Both the primary and secondary walls of typical cells of the higher plants are composed of a porous but firmly coherent matrix of anisotropic cellulose, whose finer structural details grade down to the limits of microscopic visibility-i. e., 0. l p or less. Lignin and other noncellulosic constituents may be deposited in the elongated, intercommunicating interstices of the cellulose, thus resulting in two continuous, interpenetrating systems. In heavily lignified forms, either system may be dissolved without seriously modifying the continuity or the structural pattern of the remaining system. The visible structural patterns