5 0 μηι a n d l e n g t h s a v e r a g e b e t w e e n 3 a n d 5 m m . T h e s e c e l l s s e r v e a d u a l r o l e o f p r o v i d i n g s t r e n g t h a n d m e c h a n i c a l s u p p o r t as w e l l as b e i n g the pathway b y w h i c h water a n d dissolved minerals are trans located f r o m the tree's root s y s t e m u p w a r d to the tree c r o w n . A r r a n g e d h o r i z o n t a l l y or radially i n the tree are the w o o d rays, w h i c h , as m e n t i o n e d e a r l i e r , a r e c o m p o s e d p r e d o m i n a n t l y o f s m a l l , b r i c k l i k e , a n d o f t e n l i v i n g c e l l s c a l l e d p a r e n c h y m a (see F i g u r e s 4, 7, a n d 11). T h e s e c e l l s f u n c t i o n i n r a d i a l t r a n s l o c a t i o n b u t h a v e a m a j o r r o l e as a s t o r a g e r e c e p t a c l e , a n d f r e q u e n t l y c o n t a i n e x t r a n e o u s m a t e r i a l s s u c h as s t a r c h , fats, o i l s , v a r i o u s s u g a r s , a n d i n o r g a n i c d e p o s i t i o n s s u c h as c a l c i u m o x a l a t e c r y s t a l s o r s i l i c a ( F i g u r e 12). T h e r a y s i n s o m e s p e c i e s a l s o c o n t a i n c e l l s k n o w n as ray tra cheids, w h i c h a r e s i m i l a r i n s i z e to p a r e n c h y m a b u t a r e d e a d at m a t u r i t y (see F i g u r e 11). P r e s e n c e a n d / o r t y p e o f r a y t r a c h e i d is s o m e times a very useful diagnostic feature i n identification of a particular w o o d genus o r species. W i t h or w i t h o u t ray t r a c h e i d s , rays are u s u a l l y several cells h i g h , b u t i n softwoods they are generally o n l y one c e l l w i d e ( u n i s e r i a t e ) ( F i g u r e 13), e x c e p t i n s p e c i a l c a s e s w h e r e t h e y c a n b e u p to s e v e r a l cells w i d e (multiseriate). Resin ducts o r resin canals a r e t u b e l i k e v o i d s t h a t a r e b o t h l o n g i t u d i n a l l y a n d r a d i a l l y o r i e n t e d t h r o u g h o u t t h e x y l e m o f s o m e soft w o o d s (see F i g u r e 9). T h e s e d u c t s a r e l i n e d w i t h s p e c i a l i z e d p a r e n c h y m a , c a l l e d epithelial cells, t h a t s e c r e t e i n t o t h e d u c t a s u b s t a n c e c a l l e d o l e o r e s i n ( F i g u r e 1 4 A ) (3). V e r t i c a l a n d h o r i z o n t a l r e s i n ducts are n a t u r a l a n d constant fea t u r e s o f f o u r d o m e s t i c g e n e r a : p i n e s (Pinus), s p r u c e s (Picea), l a r c h e s (Larix), a n d D o u g l a s - f i r (Pseudotsuga). R e s i n d u c t s a l s o d e v e l o p as a r e s p o n s e t o i n j u r y o r t r a u m a i n o t h e r g e n e r a , as w e l l as i n t h e f o u r l i s t e d a b o v e (2). H o r i z o n t a l r e s i n d u c t s a r e c o n t a i n e d i n s p e c i a l m u l t i s e r i a t e r a y s , c a l l e d fusiform rays b e c a u s e o f t h e i r s p i n d l e s h a p e i n tangential v i e w (Figure 14B). A few softwoods, notably the cedars a n d baldcypress, contain a n o t i c e a b l e a m o u n t o f l o n g i t u d i n a l ( s t r a n d ) p a r e n c h y m a , b u t as m e n tioned previously, these axially o r i e n t e d p a r e n c h y m a are generally sparse i n most conifers. Hardwoods. Various c e l l types are f o u n d i n hardwoods (see box). H a r d w o o d a n a t o m y is m o r e v a r i e d o r c o m p l i c a t e d t h a n t h a t o f the softwoods, b u t most structural concepts are analogous. H a r d w o o d s c o n t a i n a s u b s t a n t i a l v o l u m e o f fiber c e l l s , b u t t h e d i s t i n g u i s h i n g f e a t u r e o f a n g i o s p e r m x y l e m is t h e o c c u r r e n c e o f ves sels. T h e v e s s e l s a r e s e e n o n t h e w o o d c r o s s s e c t i o n as h o l e s o r p o r e s



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Figure 11. Light micrograph of ray structure in softwoods as seen in radial section. Key: A , homocellular ray composed of procumbent parenchyma (RP) in white fir; B, heterocellular ray with nondentate ray tracheids (RT) in black spruce; and C, heterocelluhr ray with dentate ray tracheids in red pine. (Reproduced with permission from Ref. 40. Copyright 1982, Technical Association of the Pulp and Paper Industry Press.)

i n v a r i o u s p a t t e r n s . T h u s , a l l h a r d w o o d s a r e a l s o r e f e r r e d t o as p o r o u s woods, i n contrast to the softwoods that are technically nonporous (compare F i g u r e s 9 and 10). A n i n d i v i d u a l vessel o r p o r e consists o f a vertical series of short (0.02-0.5 m m ) vessel segments, w h i c h are j o i n e d end-to-end along the g r a i n . I n d i v i d u a l v e s s e l s c a n m e a n d e r t o a l i m i t e d e x t e n t i n t h e r a d i a l o r t a n g e n t i a l d i r e c t i o n t o j o i n , t e r m i n a t e i n , o r d e p a r t from o t h e r vessels, b u t t h e i r m a j o r f u n c t i o n is t h e v e r t i c a l t r a n s l o c a t i o n o f



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Figure 12. SEM of calcium oxalate crystals in the ray parenchyma of the wood radial surface of a tropical hardwood. (Reproduced from Ref. 39. Copyright 1982, American Chemical Society.) sap. To facilitate this t r a n s l o c a t i o n , t h e e n d s o f a l l v e s s e l s e g m e n t s are perforate; that i s , t h e ends are o p e n for free f l o w o f l i q u i d s b e t w e e n cells, i n contrast to t h e situation o f c o m p l e t e l y imperforate ends o f a l l w o o d fibers. I n s o m e h a r d w o o d s t h e v e s s e l s e g m e n t e n d s a r e entirely o p e n (simple), w h i l e i n others the ends contain a series o f parallel crossbars (scalariform) o r some other d e s i g n (e.g., reticulate). T h e particular t y p e o f o p e n i n g h e r e is o f c o n s i d e r a b l e value i n w o o d

Figure 13. Light micrograph of tangential section of redwood showing uniseriate rays (UR), the most common type of ray in softwoods, together with multiseriate rays (MR). The latter, if not containing a horizontal resin duct, are rare in softwoods.



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Figure 14. SEM of resin ducts in spruce wood. (A) Vertical duct with exuded resin droplets. (Reproduced from Ref. 39. Copyright 1982, American Chemical Society.) (B) Horizontal ducts contained in fusiform rays (FR) of the wood tangential surface. s p e c i e s i d e n t i f i c a t i o n (2). (See discussion c h i t e c t u r e , " page 34).

on " V e s s e l E l e m e n t s : A r

H a r d w o o d fibers, b e c a u s e o f t h e p r e s e n c e o f v e s s e l s , o c c u p y a proportionally smaller v o l u m e o f w o o d tissue than softwood fibers M a j o r Types of Cells i n Hardwoods Vertically A.

Oriented

Fibers 1. L i b r i f o r m fibers 2. F i b e r t r a c h e i d s 3. V a s i c e n t r i c t r a c h e i d s B. Axial p a r e n c h y m a C. Vessel elements

Horizontally

Oriented

Ray parenchyma 1. P r o c u m b e n t c e l l s 2. U p r i g h t c e l l s H o m o c e l l u l a r rays: 1 or 2 H e t e r o c e l l u l a r rays: 1 a n d 2 0

U p r i g h t cells are actually o r i e n t e d parallel to the fiber axis (i.e., axially), but w i t h i n a ray they are arranged i n radial or horizontal lines (with or without p r o c u m b e n t cells). a


24


d o . T h e fibers t h e m s e l v e s a r e a l s o s m a l l e r , a n d a v e r a g e a b o u t o n e h a l f t h e w i d t h a n d o n e - t h i r d t h e l e n g t h o f s o f t w o o d t r a c h e i d s (2). T h e s c u l p t u r i n g o f h a r d w o o d fiber w a l l s d i f f e r s i n s e v e r a l r e s p e c t s f r o m t h a t o f s o f t w o o d fibers, a n d t h e s e d e t a i l s , i n a d d i t i o n to s t r u c t u r a l i n f o r m a t i o n o n s o f t w o o d fibers, are d i s c u s s e d i n the section o n " W o o d C e l l Walls."


T h e p a r e n c h y m a - c e l l content of hardwoods is, on the average, m u c h g r e a t e r t h a n t h a t o f s o f t w o o d s . T h i s s i t u a t i o n is a r e s u l t o f t h e w i d e r rays ( 1 - 5 0 cells) a n d greater r a y v o l u m e o f h a r d w o o d s , a n d also t h e r e l a t i v e l y h i g h p r o p o r t i o n o f l o n g i t u d i n a l p a r e n c h y m a (2). A d d i t i o n a l l y , the rays are a l l p a r e n c h y m a — n o ray tracheids. T h e v o l u m e r a t i o o f v e s s e l s to fibers a n d fiber w a l l t h i c k n e s s a r e t w o i m p o r t a n t factors i n f l u e n c i n g t h e h a r d n e s s a n d d e n s i t y of dif ferent h a r d w o o d s p e c i e s a n d t h e p e r m e a b i l i t y o f these w o o d s to l i q u i d s a n d g a s e s . W o o d g r a i n is a l s o p a r t l y a f u n c t i o n o f t h e s e t w o parameters. R e s e a r c h o n t h e c h e m i c a l n a t u r e o f h a r d w o o d x y l e m has r e v e a l e d that the walls of fibers a n d ray cells contain l i g n i n of one type, syringyl, w h i l e vessel walls together w i t h the surrounding m i d d l e l a m e l l a a r e r i c h i n l i g n i n o f a s e c o n d t y p e , guaiacyl, w h i c h is t h e s a m e t y p e f o u n d i n s o f t w o o d x y l e m (9, 10). T h i s b a s i c c h e m i c a l d i f f e r ence b e t w e e n hardwoods a n d softwoods c o u l d , i n some instances, potentially i n f l u e n c e p h e n o m e n a that d e p e n d on the c h e m i c a l nature o r r e a c t i v i t y o f w o o d t i s s u e at t h e c e l l u l a r l e v e l . F u r t h e r d e t a i l s o n the nature a n d d i s t r i b u t i o n of w o o d c h e m i c a l constituents are f o u n d i n C h a p t e r 2.

Wood Cell Walls Fibers. ARCHITECTURE. T h e basic skeletal substance of the w o o d cell w a l l — c e l l u l o s e — i s , i n the mature cell, aggregated into l a r g e r u n i t s o f s t r u c t u r e c a l l e d elementary fibrils that, i n t u r n , are a g g r e g a t e d t o f o r m t h r e a d l i k e e n t i t i e s k n o w n as microfibrils. The latter are r e a d i l y o b s e r v e d i n the e l e c t r o n m i c r o s c o p e a n d are u s u a l l y found together i n still larger entities that c o u l d be labeled macrofibrils, a l t h o u g h t h e y a r e s t i l l m i c r o s c o p i c ( F i g u r e 15). C o n s i d e r a b l e controversy still reigns over the size, morphology, or even existence o f e l e m e n t a r y fibrils [or s u b e l e m e n t a r y fibrils i n c a m b i a l t i s s u e (11)] a n d t h e a r c h i t e c t u r e o f m i c r o f i b r i l s (12-14). H o w e v e r , b e c a u s e at l e a s t s o m e t y p e o f t h r e a d l i k e e n t i t y is r e a d i l y d i s t i n g u i s h a b l e , w e w i l l r e f e r t o t h a t e l e m e n t o f s t r u c t u r e as a m i c r o f i b r i l a n d r e c o g n i z e its a g g r e g a t i o n i n t o m a c r o f i b r i l s . M i c r o f i b r i l s a n d m a c r o f i b r i l s c o m b i n e to f o r m sheets o f w a l l s u b s t a n c e , a n d u l t i m a t e l y these sheets or



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Figure 15. TE M of cellulose microfibrils (Mi) and macrofibrils (Ma) within the secondary fiber wall of balsam fir; direct carbon replica. lamellae form relatively discrete wall layers. This architectural s c h e m e is d i a g r a m e d i n F i g u r e 16 f o r a t y p i c a l , n o r m a l w o o d fiber. A s m e n t i o n e d e a r l i e r , t h e i n i t i a l p o r t i o n o f a fiber c e l l w a l l is m a n u f a c t u r e d i n t h e c a m b i a l z o n e a n d is r e f e r r e d t o as t h e primary wall. H e r e , c e l l u l o s e m i c r o f i b r i l s f o r m a r a n d o m , i r r e g u l a r , a n d i n t e r w o v e n n e t w o r k ( F i g u r e 17) to f a c i l i t a t e c e l l e x p a n s i o n d u r i n g t h e e n l a r g e m e n t p h a s e of f i b e r d e v e l o p m e n t . I n a d d i t i o n to c e l l u l o s e , the p r i m a r y w a l l contains a large p r o p o r t i o n of matrix carbohydrates, p a r t i c u l a r l y p e c t i c m a t e r i a l s a n d h e m i c e l l u l o s e s (see C h a p t e r 2). T h e combination of two adjacent p r i m a r y walls and the interdisposed true m i d d l e l a m e l l a z o n e is c o l l e c t i v e l y r e f e r r e d t o as t h e compound middle lamella. M i c r o s c o p i c a l l y , i t is d i f f i c u l t t o s e p a r a t e w a l l s u b stance h e r e f r o m the i n t e r f i b e r substance. At the innermost region of the p r i m a r y wall, microfibrils begin to e x h i b i t a p r e f e r r e d o r i e n t a t i o n a n d s h o w a t e n d e n c y t o a l i g n t h e m s e l v e s a b o u t t h e fiber axis as a h e l i x a n d t o h a v e a s p e c i f i c microfibril angle ( m e a s u r e d as t h e a n g u l a r d i s p l a c e m e n t f r o m t h e fiber axis). F r o m t h i s p o i n t i n w a r d t o t h e fiber l u m e n , t h e w a l l is r e f e r r e d t o as secondary wall. T h i s p o r t i o n o f t h e w a l l is i n i t i a t e d at t h e e n d o f t h e c e l l e n l a r g e m e n t p h a s e a n d is c o m p o s e d o f m a n y l a m e l l a e , e a c h w i t h a specific orientation. T h e lamellae are organized into distinguishable wall layers ( F i g u r e 18A).



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Figure 16. Schematic of what is widely considered to be (at least in ρήηciple) the general wall architecture of normal wood fibers. Key: ML, middle lamella; P, primary wall; and S S , and So, layers of the second ary wall. (Adapted from Ref. 15.) ly

2

I n n o r m a l w o o d t i s s u e , t h e fiber s e c o n d a r y w a l l c o n s i s t s o f t h r e e f a i r l y d i s t i n c t l a y e r s . T h e o u t e r m o s t l a y e r o r S i is v e r y t h i n ( 0 . 1 - 0 . 2 μπι) a n d e x h i b i t s a n a v e r a g e m i c r o f i b r i l a n g l e (for t h e l a y e r as a w h o l e ) o f a b o u t 5 0 - 7 0 ° (2). T h e b u l k o f t h e s e c o n d a r y w a l l is m a d e u p o f t h e S l a y e r , w h i c h is t y p i c a l l y s e v e r a l m i c r o m e t e r s t h i c k ( F i g u r e 18). H e r e t h e m i c r o f i b r i l s a r e u s u a l l y o r i e n t e d to t h e fiber axis at a r e l a tively small angle (5-20°). T h e thickness a n d small microfibril angle 2

Figure 17. TEM of randomly arranged microfibrils in the fiber primary wall (PW) of balsam fir; direct carbon replica.


PARHAM AND GRAY


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1.

Figure 18. TEM of normal layering (SJSJS^ in the fiber secondary wall ofloblolly pine; transverse sections. (Reproducedfrom Ref. 41. Copyright 1971, Springer-Verlag.) Key: A , portion of two earlywood fibers (ML = middle lamella); and B, lignin skeleton of region similar to that shown in A.

o f the S2 c o n t r i b u t e s i g n i f i c a n t l y to the h i g h s t r e n g t h p r o p e r t i e s o f w o o d p a r a l l e l to g r a i n (tensile a n d c o m p r e s s i o n , see C h a p t e r 5) (2). T h e i n n e r m o s t l a y e r o f n o r m a l w o o d f i b e r s , t h e S3, is g e n e r a l l y s i m i l a r to t h e S a l t h o u g h i t is p e r h a p s a l i t t l e t h i n n e r , a n d has s u b l a m e l l a e a v e r a g i n g 6 0 - 9 0 ° t o t h e fiber axis (15). l 5


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F i g u r e 16 s h o w s t h a t t h e v a r i o u s s u b l a y e r s o r l a m e l l a e i n t h e S S , a n d S c a n e x h i b i t l e f t - h a n d e d ( p a r a l l e l to m i d d l e b a r o f a n S) a n d / or r i g h t - h a n d e d ( p a r a l l e l to m i d d l e b a r o f a Z) h e l i c e s . T h i s p a r t i c u l a r v a r i a t i o n i n fiber-wall a r c h i t e c t u r e has b e e n i n v e s t i g a t e d i n v e r y f e w w o o d species, a n d g e n e r a l i z a t i o n to i n c l u d e a l l softwood a n d h a r d w o o d fibers m a y b e a b i t r i s k y . H o w e v e r , a v a i l a b l e d a t a i m p l y t h a t a l l n o r m a l w o o d fibers m a y b e c o n s t r u c t e d i n p r i n c i p l e f r o m a s i m i l a r blueprint. l 5


2

3

SCULPTURING. Pits. Softwood a n d h a r d w o o d fibers have closed e n d s , b u t a s p e c i a l w a l l f e a t u r e f a c i l i t a t e s m o v e m e n t o f t h e t r e e ' s sap s t r e a m f r o m o n e fiber t o a n o t h e r , f r o m fibers t o v e s s e l e l e m e n t s , a n d f r o m fibers t o r a y c e l l s . T h i s s p e c i a l f e a t u r e is a s m a l l o p e n i n g o r r e c e s s i n t h e fiber s e c o n d a r y w a l l k n o w n t e c h n i c a l l y as a pit. T h e m o s t o b v i o u s fiber p i t s a r e t h o s e t h a t o c c u r b e t w e e n c o n t i g u o u s s o f t w o o d fibers. T h e y a r e d o n u t - s h a p e d i n face v i e w w i t h a circular ridge of wall material overarching and bordering the actual a p e r t u r e i n t h e w a l l . T h e s e p i t s a r e k n o w n t e c h n i c a l l y as interfiber bordered pits a n d a r e l o c a t e d p r e d o m i n a n t l y o n r a d i a l fiber w a l l s (Figure 19A). Actually, pits i n contiguous cells usually occur i n m a t c h e d pairs; each of the two participating cells contributes oneh a l f of a p i t p a i r ( F i g u r e 19, Β a n d C ) . W i t h i n a g r o w t h r i n g , softwood i n t e r f i b e r pits are larger a n d m o r e a b u n d a n t i n e a r l yw o o d . I n l a t e w o o d t h e y a r e f e w e r , s m a l l e r , a n d o f t e n a p p e a r s l i t l i k e i n v e r y t h i c k - w a l l e d fibers (2). T h i s s a m e t y p e o f p i t i n h a r d w o o d fibers v a r i e s m o r p h o l o g i c a l l y w i t h t h e fiber t y p e , c h a n g i n g f r o m a n o b v i o u s l y b o r d e r e d p i t i n t h i n - w a l l e d cells to o n l y a s l i t l i k e a p e r t u r e i n fibers w i t h t h i c k w a l l s . A s all pits develop i n softwoods a n d hardwoods, a specialized p i t m e m b r a n e r e m a i n s w i t h i n t h e p i t c o m p l e x ( F i g u r e 19, D a n d E ) . T h i s m e m b r a n e is i n i t i a l l y c o n s t r u c t e d f r o m t h e c o m p o u n d m i d d l e l a m e l l a i n a l l cases, b u t i n its f u l l y d i f f e r e n t i a t e d state the m e m b r a n e c a n d i f f e r c o n s i d e r a b l y b e t w e e n v a r i o u s c e l l t y p e s , b e t w e e n soft w o o d s a n d h a r d w o o d s , a n d to s o m e e x t e n t e v e n b e t w e e n different s p e c i e s (3). I n h a r d w o o d s , p i t m e m b r a n e s a r e o b s e r v e d to b e t h i n and generally nonporous partitions of microfibrils, matrix materials, a n d l i g n i n ( F i g u r e 20). M o v e m e n t o f l i q u i d s t h r o u g h t h e p i t c o m p l e x to a n adjacent c e l l m u s t t h e r e f o r e o c c u r l a r g e l y b y diffusion r a t h e r than b y free l i q u i d translocation. F o r t u n a t e l y , h a r d w o o d s have an e f f e c t i v e a l t e r n a t e m e c h a n i s m f o r l i q u i d m o v e m e n t , at l e a s t i n t h e v e r t i c a l d i r e c t i o n , a n d t h a t m e c h a n i s m is t h e v e s s e l s y s t e m . T h e interfiber-pit m e m b r a n e s i n softwoods are substantially dif ferent from the pit m e m b r a n e s i n hardwoods. M u c h of the m e m b r a n e p e r i p h e r y is q u i t e o p e n , w i t h o n l y t h e c e n t r a l p o r t i o n b e i n g n o n p o -



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Figure 19. Anatomy of softwood interfiber pits. (Reproduced from Ref. 39. Copyright 1982, American Chemical Society.) (A) SEM of interfiber pits in earlywood as seen on the wood radial face. Note the donut-shaped borders. (É and C) SEM of pit pairs between adjacent fibers; cross-sectional surface. (D) SEM ofboraered-pit membranes (PM) in face view of a split wood radial surface. (E) Light micrograph of pit pairs as seen in cross section with a light microscope. Key: PM, pit membranes; PB, pit border; and PA, pit aperture. r o u s ( F i g u r e 2 1 A ) . T h e e x a c t m o r p h o l o g y o f t h e m e m b r a n e is s p e c i e s d e p e n d e n t (2). In the standing, l i v i n g tree the b o r d e r e d - p i t m e m b r a n e s b e t w e e n s o f t w o o d fibers a c t as valves t o p r e v e n t t h e s p r e a d o f a i r o r bubbles into sap-filled cells i n the event of tree i n j u r y a n d potential r u p t u r e to v e r t i c a l w a t e r c o l u m n s . U n f o r t u n a t e l y , t h e y p e r f o r m a similar function i n the processing of w o o d into c o m m e r c i a l products. For e x a m p l e , d u r i n g w o o d d r y i n g , substantial c a p i l l a r y a n d surface t e n s i o n f o r c e s a r e d e v e l o p e d u p o n w a t e r r e t r e a t f r o m t h e fiber l u mens t h r o u g h the pits, a n d the m e m b r a n e s m o v e effectively (partic u l a r l y i n e a r l y w o o d ) to seal t h e a p e r t u r e s i n t h e d i r e c t i o n o f w a t e r



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Figure 20. SEM of intervessel-pit membranes in hardwoods. (A) Vessel in western red alder. The cell wall at the lumen is partly torn away to reveal the nonporous nature of the pit membranes (PM) (PA = pit aperture). (B) High magnification of pits in Anthocephalus, a tropical species. The secondary wall (S) at the lumen has been removed to expose the nonporous pit membranes (PM) and a special structure known as vestures. The latter can be found in the pit complex of various hardwoods (2). (Reproduced with permission from Ref. 40. Copyright 1982, Technical Association of the Pulp and Paper Industry Press. ) m o v e m e n t ( F i g u r e 2 1 , B - D ) . T h i s c o n d i t i o n , k n o w n as pit aspiration, g r e a t l y i m p e d e s t h e s u b s e q u e n t m o v e m e n t o f f l u i d s o r gases t h r o u g h t h e w o o d t i s s u e i n q u e s t i o n (2). P i t a s p i r a t i o n o c c u r s n a t u r a l l y i n conifers u p o n c o n v e r s i o n o f s a p w o o d to h e a r t w o o d (discussed later). H o w e v e r , a s p i r a t i o n is a n a l m o s t u n a v o i d a b l e c o n s e q u e n c e o f a n y c i r c u m s t a n c e that p r o m o t e s w o o d d r y i n g , i . e . , the creation of water/ air interfaces w i t h i n the w o o d structure. P i t s w i t h s o m e f o r m o f b o r d e r a l s o o c c u r at i r r e g u l a r i n t e r v a l s a l o n g s o f t w o o d fibers w h e r e t h e fibers c o n t a c t r a y c e l l s ( F i g u r e 22). S u c h p i t s a r e v e r y r a r e i n h a r d w o o d fibers, b u t i n s o f t w o o d s t h e y a r e a b u n d a n t a n d c o n s p i c u o u s , e s p e c i a l l y i n e a r l y w o o d . T h e s e pits are k n o w n t e c h n i c a l l y as r a y cross-field pits (see F i g u r e 11), a n d t h e y


PARHAM A N D GRAY


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1.

Figure 21. Softwood bordered-pit membranes of western hemlock. (Reproduced from Ref 39. Copyright 1982, American Chemical Society.) (A) SEM of unaspirated pit in earlywood. Note porous periphery of the membrane. Rodlike bacteria are also present here, apparently filtered out onto the membrane during sample preparation; split wood radial surface. (B) Light micrograph of aspirated pits (A?) in late wood; cross section. (C) Light micrograph of aspirated pit and unaspirated pit (UP) in earlywood; cross section. (D) Fully aspirated pit in earlywood. Note the reduction in porosity upon aspiration. (Compare to A above.)



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Figure 22. SEM of ray cross-field pits in softwoods as seen on the wood radial surface. Key: A , pits to ray parenchyma (RP) in western white fir; and B, pits to ray parenchyma and ray tracheids (RT) in lodgepole pine. p r o v i d e m u c h o f t h e i n f o r m a t i o n n e e d e d f o r i d e n t i f i c a t i o n o f soft w o o d s (2). M e m b r a n e s w i t h i n ray cross-field pits are essentially nonporous p a r t i t i o n s o f compound middle lamella, although the m e m b r a n e ar c h i t e c t u r e v a r i e s t o s o m e e x t e n t , d e p e n d i n g o n w h e t h e r fiber c o n t a c t is m a d e t o r a y p a r e n c h y m a o r r a y t r a c h e i d s . I n e i t h e r c a s e , c r o s s field p i t s d o p r o v i d e a p a t h o f c o m m u n i c a t i o n b e t w e e n fibers a n d r a y s t h a t is at l e a s t s o m e w h a t m o r e e a s i l y t r a v e r s e d t h a n o n e r e q u i r i n g passage of materials t h r o u g h e n t i r e c e l l walls. R a d i a l transport o f l i q u i d s is f a c i l i t a t e d b y d i f f u s i o n t h r o u g h s i m p l e p i t s ( i . e . , w i t h o u t borders) located i n the side a n d e n d walls of contiguous p a r e n c h y m a cells a n d by s m a l l b o r d e r e d pits b e t w e e n contiguous ray tra c h e i d s (2). Spiral Thickenings. T h e fibers of a very few species woods a n d hardwoods are l i n e d w i t h helically o r i e n t e d ridges m a t e r i a l ( F i g u r e 23). T h e s e r i d g e s , w h i c h c a n b e r o p e l i k e i n w o o d s , a r e a s s o c i a t e d w i t h , a n d a r e a n i n t e g r a l p a r t of, t h e


o f soft of wall certain S wall 3


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Figure 23. SEM of spiral thickenings in the fibers of Doughs-fir; wood radial surfaces. Key: A , spirals in the vicinity of ray cross-field pits in earlywood; and B, nigh magnification of spirals in the last htewood fiber of one year and the first earlywood fiber of the next year. Pits shown in Β are interfiber-bordered pits. l a y e r (2, 16). T h e e x a c t m o r p h o l o g y o f s u c h s p i r a l s v a r i e s w i t h t h e species i n q u e s t i o n , b u t i n a l l cases t h e y p r o b a b l y d o not h a v e a detectable i n f l u e n c e o n w a l l physics or c h e m i c a l reactivity. T h e y are, h o w e v e r , a f e a t u r e o f d i a g n o s t i c v a l u e . O n t h i s b a s i s , D o u g l a s - f i r is readily distinguished from other commercial, domestic softwood timbers. Warts. W a r t s are c o n e l i k e or d r o p l i k e protuberances, s o m e times f o u n d c o v e r e d w i t h an a m o r p h o u s d e p o s i t i o n , that are scattered i n a r a n d o m p a t t e r n o n t h e i n n e r fiber-wall s u r f a c e i n m o s t s o f t w o o d s a n d t h e fibers o f s o m e h a r d w o o d s p e c i e s ( F i g u r e 24). T h e w a r t s t r u c t u r e , k n o w n c o l l e c t i v e l y as t h e warty layer (17), is m a n u f a c t u r e d b y t h e l i v i n g c e l l p r o t o p l a s t b e f o r e c e l l a u t o l y s i s (18,19). T h e w a r t y l a y e r is l i g n i n l i k e i n n a t u r e b u t h a s n o a p p a r e n t p h y s i o l o g i c a l r o l e ; i t p r o b -



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Figure 24. TEM of warty layer in softwood (A) and hardwood (B) fibers; direct carbon replicas. (A) Lumen surface of fiber in bahamfir. Note the amorphous substance masking the S . (Reproduced with permission from Ref. 20. Copyright 1974, Society of Wood Science and Technology. ) (B) Lumen surface of a warty fiber in sycamore. The slits shown here are pit apertures. (Reproduced with permission from Ref 23. Copyright 1974, Springer- Verlag. ) 3

a b l y also has l i t t l e o r n o effect o n w o o d p h y s i c a l b e h a v i o r . H o w e v e r , l i g n i n i n t h e w a r t y l a y e r d o e s s e e m t o b e m o r e h i g h l y c o n d e n s e d (or b o n d e d t h r e e - d i m e n s i o n a l l y ) t h a n t h e l i g n i n i n t h e r e s t o f t h e fiber w a l l (20, 21), a n d p e r h a p s this s i t u a t i o n c o u l d i n f l u e n c e w a l l c h e m i c a l r e a c t i v i t y o r p e n e t r a b i l i t y at t h e c e l l u l a r l e v e l . Vessel Elements. ARCHITECTURE. T h e cell wall of vessel ele m e n t s a p p e a r s t o b e c o n s t r u c t e d a l o n g t h e s a m e g e n e r a l s c h e m e as w o o d fibers. H o w e v e r , t h e l a y e r i n g i s g e n e r a l l y m o r e c o m p l i c a t e d , and t h e p r e s e n c e i n m a n y species o f n u m e r o u s i n t e r v e s s e l b o r d e r e d



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pits causes m a r k e d d e v i a t i o n s i n the wall's i n t e r n a l m i c r o f i b r i l l a r ar r a n g e m e n t (22). SCULPTURING. Pits. T h e regions on vessel elements where c o n t a c t is m a d e w i t h a d j a c e n t v e s s e l s , fibers, a n d p a r e n c h y m a a r e d i s t i n c t l y p i t t e d ( F i g u r e 25). T h e a r r a n g e m e n t , s i z e , a n d s h a p e o f these pits are often species-dependent a n d are thus valuable i n w o o d i d e n t i f i c a t i o n efforts (2). H o w e v e r , h a r d w o o d p i t s a r e n o t a n o p e n r o u t e for r a p i d i n t e r c e l l u l a r t r a n s p o r t ( F i g u r e 20); c o n s e q u e n t l y , t h e r e is a n e e d f o r v e s s e l e l e m e n t p e r f o r a t i o n s as a m o r e e f f e c t i v e r o u t e for f l u i d t r a n s l o c a t i o n . Perforations. T h e o p e n a r e a s at t h e e n d s o f v e s s e l e l e m e n t s are c a l l e d perforation plates. T h e form of these e n d - w a l l regions varies b e t w e e n species a n d sometimes b e t w e e n e a r l y w o o d a n d late-

Figure 25. SEM of vessel pitting in hardwoods. (A) Vessel/vessel pitting (VP) and vessel/ray parenchyma pitting (RP) as seen from a transverse/ radial perspective in cottonwood (F = fibers). (B) Individual vessel element of cottonwood isolated by chemical pulping. (C) Isolated earlywood vessel element of white oak (FP = fiber!vessel pitting).


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w o o d o f t h e s a m e s p e c i e s (2). H o w e v e r , i n a l l cases t h e p e r f o r a t i o n p l a t e s a l l o w t h e f r e e f l o w o f l i q u i d s o r gases b e t w e e n v e r t i c a l l y c o n t i g u o u s v e s s e l e l e m e n t s a l o n g t h e s t e m axis. M a j o r t y p e s o f p e r f o ration plates are illustrated i n F i g u r e 26. T h e i r variability a m o n g d i f f e r e n t w o o d s p e c i e s is b e n e f i c i a l i n efforts to i d e n t i f y a p a r t i c u l a r w o o d source.


Spiral Thickenings. T h e s e structures are c o m m o n i n the ves sels o f m a n y h a r d w o o d s (e.g., m a p l e , c h e r r y , b a s s w o o d , b u c k e y e , s o u t h e r n m a g n o l i a , a n d m a d r o n e ) . T h e p a r t i c u l a r f o r m is v a l u a b l e t o the w o o d a n a t o m i s t for species i d e n t i f i c a t i o n , b u t t h e p r e s e n c e o r a b s e n c e o f s p i r a l s h a s n o a p p a r e n t effect o n w o o d b e h a v i o r (2). Warts. T h e p r e s e n c e o f a v e s s e l e l e m e n t w a r t y l a y e r is s p e c i e s d e p e n d e n t , a n d w h e r e i t is f o u n d , i t t e n d s t o b e a s s o c i a t e d m o r e w i t h t h e o c c u r r e n c e o f s c a l a r i f o r m - t y p e p e r f o r a t i o n plates ( I , 23). A s i n t h e c a s e o f fibers, v e s s e l w a r t s h a v e n o o b v i o u s r o l e at t h e c e l l u l a r

Figure 26. SEM of major types of perforation plates in hardwood vessel elements. (A) Wood radial section of redgum showing scalariform (ladderlike) perforation plates in three vessel elements (RP = ray cross-field pits). (B) Portion of a maple vessel element containing a simple perforation plate; chemically pulped wood. (C and D) Portions of vessel elements from yellow poplar (C) and paper birch (D) showing scalariform plates with very few bars and with numerous bars; chemically pulped wood.


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l e v e l a n d are l i k e l y to h a v e n o d e c i p h e r a b l e i n f l u e n c e o n w o o d p r o p erties. Other Cells. T h e w a l l architecture/sculpturing of ray tracheids i n s o f t w o o d s a n d o f t h e r a y a n d a x i a l p a r e n c h y m a c e l l s i n b o t h soft w o o d s a n d h a r d w o o d s is d i s c u s s e d at l e n g t h i n o t h e r t e x t s (2 a n d


references c i t e d therein). D e t a i l s s h o w i n g variability i n c l u d e pit type, w a l l l a y e r i n g , d e g r e e of l i g n i f i c a t i o n , a n d w a l l s c u l p t u r i n g p e c u l i a r to a few species (e.g., dentations i n ray tracheids of the h a r d pines). A l t h o u g h these details are of botanical a n d academic interest, t h e i r exact f o r m a n d v a r i a b i l i t y are o f l i t t l e r e l e v a n c e to t h e r e m a i n d e r o f this text a n d the m a j o r aspects o f w o o d c h e m i s t r y a n d w o o d behavior.

Wood Physical Characteristics T h e weight a n d strength properties of w o o d , together w i t h the b e h a v i o r o f w o o d i n r e s p o n s e to w e a t h e r , c h e m i c a l t r e a t m e n t , f i r e , or m i c r o b i a l organisms, are i n f l u e n c e d greatly b y the wood's water c o n t e n t a n d t h e m a s s o f w o o d t i s s u e p e r u n i t v o l u m e (its d e n s i t y ) . T h e p h y s i c a l c h a r a c t e r i s t i c s o f w o o d t i s s u e at g i v e n l e v e l s o f m o i s t u r e a n d d e n s i t y w i l l b e g i v e n s p e c i f i c a t t e n t i o n i n C h a p t e r 3. H o w e v e r , s o m e basic aspects of w o o d p h y s i c s are i n c l u d e d h e r e to p r o v i d e a m o r e c o m p r e h e n s i v e i n t r o d u c t i o n to t h e t e c h n i c a l n a t u r e o f s o l i d wood. Hygroscopicity. T h e c h e m i c a l nature of w o o d substance, par ticularly that of the polysaccharides, r e n d e r s w o o d c e l l walls h y g r o s c o p i c (or h y d r o p h i l i c ) . T h e h y d r o x y l g r o u p s o n t h e c e l l u l o s e a n d h e m i c e l l u l o s e m o l e c u l e s are r e s p o n s i b l e for this great affinity for w a t e r a n d have a v e r y s t r o n g p r o p e n s i t y to f o r m h y d r o g e n b o n d s (2). L i g n i n , o n t h e o t h e r h a n d , p o s s e s s e s c o m p a r a t i v e l y f e w f r e e h y d r o x y Is, a n d as a r e s u l t is m u c h l e s s h y g r o s c o p i c . I n fact, f o r a l l p r a c t i c a l p u r p o s e s , l i g n i n is g e n e r a l l y c o n s i d e r e d t o b e e s s e n t i a l l y h y d r o p h o b i c (or l i p o p h i l i c ) . A s a c o n s e q u e n c e o f its h y d r o p h i l i c i t y , w o o d t i s s u e w i l l s e e k t o m a i n t a i n , t h r o u g h e i t h e r g a i n o r loss o f m o i s t u r e , a n e q u i l i b r i u m m o i s t u r e c o n t e n t w i t h the s u r r o u n d i n g a t m o s p h e r e . I f the w o o d takes on water, the c e l l w a l l s p r o c e e d to s w e l l u n t i l the c e l l walls b e c o m e w a t e r - s a t u r a t e d . T h e l a t t e r m o i s t u r e c o n t e n t is c a l l e d t h e w o o d ' s f i b e r s a t u r a t i o n p o i n t . I n c o n t r a s t , loss o f w o o d w a t e r ( b e l o w t h e fiber saturation p o i n t ) , d u e to d i f f u s i o n a n d e v a p o r a t i o n , r e s u l t s i n w o o d shrinkage. T h e w a t e r c o n t e n t o f t h e w o o d c e l l w a l l has a s t r o n g i n f l u e n c e on the wood's mechanical properties, and a higher moisture content, at l e a s t b e l o w t h e fiber s a t u r a t i o n p o i n t , a n d n o r m a l l y is i n v e r s e l y r e l a t e d t o m o s t s t r e n g t h p r o p e r t i e s (see C h a p t e r 5). T h i s s i t u a t i o n is easily r e c o n c i l e d i f one considers that the takeup of water b e l o w the


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fiber s a t u r a t i o n p o i n t r e s u l t s i n a p u s h i n g a p a r t o f p o l y s a c c h a r i d e m o l e c u l e s as w a t e r is i m b i b e d , a n d t h a t t h e d r y i n g o f w e t w o o d , i f resulting i n w o o d shrinkage, promotes coalescence and b o n d i n g of the wood's internal architecture or ultrastructure.


O t h e r ways i n w h i c h the presence b e h a v i o r w i l l b e d i s c u s s e d later.

of water influences

wood

Density and Specific Gravity. DEFINITION AND CALCULA TION. A w o o d p r o p e r t y that furnishes one of the most useful indices to t h e p r e d i c t e d b e h a v i o r a n d t r e a t a b i l i t y o f w o o d is w o o d ' s bulk density—the mass or w e i g h t of w o o d substance p e r unit v o l u m e , u s u a l l y e x p r e s s e d as g r a m s p e r c u b i c c e n t i m e t e r o r k i l o g r a m s p e r c u b i c m e t e r (2, 24). U n f o r t u n a t e l y , as d e f i n e d a b o v e , b u l k w o o d d e n sity has t w o m a j o r p r o b l e m s associated w i t h its m e a s u r e m e n t . T h e f i r s t is t h e c o n s t a n t q u e s t i o n : at e x a c t l y w h i c h m o i s t u r e c o n t e n t s h o u l d t h e w e i g h t a n d v o l u m e o f t h e s a m p l e b e d e t e r m i n e d ? T h i s is a l w a y s a c o n s i d e r a t i o n b e c a u s e t h e v o l u m e (at l e a s t b e l o w t h e fiber s a t u r a t i o n p o i n t ) a n d w e i g h t o f w o o d t i s s u e c h a n g e w i t h its m o i s t u r e c o n t e n t . T h e s e m e a s u r e m e n t s c a n b e m a d e at a g i v e n m o i s t u r e c o n tent, but accurately a c h i e v i n g a n d m a i n t a i n i n g a particular moisture c o n t e n t is n o t s t r a i g h t f o r w a r d a n d c e r t a i n l y n o t c o n v e n i e n t . The second p r o b l e m associated w i t h the d e t e r m i n a t i o n of w o o d d e n s i t y is s t r i c t l y o n e o f l o g i s t i c s ; t h a t i s , e x a c t l y h o w s h o u l d t h e necessary measurements be made, particularly that of v o l u m e ? If all w o o d samples of i n t e r e s t c o u l d be easily d r e s s e d to perfect g e o m e t rical shapes, v o l u m e m e a s u r e m e n t w o u l d not be a p r o b l e m . H o w e v e r , t h i s is n o t t h e c a s e , a n d d e n s i t y i n f o r m a t i o n is o f t e n d e s i r e d for large specimens, irregularly shaped specimens, or small samples of earlywood or latewood of a single g r o w t h increment. To c i r c u m v e n t t h e s e t w o p r o b l e m s , t h e w o o d t e c h n o l o g i s t u s e s a s p e c i a l o r a r t i f i c i a l p a r a m e t e r k n o w n as basic density. I t is a n a r t i ficial p a r a m e t e r b e c a u s e t h e w e i g h t a n d v o l u m e m e a s u r e m e n t s r e q u i r e d f o r its c a l c u l a t i o n a r e m a d e o n e x t r e m e l y d i f f e r e n t w o o d c o n d i t i o n s — t h e c o m p l e t e l y d r y [or o v e n - d r y ( O D ) ] state for w e i g h t m e a s u r e m e n t s a n d t h e c o m p l e t e l y w e t (or w a t e r - s a t u r a t e d ) state for v o l u m e m e a s u r e m e n t s . I n this way, e v e n t h o u g h w o o d changes i n weight and v o l u m e w i t h changes i n moisture content, measurements o f w e i g h t a n d v o l u m e o f a g i v e n s a m p l e a r e p o s s i b l e at c o n d i t i o n s t h a t a r e as n e a r l y c o n s t a n t a n d r e p r o d u c i b l e as c a n b e o b t a i n e d w i t h w o o d t i s s u e (2). A n a d d i t i o n a l a d v a n t a g e o f u s i n g w a t e r - s w o l l e n o r g r e e n w o o d v o l u m e is t h a t t h e s i m p l e t e c h n i q u e o f w a t e r d i s p l a c e m e n t c a n b e u s e d to m e a s u r e t h e v o l u m e o f l a r g e , s m a l l , a n d / o r i r r e g u l a r l y s h a p e d s a m p l e s . T h i s c o n v e n i e n c e , t o g e t h e r w i t h t h e fact t h a t 1 c m (or 1 m L ) o f w a t e r w e i g h s 1 g, p e r m i t s t h e c o n v e r s i o n o f b a s i c d e n s i t y e x p r e s s e d as O D w t / g r e e n v o l to a n e q u i v a l e n t f r a c t i o n 3


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e x p r e s s e d as O D w t / w t w a t e r d i s p l a c e d . T h e l a t t e r f r a c t i o n is a p u r e n u m b e r ( i . e . , w i t h o u t u n i t s ) , a n d as s u c h , is e q u i v a l e n t t o a r e l a t e d term—specific gravity. T h e s p e c i f i c g r a v i t y o f a m a t e r i a l is d e f i n e d as t h e r a t i o o f t h e d e n s i t y o f t h e m a t e r i a l t o t h e d e n s i t y o f w a t e r (2). I f t h i s c o n c e p t is b r o a d e n e d t o i n c o r p o r a t e t h e p a r a m e t e r o f basic density, another t e r m is o b t a i n e d : basic s p e c i f i c g r a v i t y , w h i c h is d e f i n e d as:

specific


gravity

-

basic density density of water

_

^J^^QJ green vo

O D w

t

wt

displaced water

Consequently, the terms basic density a n d basic specific gravity give the same information, a n d they are different o n l y i n the f u n d a m e n t a l s e n s e t h a t b a s i c s p e c i f i c g r a v i t y is a p u r e n u m b e r a n d b a s i c d e n s i t y is n o t . T h e c h o i c e o f o n e t e r m o v e r t h e o t h e r f o r d e s c r i p t i o n s o f w o o d q u a l i t y is a m a t t e r o f p r e f e r e n c e a n d v a r i e s w i t h p a r t i c u l a r a u t h o r s o r i n v e s t i g a t o r s . H o w e v e r , t h e a s s i g n m e n t o f u n i t s t o t h e s e t e r m s is a p p r o p r i a t e o n l y for basic density. A v a i l a b l e t e c h n i q u e s m a k e i t p o s s i b l e to m e a s u r e t h e t r u e d e n s i t y o f w o o d (at a g i v e n m o i s t u r e c o n t e n t ) . T h e s e i n c l u d e s p e c i a l p r o c e d u r e s t o o b t a i n d r y - w o o d v o l u m e s ( 2 5 ) , as w e l l as s p e c i a l m e t h o d s u s i n g β - r a y o r X - r a y t e c h n o l o g y (26). H o w e v e r , t h e s e p r o cedures are not easily a p p l i e d i n a r o u t i n e fashion, o n a large scale, or on large w o o d specimens. A n y p r o c e d u r e for w o o d specific g r a v i t y or d e n s i t y b a s e d o n measuring weight a n d v o l u m e can be considered accurate only i f the w o o d s a m p l e has b e e n first e x t r a c t e d w i t h s u i t a b l e o r g a n i c s o l v e n t s to r e m o v e e x t r a n e o u s r e s i n s , o i l s , fats, g u m s , e t c . (27). T h e s e m a terials b u l k c e l l w a l l s , b l o c k p o t e n t i a l sites for w a t e r a d s o r p t i o n / a b sorption, alter potential w o o d swelling/shrinkage, and thereby inter fere w i t h the accurate characterization of w o o d tissue.

SIGNIFICANCE.

T h e basic density of w o o d varies w i t h cell size, cell w a l l thickness, a n d the v o l u m e proportion of cells of a g i v e n t y p e . I t affects w o o d s h r i n k a g e a n d s w e l l i n g , m a c h i n a b i l i t y , s u r f a c e texture and microsmoothness, gluability, penetrability of fluids and gases, a n d i n o t h e r r e s p e c t s , g o v e r n s t h e d e g r a d a t i o n o f w o o d b y c h e m i c a l s , fire, a n d m i c r o o r g a n i s m s . I n p a r t i c u l a r , t h e s t r e n g t h o f w o o d a n d i t s stiffness c l o s e l y p a r a l l e l c h a n g e s i n t h e b a s i c d e n s i t y . B a s i c d e n s i t y is n o t a n i n d e p e n d e n t p r e d i c t o r o f w o o d p h y s i c a l p e r f o r m a n c e b e c a u s e t o t a l w o o d b e h a v i o r is p r o f o u n d l y a f f e c t e d b y i t s m o i s t u r e c o n t e n t (2). N e v e r t h e l e s s , i t m a y b e p o s s i b l e t o l e a r n m o r e a b o u t t h e n a t u r e o f a g i v e n w o o d s a m p l e b y d e t e r m i n i n g its b a s i c d e n s i t y t h a n b y a n y o t h e r s i n g l e m e a s u r e m e n t (24).


40

THE CHEMS ITRY OF SOLID WOOD

I n s o f t w o o d s , b a s i c d e n s i t y is s t r o n g l y r e l a t e d to t h e v o l u m e p r o p o r t i o n o f l a t e w o o d a n d i t s a v e r a g e fiber w a l l t h i c k n e s s . H o w e v e r , h a r d w o o d b a s i c d e n s i t y d e p e n d s n o t o n l y o n fiber w a l l t h i c k n e s s b u t a l s o i n v o l v e s t h e v o l u m e r a t i o o f fibers t o v e s s e l s . N a t i v e c o m m e r c i a l w o o d s fall m o s t l y i n t h e basic d e n s i t y range of 0 . 3 5 - 0 . 6 5 g / c m , a l t h o u g h n a t i v e s p e c i e s c a n b e as l o w as 0 . 2 1 g / c m ( c o r k w o o d ) a n d as h i g h as 1.04 g / c m ( b l a c k i r o n w o o d ) (2). 3

3

3

W o o d s w i t h basic d e n s i t y values (means for the species) that fall in the range of < 0 . 3 6 , 0 . 3 6 - 0 . 5 0 , a n d > 0 . 5 0 g / c m are c o n s i d e r e d l i g h t , m o d e r a t e l y l i g h t to m o d e r a t e l y heavy, a n d heavy, r e s p e c t i v e l y , a n d i n c l u d e b o t h t e m p e r a t e a n d t r o p i c a l w o o d s (2). H o w e v e r , f o r a g i v e n s p e c i e s , t h e r e is c o n s i d e r a b l e v a r i a b i l i t y a b o u t a n y p u b l i s h e d a n d a c c e p t e d m e a n . S p e c i f i c a l l y , at l e a s t f o r m o s t N o r t h A m e r i c a n woods, the expected coefficient of variation (i.e., standard deviation d i v i d e d b y t h e m e a n ) is a b o u t 1 0 % (24). T h u s , i f t h e 9 5 % p r o b a b i l i t y l e v e l is t o b e c o n s i d e r e d , a r e a s o n a b l e e s t i m a t e o f t h e t o t a l e x p e c t e d r a n g e o f v a r i a b i l i t y w o u l d b e t h e m e a n b a s i c d e n s i t y ± ( 1 0 % x 1.96 X m e a n basic density). Table I presents the ranges of basic density that m i g h t b e a n t i c i p a t e d for s e v e r a l i m p o r t a n t U . S . w o o d s .


3

A t the cellular level, the true density of d r y cell wall substance ( i . e . , w i t h i n t h e c e l l w a l l ) h a s b e e n d e t e r m i n e d to b e a b o u t 1.5 g/ c m , v a r y i n g to s o m e extent w i t h t h e m e t h o d o f m e a s u r e m e n t a n d s p e c i e s (2). T h e r e a r e v o i d s w i t h i n t h e d r y w o o d c e l l w a l l , b u t t h e v o i d v o l u m e h e r e ( i . e . , m i c r o p o r e s ) is r e p o r t e d to b e o n l y a b o u t 2 - 4 % . H o w e v e r , t h i s figure w o u l d b e e x p e c t e d to i n c r e a s e as w o o d m o i s t u r e c o n t e n t is i n c r e a s e d t o t h e fiber s a t u r a t i o n p o i n t (28). 3

T h e importance of w o o d moisture a n d basic density i n deter mining wood behavior will become more evident in subsequent chap t e r s . S u f f i c e i t h e r e to say t h a t v a r i a t i o n i n t h e a m o u n t o f c e l l w a l l s u b s t a n c e at a g i v e n m o i s t u r e c o n t e n t t h a t m u s t b e t r a v e r s e d b y a penetrating l i q u i d or c h e m i c a l , m i c r o b e , etc., can d e t e r m i n e the rate o f r e a c t i o n as w e l l as t h e e x t e n t o f r e a c t i o n o r t h e c h a n g e i n t h e character of the w o o d i n question.

Wood Variability Causes. D i f f e r e n t s p e c i m e n s o f w o o d e v e n from t h e s a m e t r e e a r e n e v e r i d e n t i c a l a n d a r e s i m i l a r o n l y w i t h i n b r o a d l i m i t s (2). W i t h i n t h e l a r g e r c a t e g o r i e s o f s o f t w o o d s a n d h a r d w o o d s , s u c h v a r i a b i l i t y is e x t e n d e d to different trees o f the same species a n d to different g e n e r a a n d f a m i l i e s . A l l o f t h i s v a r i a b i l i t y o c c u r s n a t u r a l l y a n d is t h e c o m b i n e d r e s u l t o f t r e e g e n e t i c s , t h e e n v i r o n m e n t , a n d t h e age o f t h e v a s c u l a r c a m b i u m ( i . e . , t r e e age). O b s e r v e d c o n s e q u e n c e s are changes i n the type, n u m b e r , a n d f o r m of w o o d cells. Additionally, such changes are not i n f r e q u e n t l y a c c o m p a n i e d b y v a r y i n g w o o d chemistry and cell wall ultrastructure. In The Chemistry of Solid Wood; Rowell, Roger; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.

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Table I. Basic Density (BD) of Some Important U . S . Woods and the E x p e c t e d R a n g e of Variability


Species Slash pine Longleaf pine Loblolly pine Douglas-fir Western hemlock Ponderosa pine W h i t e fir W. white pine

Mean BD" (g/cm )

Expected

3

Softwoods 0.54 0.54 0.47 0.45 0.42 0.38 0.37 0.35

Range ' 1

0.43-0.65 0.43-0.65 0.38-0.56 0.36-0.54 0.34-0.50 0.31-0.45 0.30-0.44 0.28-0.42

Hardwoods Shagbark hickory N . r e d oak Sugar maple W h i t e ash S. r e d o a k Redgum Blackgum Yellow poplar W. red alder Q u a k i n g aspen

0.51-0.77 0.45-0.67 0.45-0.67 0.44-0.66 0.42-0.62 0.37-0.55 0.37-0.55 0.34-0.50 0.30-0.44 0.28-0.42

0.64 0.56 0.56 0.55 0.52 0.46 0.46 0.42 0.37 0.35

B a s e d o n o v e n - d r y weight a n d g r e e n v o l u m e (g/cm ). A s s u m e s a coefficient of variation o f 1 0 % (see Ref. 24). A d d i t i o n a l information was calculated from data i n Ref. 2. a

3

b

S u p e r i m p o s e d o n n a t u r a l w o o d v a r i a b i l i t y is t h a t v a r i a b i l i t y i n d u c e d b y t h e p r o f e s s i o n a l f o r e s t e r , w h o is c h a r g e d w i t h t h e t a s k o f silviculture—the s c i e n c e of p r o d u c i n g a n d m a i n t a i n i n g a forest. T h e p r a c t i c e s o f s c i e n t i f i c f o r e s t m a n a g e m e n t a r e u s u a l l y a i m e d at i n creasing tree g r o w t h rate a n d w o o d p r o d u c t i o n , increasing the av erage l e n g t h of the branch-free bole, a n d s i m u l t a n e o u s l y m a i n t a i n i n g a suitable, if not i m p r o v e d , l e v e l of w o o d quality. W e l l - e s t a b l i s h e d p r o c e d u r e s i n c l u d e , b u t are not l i m i t e d to, r e g u l a t i o n of the n u m b e r of stems p e r acre (stand density), p r u n i n g the l o w e r branches of i n d i v i d u a l s t e m s , f e r t i l i z a t i o n , a n d i r r i g a t i o n (8). A s i n t h e c a s e o f natural tree variability, i m p o s e d treatments cause changes i n w o o d physical characteristics t h r o u g h alteration of the form a n d n u m b e r of various c e l l types. W o o d c h e m i s t r y m a y also b e affected b y s u c h treatments. W i t h i n a g i v e n tree, c e r t a i n aspects of w o o d variability are r e lated to t h e p h y s i c a l p o s i t i o n of w o o d tissue i n the b o l e . O n e m a j o r a s p e c t o f w o o d v a r i a b i l i t y is f o u n d a l o n g t h e t r e e r a d i u s ; w o o d i n t h e c e n t e r o f t h e t r e e is d i f f e r e n t f r o m t h e w o o d m u c h f u r t h e r o u t . T h i s In The Chemistry of Solid Wood; Rowell, Roger; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.

42



situation t h e n leads d i r e c t l y to a s e c o n d m a j o r aspect. T h a t is, b e c a u s e w o o d is l a i d o n t o t h e d e v e l o p i n g s t e m i n t h e f o r m o f i n v e r t e d h o l l o w c o n e s (see F i g u r e 8) v a r i a t i o n i n t h e r a d i a l d i r e c t i o n is r e s p o n s i b l e for a g r a d u a l c h a n g e i n w o o d characteristics w i t h i n c r e a s i n g tree height or position u p the tree bole. Changes i n w o o d properties resulting from natural or i n d u c e d tree variability often i n c l u d e a change i n w o o d basic density. Asso c i a t e d changes i n w o o d a n a t o m y i n c l u d e a l t e r e d g r o w t h rates (ring w i d t h or n u m b e r of rings p e r inch), fiber w a l l thickness, earlywood to l a t e w o o d t r a n s i t i o n , p e r c e n t l a t e w o o d , a n d v e s s e h f i b e r r a t i o . T h e e n s u i n g changes i n b e h a v i o r or reactivity of w o o d i n the affected trees w i l l v a r y accordingly, f o l l o w i n g the same trends for c h a n g i n g basic density discussed previously.

HEARTWOOD AND SAPWOOD.

Major Types. W h e n a t r e e is y o u n g (a r e l a t i v e t e r m , h o w e v e r ) , u p w a r d c o n d u c t i o n o f sap i n t h e x y l e m is p o s s i b l e t h r o u g h t h e e n t i r e c r o s s - s e c t i o n a l p l a n e . ( U p w a r d c o n d u c t i o n m a y not always i n v o l v e the e n t i r e b o l e cross section, w h i c h is d e p e n d e n t o n t r e e t y p e , b u t at l e a s t i t is p o s s i b l e . ) A d d i tionally, i n the four softwood g e n e r a m e n t i o n e d earlier, r e s i n canals are n o r m a l l y f u n c t i o n a l , a n d i n a l l trees, most storage p a r e n c h y m a a r e s t i l l l i v i n g . A t t h i s p o i n t , t h e x y l e m is k n o w n a p p r o p r i a t e l y as sapwood, w h i c h is n o r m a l l y l i g h t - c o l o r e d ( F i g u r e 27). A s t r e e s g r o w o l d e r , t h e c e n t r a l p o r t i o n o f m o s t t r e e s is e v e n t u a l l y a l t e r e d , p r i m a r i l y c h e m i c a l l y b u t a l s o s t r u c t u r a l l y t o s o m e e x t e n t , to t h e p o i n t w h e r e it m u s t b e d i s t i n g u i s h e d f r o m n o r m a l s a p w o o d . I n its a l t e r e d state this c e n t r a l r e g i o n o f t h e tree b o l e , v a r i a b l e i n size a n d o t h e r c h a r a c t e r i s t i c s , is t e r m e d heartwood a n d c o n t i n u e s to e n l a r g e w i t h t i m e (2). T h e characteristics o f h e a r t w o o d that serve to d i s t i n g u i s h it f r o m n o r m a l s a p w o o d are the f o l l o w i n g :

Figure 27. Heartwood distribution (H) as seen in tree cross sections from two blackjack oaks. Note that the heartwood zone increases with tree age. (Photo by W. J. McCleary.)


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1. T h e e n t i r e r e g i o n is n o r m a l l y i n f i l t r a t e d a n d i n c r u s t e d w i t h organic extractives that are largely p o l y p h e n o l i c i n nature. T h e s e materials are d e r i v e d f r o m the c o n v e r s i o n of starch, sugars, a n d organic extractives present i n the sapwood parenchyma. U p o n heartwood formation, the p a r e n c h y m a cells usually die, and their contents t h e n p r o c e e d to i n f i l t r a t e c e l l w a l l s , i n c r u s t p i t m e m b r a n e s ( F i g u r e 28), a n d c a n e v e n p l u g v e s s e l s i n t h e a f f e c t e d a r e a s (2). 2. T h e m o i s t u r e c o n t e n t o f h e a r t w o o d i n s o f t w o o d t r e e s is r e d u c e d to a l e v e l m u c h l o w e r t h a n that of n o r m a l sapw o o d (2, 24). D u r i n g t h e m o i s t u r e r e d u c t i o n p e r i o d , the m e m b r a n e s of b o r d e r e d pits i n sapwood fibers have a s t r o n g t e n d e n c y to b e c o m e a s p i r a t e d . T h i s s i t u a t i o n , together w i t h that of p i t m e m b r a n e i n c r u s t a t i o n , greatly r e d u c e s t h e n a t u r a l p e r m e a b i l i t y o f h e a r t w o o d t i s s u e to l i q u i d s a n d gases. 3. T h e v e s s e l s i n m a n y h a r d w o o d s p e c i e s ( e . g . , w h i t e oaks and hickories) become plugged w i t h bubblelike intru s i o n s f r o m a d j a c e n t p a r e n c h y m a c e l l s ( F i g u r e 29). T h e s e s t r u c t u r e s , t e r m e d tyloses ( p l u r a l ) , i n v a d e t h e v e s s e l l u m e n t h r o u g h vessel e l e m e n t / p a r e n c h y m a pits and represent e r u p t i v e outgrowths of the parenchyma's l i v i n g c o n t e n t s (29). T y l o s e s g r e a t l y r e d u c e w o o d p e r meability. 4.

H e a r t w o o d m a y take o n a distinctive color, i n c l u d i n g shades of b r o w n , y e l l o w , orange, or r e d (e.g., s o u t h e r n p i n e , Douglas-fir, r e d w o o d , oak, black w a l n u t , a n d

Figure 28. SEM of an incrusted, aspirated-pit membrane in western hemlock heartwood. (Reproduced from Ref 39. Copyright 1982, American Chemical Society.)



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Figure 29. Tyloses in white oak heartwood. (A) SEM of transverse/tangential view showing vessels plugged with numerous tyloses (T). (Reproduced from Ref 39. Copyright 1982, American Chemical Society); (B) Light micrograph of radial section ofearly wood showing a plugged vessel.

b l a c k c h e r r y ) . H o w e v e r , i n o t h e r s p e c i e s , t h i s z o n e is n o t n o t i c e a b l y c h a n g e d i n c o l o r ( e . g . , s p r u c e , fir, h e m l o c k , a s p e n , a n d c o t t o n w o o d ) (2). N e v e r t h e l e s s , t h e h e a r t w o o d e v e n i n these species still contains an ab n o r m a l l y h i g h e x t r a c t i v e s c o n t e n t as w e l l as e x h i b i t i n g the physical alterations described i n characteristics 1 3. J u s t t h e e x t r a c t i v e s t h e m s e l v e s , w h e t h e r c o l o r e d o r not, c a n h a v e a n effect o n w o o d r e a c t i v i t y o r t r e a t m e n t p r o c e s s e s s u c h as finishing, preservation, gluing, and the production of p o l y m e r / w o o d composites. 5. T h e h e a r t w o o d e x t r a c t i v e s i n c e r t a i n s p e c i e s ( e . g . , r e d w o o d a n d cedars) are toxic, to s o m e e x t e n t , a n d r e n d e r the w o o d m o r e resistant to d e c a y m i c r o o r g a n i s m s a n d /


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o r i n s e c t s (2). H o w e v e r , w o o d d u r a b i l i t y is d u e t o t h e specific toxicity a n d a m o u n t o f the extractives, rather t h a n to h e a r t w o o d c o l o r p e r se.


REACTION WOOD.

W h e n a t r e e s t e m o r b r a n c h is b r o u g h t o u t of its n o r m a l e q u i l i b r i u m p o s i t i o n i n space b y an outside force, a n a c c e l e r a t e d r a d i a l g r o w t h is p r o m o t e d o n e i t h e r t h e l o w e r o r u p p e r side o f t h e s t e m o r b r a n c h i n q u e s t i o n . S u c h g r o w t h is i n i t i a t e d b y t h e a f f e c t e d t r e e p a r t as a m e a n s t o r e a c h i e v e i t s n a t u r a l o r e q u i l i b r i u m p o s i t i o n . I n d o i n g so, t h e s t e m o r b r a n c h m a y d e v e l o p e c c e n t r i c o r e l l i p t i c a l g r o w t h r i n g s (as s e e n i n c r o s s s e c t i o n ) , a n d t h e w i d e s t s i d e o f t h e e c c e n t r i c i t y is c o m p o s e d o f c e l l s a l t e r e d b o t h s t r u c t u r a l l y and c h e m i c a l l y from n o r m a l w o o d , adjacent or side w o o d , a n d o p posite w o o d . T h e a t y p i c a l cells o r tissues g e n e r a t e d i n t h e regions of a c c e l e r a t e d g r o w t h a r e k n o w n c o l l e c t i v e l y as reaction wood b e c a u s e the tissues r e f l e c t t h e t r e e s response o r r e a c t i o n to a n alteration i n its u s u a l e n v i r o n m e n t (30). G r a v i t a t i o n a l a n d h o r m o n a l s t i m u l i a r e i n v o l v e d , b u t t h e precise physiology g o v e r n i n g reaction w o o d for mation remains to b e clarified. R e a c t i o n w o o d i n s o f t w o o d s is n o r m a l l y c o n c e n t r a t e d o n t h e u n d e r s i d e o r l o w e r side o f t h e affected tree o r b r a n c h . Because t h e w o o d i n s u c h regions appears to b e subjected to c o m p r e s s i o n a l forces, t h i s w o o d is g i v e n t h e n o n t e c h n i c a l d e s i g n a t i o n o f compression wood ( F i g u r e 30). I n h a r d w o o d s t h e l o c a t i o n a n d c o n c e n t r a t i o n o f r e a c t i o n w o o d t i s s u e is n o r m a l l y o p p o s i t e t h a t i n s o f t w o o d s , i . e . , o n t h e u p p e r s i d e o f b r a n c h e s a n d l e a n i n g s t e m s , a n d t h e t i s s u e is c a l l e d tension wood ( F i g u r e 3 1 ) . R e a c t i o n w o o d i n s o f t w o o d s a n d h a r d w o o d s is a l s o located i n the vicinity of points o n any tree bole where branches originate. Although concentrated i n a n d around branches and i n obviously leaning tree boles, reaction w o o d zones can be frequently scattered

Figure 30. Compression wood in the stem cross section of a severely leaning Doughs-fir. Note the growth ring eccentricity (NW = normal wood). (Reproduced from Ref 39. Copyright 1982, American Chemical Society. J



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Figure 31. Cross-sectional surface of a young quaking aspen stem. Note the white arcs of tension wooa(TW) (NW = normal wood). (Reproduced from Ref. 39. Copyright 1982, American Chemical Society.)

t h r o u g h o u t a n y s t e m cross-section i n short arcs, r e f l e c t i n g t h e v a r i ation i n stresses i m p o s e d o n a tree w i t h t i m e . T h e formation, structure, and chemistry o f c o m p r e s s i o n w o o d h a v e b e e n g i v e n c o n s i d e r a b l e a t t e n t i o n (see R e f e r e n c e 3 0 and r e f e r e n c e s c i t e d t h e r e i n ) . C o m p r e s s i o n w o o d t i s s u e exhibits both major a n d subtle differences from n o r m a l softwood x y l e m . T h e s e d i f f e r e n c e s (Table II) a r e e i t h e r d i r e c t l y o r i n d i r e c t l y responsible for a n y d i s t i n g u i s h a b l e changes i n the b e h a v i o r a n d treat ability of c o m p r e s s i o n w o o d versus the b e h a v i o r generally n o t e d for normal wood.

COMPRESSION WOOD.

P e r h a p s the most i n f l u e n t i a l properties of compression w o o d are its h i g h l i g n i n a n d l o w c e l l u l o s e c o n t e n t s , i t s t h i c k a n d u l t r a s t r u c t u r ally m o d i f i e d fiber walls, a n d t h e r e d u c t i o n of total m i d d l e l a m e l l a s u b s t a n c e ( a n d i n t e r c e l l u l a r a d h e s i o n ) d u e to r o u n d e d f i b e r o u t l i n e s a n d i n t e r c e l l u l a r spaces ( F i g u r e 32). A t t e n d a n t w o o d features i n c l u d e (usually) h i g h e r - t h a n - n o r m a l basic d e n s i t y a n d w o o d hardness, ex t r e m e l y h i g h l o n g i t u d i n a l shrinkage, l o w e r moisture h o l d i n g capacity, brashness (abruptness) i n m e c h a n i c a l fracture, a n d altered strength properties (usually lower than normal). I n s e v e r e l y l e a n i n g s o f t w o o d t r e e s , c o m p r e s s i o n w o o d is f o r m e d i n v e r y l a r g e p r o p o r t i o n s a n d m a i n l y u n i l a t e r a l l y , g i v i n g r i s e to t h e aforementioned eccentric g r o w t h rings. I n relatively straight or erect t r e e s , c o m p r e s s i o n w o o d is f o u n d i n s m a l l e r , s c a t t e r e d a r c s o r l u n e s , p a r t i c u l a r l y n e a r t h e p i t h o r t r e e c e n t e r . Its d a r k r e d c o l o r i n d o m e s t i c softwoods r e a d i l y distinguishes c o m p r e s s i o n w o o d along the tree bole a n d at t h e b a s e o f a l l b r a n c h e s a n d a r o u n d k n o t s . I t also a p p e a r s to be a subtle b u t constant feature of y o u n g , rapidly g r o w i n g conifers a n d m a y b e p r o m o t e d i n y o u n g o r o l d e r trees r e m a i n i n g after h e a v y t h i n n i n g o f a c r o w d e d f o r e s t s t a n d (2, 31). R e a c t i o n w o o d is n o t f o u n d as c o n s i s t e n t l y i n

TENSION WOOD.


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h a r d w o o d s as i n s o f t w o o d s . T h a t i s , t e n s i o n w o o d i s n o t d e t e c t e d i n s o m e s p e c i e s , at l e a s t i n a d e f i n i t i v e m a n n e r , y e t i t c a n m a n i f e s t i t s e l f differently i n different species, w i t h i n t h e same tree, or even w i t h i n t h e s a m e g r o w t h r i n g (32, 33). I t m a y c o n s i s t o f l a r g e b a n d s s i m i l a r to t h o s e o f c o m p r e s s i o n w o o d , b u t t e n s i o n w o o d is c o m m o n l y d i s t r i b u t e d (particularly i n fast-growing, y o u n g , and/or relatively straight trees) i n a m o r e d i f f u s e p a t t e r n o f s m a l l e r areas (33). I n s o m e o f t h e s e l a t t e r c a s e s , i t m a y b e so d i f f u s e as t o b e d e t e c t a b l e o n l y b y m i c r o s c o p i c a l e x a m i n a t i o n (34). T h e salient features o f t e n s i o n w o o d are s u m m a r i z e d i n Table I I . M o s t notable are t h e increased v o l u m e of fibers, t h e h i g h cellulose content, l o w lignin content, and the special wall architecture of ten sion w o o d fibers. D e p e n d i n g o n species, g r o w t h rate, a n d severity of tree lean, t e n s i o n w o o d f i b e r s c a n e x h i b i t o n e o f s e v e r a l f o r m s . I n m o s t cases the formation o f tension w o o d involves t h e deposition of a loosely a t t a c h e d l a y e r at ( n o r m a l l y ) t h e f i b e r l u m e n ( F i g u r e 3 3 ) , a l a y e r t h a t is a b o u t 9 8 % c e l l u l o s e a n d w h o s e m i c r o f i b r i l s a r e a l i g n e d e s s e n t i a l l y p a r a l l e l to t h e f i b e r axis. A s seen i n u n e m b e d d e d , m i c r o t o m e d w o o d cross s e c t i o n s s t a i n e d f o r l i g h t m i c r o s c o p y , t h i s s p e c i a l w a l l l a y e r is almost always d i s t o r t e d o r c o n v o l u t e d a n d separated f r o m t h e rest o f t h e s e c o n d a r y w a l l t o g i v e t h e i m p r e s s i o n o f b e i n g soft o r p e r h a p s gelatinous ( F i g u r e 33). B a s e d o n these observations, tension w o o d f i b e r s h a v e b e e n r e f e r r e d t o as gelatinous fibers o r s i m p l y G-fibers (meaning they contain a gelatinous or G-layer) a n d have been used as a m e a n s b y w h i c h t o d e f i n e t h e e x i s t e n c e a n d l o c a t i o n o f t e n s i o n wood. I n some species t h e G-layers are m i s s i n g i n t h e tension w o o d zones, b u t s u c h zones still t e n d to have r e d u c e d Ugnin contents (2, 35). T h e e x t r a c e l l u l o s e c o n t e n t o f t e n s i o n w o o d t i s s u e is m o s t c o m m o n l y d u e to t h e presence o f fiber G - l a y e r s . H o w e v e r , t h e layers themselves a r e n o t really gelatinous. O n t h e contrary, they are q u i t e h i g h l y c r y s t a l l i n e , a n d t h i s fact, t o g e t h e r w i t h t h e a x i a l o r i e n t a t i o n o f their microfibrils, renders this layer easily distorted i n t h e horizontal p l a n e ( i . e . , n o r m a l t o t h e f i b e r axis). T h e o t h e r significant s t r u c t u r a l feature o f t e n s i o n w o o d fibers is the nature o f the rest o f the secondary w a l l , w h i c h m a y lack a n S o r S a n d S (36) ( F i g u r e 3 4 ) . 3

3

2

T h e reduced vessel v o l u m e of tension wood, together with thick e n e d fiber walls, c a n lead to a h i g h e r than n o r m a l basic density. T h i s general situation, coupled w i t h a difference in w o o d chemistry, could cause a v a r i a b l e response o f s u c h tissue to b o t h c h e m i c a l a n d p h y s i c a l treatments o r to m i c r o b i a l degradation w h e n c o m p a r e d to n o r m a l hardwood xylem. A l t h o u g h m u c h i s k n o w n a b o u t r e a c t i o n w o o d , t h e s p e c i f i c ef fects o f its c h a r a c t e r i s t i c s o n s u c h p r o c e s s e s as w o o d p r e s e r v a t i o n , .•merican U i e m i c a f In The Chemistry of Solid Wood; Rowell, Roger; Society Advances in Chemistry; American Chemical Library Society: Washington, DC, 1984.


Anatomical features

Macroscopic features

Characteristics Wood

1. M o r e p r e v a l e n t i n m i d d l e o r at e n d o f growth increments. 2. A b r u p t e a r l y w o o d / l a t e w o o d t r a n s i t i o n s b e c o m e more gradual, a n d vice versa. 3. T i s s u e i s r e d d i s h c o l o r a n d d a r k e r t h a n n o r m a l w o o d (in n o r t h e r n hemisphere). 1. F i b e r c r o s s - s e c t i o n a l o u t l i n e s t e n d t o b e r o u n d e d , p a r t i c u l a r l y i n l a t e w o o d areas. 2. I n t e r c e l l u l a r s p a c e s b e t w e e n f i b e r s a n d b e t w e e n fibers a n d r a y c e l l s a r e common, varying with compression wood severity. 3. T h e t o t a l a m o u n t o f m i d d l e l a m e l l a is less t h a n i n n o r m a l w o o d . 4. I n t e r f i b e r p i t s a r e f e w e r a n d s l i t l i k e .

Compression

Wood

Wood

1. D e v e l o p s m o r e r e a d i l y i n e a r l y w o o d . 2. I f c o n c e n t r a t e d i n t o l a r g e a r e a s , i s w h i t i s h i n color c o m p a r e d to n o r m a l wood. 3. W o o l y s u r f a c e o f b o a r d s s a w n w h i l e g r e e n , d u e t o fiber p u l l - o u t . 1. V o l u m e r a t i o o f fibers t o v e s s e l s i s increased a n d vessels m a y b e s m a l l e r i n diameter. 2. M a y o c c u r as l a r g e b a n d s , s m a l l a n d d i f f u s e l y s c a t t e r e d a r e a s , a n d / o r as i n d i v i d u a l f i b e r s o r fiber c l u s t e r s detectable only w i t h a microscope.

Tension

Table II. M a j o r Structural a n d Chemical Characteristics of Reaction


In The Chemistry of Solid Wood; Rowell, Roger; Advances in Chemistry; American Chemical Society: Washington, DC, 1984. 2

1. V e r y h e a v i l y l i g n i f i e d ( a b o u t 3 0 - 4 0 % more than normal), but middle lamella contains less l i g n i n t h a n i n n o r m a l w o o d tissue. 2. L i g n i n i t s e l f is m o r e h i g h l y c o n d e n s e d than in normal wood. 3. C e l l u l o s e c o n t e n t is 2 0 - 2 5 % less t h a n i n n o r m a l w o o d a n d is less c r y s t a l l i n e . 4. L e s s g a l a c t o g l u c o m a n n a n a n d m o r e galactan than n o r m a l ; other p o l y s a c c h a r i d e s a r e s i m i l a r to t h o s e i n normal wood. 5. P o s s i b l y h i g h e r e x t r a c t i v e s c o n t e n t , b u t t h i s is s p e c i e s - d e p e n d e n t .

2

Chemistry

2

3

S l a y e r is m i s s i n g ; S j l a y e r is considerably thicker than normal. 2. S l a y e r i n v a s t m a j o r i t y o f s p e c i e s contains helical cavities i n the same d i r e c t i o n as S m i c r o f i b r i l s . 3. T h e S o r i e n t a t i o n is c o m m o n l y 30—50°, greater than i n n o r m a l wood. 4. T h e w a r t y l a y e r is s t i l l p r e s e n t .

1.

Ultrastructure

2

2

3

x

1. T e n s i o n w o o d fiber l a y e r i n g v a r i e s w i t h species and tension w o o d severity, a n d may consist of Sj + S + S + G , S + S + G , o r Si + G , w h e r e G is t h e gelatinous layer. 2. G - l a y e r s a r e o r i e n t e d e s s e n t i a l l y p a r a l l e l to t h e fiber a x i s . 3. G - f i b e r s m a y c o n t a i n m i n u t e w a l l dislocations or axial compression failures. 1. A s a p e r c e n t a g e o f t o t a l w o o d , t e n s i o n w o o d has m o r e c e l l u l o s e , l e s s l i g n i n , a n d fewer xylose residues than n o r m a l w o o d . 2. F i b e r s e c o n d a r y w a l l e x t e r i o r to t h e G l a y e r is j u s t as l i g n i f i e d , i f n o t m o r e so, than in normal w o o d fibers. 3. A m o u n t s o f s t o r e d s u g a r s , i n c l u d i n g starch, are l o w e r t h a n for n o r m a l w o o d tissue.




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Figure 32. Cross-sectional views of normal and compression wood. (A) Light micrograph of normal wood of loblolly pine at a growth ring boundary. Key: EW, earlywood; and LW, latewood. (Reproduced with permission from Ref. 41. Copynght 1971, Spnnger-Verlag.) (B) Light micrograph of compression wood of loblolly pine. Note the rounded fiber outlines and the presence of intercellular spaces (IS). (Reproduced with permission from Ref. 41. Copyright 1971, Springer-Verlag.) (C) SEM of Doughs-fir compression wood. Note the helical fissures (HF) extending radially within the fiber wall. ( Reproduced from Ref. 39. Copyright 1982, American Chemical Society.) (D) High magnification (TEM) of a comression wood fiber in loblolly pine. Note the absence of an S layer. The elical fissures here terminate in the outer region of the S layer. Key: ML, middle lamella; IS, intercelluhr space; and L, lumen. 3

2



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Figure 33. Cross-sectional views of tension wood in a young quaking aspen stem. (Reproduced from Ref. 39. Copyright 1982, American Chemical Society. )(A) Light micrograph of a section that was selectively stained to differentiate the gelatinous layers in G-fibers. (B) SEM of a surface of tension wood fiber zone. The G-layers, which are loosely attached to the rest of the fiber wall, were dislodged during specimen preparation and drying.

finishing, g l u i n g , o r i t s r e s i s t a n c e t o fire, w e a t h e r i n g , o r d e c a y a r e not clearly d o c u m e n t e d yet. H o w e v e r , some trends i n t h e general behavior of reaction wood can be predicted from a broad knowledge o f t h e n a t u r a l v a r i a b i l i t y a n d b e h a v i o r o f n o r m a l w o o d . O n e fact i s c l e a r ; t h e o c c u r r e n c e o f r e a c t i o n w o o d is a m a j o r c o n t r i b u t i o n t o x y l e m n o n u n i f o r m i t y , a factor that always tends to d e t e r t h e accurate prediction of w o o d quality.


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F i g u r e 34. Possible types qf tension wood fibers found among hardwood species. (Adapted from Ref. 37.)

différera

JUVENILE AND MATURE WOOD. The wood produced in young trees, the wood formed during the early or juvenile years of older trees (including the tree top and entire tree core), and most branches differ in several respects from the wood formed in the outer trunk of the same tree when the tree is more mature (Figure 35). These dissimilarities include cell size, wall thickness and microfibril orientations, varying proportions of particular ceU types, growth rate and ring width, knot volume, in situ moisture content (varying inversely with percent heartwood), and wood chemistry. The differences are responsible for the often somewhat peculiar physical behavior and chemical nature of juvenile wood including its basic density, bending strength, elasticity, and shrinkage (2). A complete explanation of why wood formed near the pith during the early Me of a tree, also called corewood, is not the exact type

Figure 3S. The general location of juvenile wood and knotwood in tree stems. (A) Cross-section qf a Sitka spruce showing portions qf two branch traces (knots). (Reproduced from Ref. 39. Copyright 1982, American Chemical Society.) (B) Schematic indicating the typical location qf juvenile wood or corewood in softwood trees.



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p r o d u c e d w h e n t h e t r e e is m o r e m a t u r e , a l s o c a l l e d outerwood, re m a i n s to b e s p e c i f i e d . H o w e v e r , t h e s i t u a t i o n appears to b e r e l a t e d i n s o m e w a y s to t h e s e x u a l m a t u r i t y o f t h e t r e e o r species i n q u e s t i o n , m a t u r i t y b e i n g w h e n t h e t r e e is p h y s i o l o g i c a l l y c a p a b l e o f p r o d u c i n g flowers, fruits, a n d seeds. A d d i t i o n a l l y , the p r o x i m i t y of the vascular c a m b i u m to t h e t r e e c r o w n (leaves a n d b u d s ) also s e e m s to exert a particularly strong influence on w o o d cell characteristics d u r i n g the first 5 - 2 0 y e a r s o f t r e e g r o w t h (2). T h u s , t h e c h a n g e s i n w o o d c h a r a c t e r i s t i c s i n a g i v e n t r e e from p i t h t o b a r k ( i . e . , w i t h i n c r e a s i n g t r e e age) a r e f u n d a m e n t a l l y r e l a t e d t o t h e age o r m a t u r i t y o f t h e c a m b i u m i t s e l f , as w e l l as t o t h e effects o n c a m b i a l a c t i v i t y c a u s e d b y n a t u r a l and i n d u c e d changes i n the e n v i r o n m e n t . B o t h softwoods a n d h a r d w o o d s a r e a f f e c t e d , b u t j u v e n i l i t y is e s p e c i a l l y w e l l m a r k e d i n t h e x y l e m of softwoods. In either softwoods or hardwoods the trends i n w o o d variability f r o m p i t h to b a r k i m p o s e a p r o g r e s s i v e b u t i n v e r s e change o f s i m i l a r t y p e w i t h i n c r e a s i n g d i s t a n c e u p t h e t r e e , b e c a u s e n e w w o o d is l a i d o n t o a d e v e l o p i n g t r e e b o l e i n t h e f o r m o f i n v e r t e d h o l l o w c o n e s (see F i g u r e 8). I n o t h e r w o r d s , j u v e n i l e w o o d f e a t u r e s d i m i n i s h o u t w a r d l y a l o n g t h e t r e e r a d i u s at a g i v e n l e v e l i n t h e t r e e b u t a r e a c c e n t u a t e d w i t h i n c r e a s i n g h e i g h t i n t h e t r e e b o l e ( F i g u r e 35). Physical Properties. J u v e n i l e w o o d t y p i c a l l y (but not always) exhibits w i d e r growth rings, a higher earlywood/latewood ratio, lower basic density (variable w i t h species), h i g h e r m o i s t u r e content, a n d m u c h greater longitudinal shrinkage than mature, n o r m a l sapwood o f t h e s a m e t r e e (2). A b o u t t w o - t h i r d s o f c o m m e r c i a l s o f t w o o d s a n d hardwoods show a r a p i d l y increasing basic density i n the radial d i r e c t i o n , l e v e l i n g o u t i n m a t u r e w o o d o r i n c r e a s i n g g r a d u a l l y for m a n y years. O t h e r tree species show s o m e k i n d of parabolic t r e n d , starting o u t at t h e p i t h e i t h e r h i g h e r o r l o w e r t h a n i n o u t e r w o o d (2). In softwoods, the generally h i g h e r basic density ( 1 0 - 1 5 % ) of o u t e r w o o d is d u e t o t h i c k e r w a l l e d fibers a n d a n i n c r e a s e d p r o p o r t i o n of latewood. H o w e v e r , i n h a r d w o o d s , j u v e n i l e w o o d a n d m a t u r e w o o d d e n s i t i e s v a r y n o t o n l y w i t h fiber w a l l t h i c k n e s s b u t a l s o w i t h the ratio of fiber/vessel v o l u m e . Radial patterns i n basic density of h a r d w o o d s a r e a l s o less c o n s i s t e n t t h a n i n t h e s o f t w o o d s , a p p a r e n t l y d e p e n d e n t t o a l a r g e e x t e n t o n w h e t h e r t h e s p e c i e s is r i n g - , s e m i ring-, or diffuse-porous, the tree's g r o w t h rate a n d p e r c e n t of late w o o d , a n d t h e v e r t i c a l p o s i t i o n i n t h e t r u n k (2). W o o d t i s s u e i n w h i c h t h e f i b e r / v e s s e l r a t i o o r f i b e r w a l l t h i c k n e s s is g r e a t e r t h a n s u r r o u n d i n g areas w i l l , i n g e n e r a l , h a v e a h i g h e r basic density. A t t h e u l t r a s t r u c t u r a l l e v e l , j u v e n i l e w o o d fibers h a v e a m u c h g r e a t e r S m i c r o f i b r i l a n g l e t h a n n o r m a l m a t u r e w o o d fibers. T h e n e t 2


54


r e s u l t o f t h i s s i t u a t i o n is t h e g r e a t e r t h a n u s u a l w o o d s h r i n k a g e a l o n g the grain. T h e h i g h microfibril angle of j u v e n i l e w o o d can i n part be a t t r i b u t e d to t h e c o m m o n o c c u r r e n c e o f c o m p r e s s i o n w o o d a n d t e n s i o n w o o d n e a r t h e p i t h . H o w e v e r , e v e n t h e n o r m a l w o o d fibers i n j u v e n i l e w o o d h a v e a n i n h e r e n t l y h i g h S a n g l e , p a r t i c u l a r l y i n soft w o o d s (2).


2

Chemistry. F o r b o t h softwoods a n d hardwoods, the i n t e r p r e tation of general trends i n changing w o o d c h e m i s t r y along the tree r a d i u s is c o m p l i c a t e d b y t h e p o t e n t i a l o c c u r r e n c e a n d d i s t r i b u t i o n o f reaction wood. Nevertheless, hardwoods show relatively little change i n c e l l u l o s e o r l i g n i n c o n t e n t s f r o m p i t h to b a r k a n d f r o m tree base t o t o p (2). H o w e v e r , s o f t w o o d s h a v e t r e e c o r e s t h a t a r e a b o u t 3 - 2 0 % l o w e r i n c e l l u l o s e a n d u p to s e v e r a l p e r c e n t h i g h e r i n l i g n i n c o n t e n t ( p r o b a b l y d u e at least i n p a r t to t h e p r e s e n c e o f c o m p r e s s i o n w o o d ) (2). T h e r e is l i m i t e d i n f o r m a t i o n r e g a r d i n g p o t e n t i a l d i f f e r e n c e s i n the nature a n d amount of j u v e n i l e w o o d and mature w o o d h e m i c e l l u l o s e s (2). W h a t d a t a a r e a v a i l a b l e i n d i c a t e t h a t t h e r e c a n b e l i m i t e d c h a n g e s i n t h e r e l a t i v e a m o u n t s o f s o m e s i m p l e s u g a r s , b u t t h e r e is a p p a r e n t l y little or n o significant difference for e i t h e r softwoods or hardwoods. T h e amount a n d nature of extractives i n n o r m a l j u v e n i l e w o o d are g e n e r a l l y s i m i l a r to those of m a t u r e s a p w o o d i n t h e s a m e t r e e , except that extractives near the tree c e n t e r can be a l t e r e d w i t h t i m e in m a n y trees b y the d e v e l o p m e n t of heartwood. Specifically, the amount a n d toxicity of j u v e n i l e w o o d extractives may both increase f r o m t h e p i t h t o t h e h e a r t w o o d / s a p w o o d b o u n d a r y (2). H o w e v e r , resin extractives are a s o m e w h a t special case. I n trees w i t h or w i t h o u t h e a r t w o o d , r e s i n c o n t e n t is r e p o r t e d t o b e h i g h e s t n e a r t h e p i t h a n d at t h e t r e e b a s e . I t t h e n d e c r e a s e s o u t w a r d a n d u p w a r d i n t h e s t e m (2). F r o m the o u t e r w o o d of o l d e r trees, l u m b e r , v e neer, or c h i p s can be cut w i t h o u t the i n c l u s i o n of natural w o o d defects c a l l e d knots. K n o t s a r e r e s i d u a l , e m b e d d e d p o r t i o n s o f b r a n c h e s , o r m o r e specifically, b r a n c h bases. A l t h o u g h knots are usually c o n c e n trated i n w o o d near the p i t h , i.e., i n c r o w n - f o r m e d w o o d , they are c h a r a c t e r i s t i c o f w o o d i n a n y s t e m r e g i o n t h a t is m a n u f a c t u r e d w h i l e in the p r o x i m i t y of branches ( F i g u r e 35A).

KNOTWOOD.

K n o t s r e p r e s e n t w h a t is left a f t e r t h e d e a t h a n d n a t u r a l loss o r p r u n i n g of tree b r a n c h e s , w h i c h takes place o n the l o w e r tree bole w i t h i n c r e a s i n g tree age. N a t u r a l p r u n i n g takes place r e l a t i v e l y e a r l y i n c r o w d e d f o r e s t s t a n d s ( p o l e s t a n d s ) , a n d k n o t v o l u m e i n t h e s e cases is m i n i m a l . P r u n i n g is d e l a y e d i n m o r e w i d e l y s p a c e d o r o p e n - g r o w n s t a n d s (8). H o w e v e r , i n o p e n - g r o w n s t a n d s , a r t i f i c i a l p r u n i n g b y t h e forest m a n a g e r c a n b e u s e d to p r o d u c e trees w i t h a greater p r o p o r t i o n of knot-free wood.


1.

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B r a n c h bases b e c o m e g r a d u a l l y e m b e d d e d i n a n y e n l a r g i n g tree b o l e . R e s u l t i n g k n o t s m a y b e d e s i g n a t e d as tight ( r e d o r i n t e r g r o w n ) if g r o w t h o f t h e s t e m a r o u n d t h e b r a n c h base takes place w h i l e that p o r t i o n o f t h e b r a n c h is s t i l l l i v i n g . C o n v e r s e l y , i f t h e b r a n c h b a s e is already d e a d f r o m t h e t i m e it b e c o m e s e m b e d d e d , t h e r e w i l l b e a b r e a k i n t i s s u e c o n t i n u i t y w i t h t h e s t e m , a n d t h e r e s u l t w i l l b e a loose ( b l a c k o r e n c a s e d ) k n o t (2). T h e l o o s e k n o t is m o r e l i k e l y t o l e a v e holes u p o n processing of t h e tree into w o o d products. T h e ultimate n u m b e r , size, a n d type of knot w i l l d e p e n d o n the n u m b e r a n d size o f l i m b s f r o m w h i c h t h e y o r i g i n a t e , l i m b a g e at d e a t h , a n d l e n g t h o f t i m e t h e d e a d l i m b s t u b s p e r s i s t o r r e m a i n o n t h e t r e e (2). K n o t w o o d is e x t r e m e l y d e n s e a n d h a r d , c o n t a i n s a h i g h p e r c e n t a g e o f r e a c t i o n w o o d , a n d i n s o m e s o f t w o o d s is v e r y r e s i n o u s (for species w i t h n o r m a l r e s i n ducts). T h e stem regions b e l o w a n d above knots a r e h i g h i n r e a c t i o n w o o d , a n d these regions also e x h i b i t s e v e r e l y d i s t o r t e d g r a i n . T h e e x a c t areas a f f e c t e d v a r y w i t h k n o t s i z e , b u t t h e g r a i n n o r m a l l y passes i n w i d e s w e e p s to e i t h e r side. T h e presence of knots a n d associated differences i n w o o d properties are g e n e r a l l y c o n s i d e r e d d e t r i m e n t a l to processes i n v o l v i n g w o o d m a c h i n i n g , f i n i s h i n g , g l u i n g , a n d to most w o o d s t r e n g t h p r o p e r t i e s , p a r t i c u l a r l y b e n d i n g s t r e n g t h (2).

Literature Cited

M.

1. Esau, K. "Anatomy of Seed Plants"; John Wiley & Sons: New York, 1977. 2. Panshin, A. J.; de Zeeuw, C. "Textbook of Wood Technology"; McGraw -Hill: New York, 1980. 3. Sjostrom, E , "Wood Chemistry—Fundamentals and Applications"; Aca demic Press: New York, 1981. 4. Berlyn, G.; Mark, R. E . For. Prod. J. 1965, 16(3), 140. 5. Fergus, B. J.; Proctor, A. R.; Scott, J.; Goring, D . A. I. Wood Sci. Technol. 1969, 3(2), 117. 6. Alvim, Ρ. T. in "The Formation of Wood in Forest Trees," Zimmermann, H., E d . ; Academic Press: New York, 1964; p. 479. 7. Fritts, H. C. "Tree Rings and Climate"; Academic Press: New York, 1976. 8. Larson, P. R. "Wood Formation and the Concept of Wood Quality"; Bull. No. 74; Yale University Press: New Haven, Conn., 1969. 9. Fergus, B. J.; Goring, D. A. I. Holzforschung 1970, 24(4), 118. 10. Musha, Y.; Goring, D. A. I. Wood Sci. Technol. 1975, 9(1), 45. 11. Hanna, R. B.; Côté, W. A. Cytobiologie 1974, 10, 102. 12. Heyn, A. N. J. Tappi 1977, 60(11), 159. 13. Berlyn, G. For. Prod. J. 1964, 14(10), 467. 14. Gardner, K. H.; Blackwell, J. Biopolymers 1974, 13(10), 1975. 15. Dunning, C. E. Wood Sci. 1968, 1(2), 65. 16. Parham, R. Α.; Kaustinen, H. Bull. IAWA 1973, 1973/2, 8. 17. Liese, W. in "Cellular Ultrastructure of Woody Plants," Côté, W. Α., E d . ; Syracuse University Press: Syracuse, 1965; p. 251. 18. Cronshaw, J. Protoplasma 1965, 60, 233. 19. Kutscha, N. P.; Schwarzmann, J. M. Holzforschung 1975, 29(3), 80. 20. Baird, W. M.; Johnson, Μ. Α.; Parham, R. A. Wood Fiber 1974, 6(3), 211. 21. Parameswaran, N.; Liese, W. Wood Sci. Technol. 1977, 11, 313. 22. Kishi, K.; Harada, H.; Saiki, H. J. Jpn. Wood Res. Soc. 1979, 25(8), 521. 23. Parham, R. Α.; Baird, W. M. Wood Sci. Technol. 1974, 8(1), 1.



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24. Haygreen, J. G.; Bowyer, J. L. "Forest Products and Wood Science"; Iowa State University Press: Ames, 1982. 25. Stamm, A. J. "Wood and Cellulose Science"; Ronald Press Co.: New York, 1964. 26. Polge, H. Wood Sci. Technol. 1978, 12, 187. 27. Taras, Μ. Α.; Saucier, J. R. For. Prod. J. 1967, 17(9), 97. 28. Kellogg, R. M.; Wangaard, F. F. Wood Fiber 1969, 1(3), 180. 29. Meyer, R. W. For. Prod. J. 1967, 17(12), 50. 30. Timell, T. E. in "Chemistry and Morphology of Wood Components", Vol. 1; Proc. Int. Symp. Wood and Pulping Chemistry, Rep. 38; SPCI: Stock holm, 1981; p. 99. 31. Barger, R. L.; Ffolliott, P. F. Wood Sci. 1976, 8(3), 201. 32. Hughes, F. E. For. Abstr. 1965, 26(1,2), 2, 179. 33. Berlyn, G. Iowa State J. Sci. 1961, 35(3), 367. 34. Robards, A. W.; Purvis, M. J. Stain Technol. 1964, 39, 309. 35. Côté, W. Α.; Day, A. C.; Timell, T. E. Wood Sci. Technol. 1969, 3(4), 257. 36. Saki, Η.; Ono, K. Bull. Kyoto Univ. For. 1971, 42, 210. 37. Dadswell, Η. E.; Wardrop, A. B.; Watson, A. J. in "Fundamentals of Papermaking Fibers"; Bolam, F., E d . ; Tech. Sect. BPBMA: London, 1958; p. 187. 38. Parham, R. A. Wood Sci. 1974, 6(4), 308. 39. Gray, R. L.; Parham, R. A. CHEMTECH 1982, 12(4), 232. 40. Parham, R. Α.; Gray, R. L. "The Practical Identification of Wood Pulp Fi bers"; TAPPI Press: Atlanta, Ga.; 1982. 41. Parham, R. Α.; Côté, W. A. Wood Sci. Technol. 1971, 5(1), 49. NOTE: Inc.

This is Contribution No. 236 from the Research Center of ITT Rayonier

RECEIVED

for review May 2, 1983.

ACCEPTED

July 1, 1983.


Formation and Structure of Wood - American Chemical Society

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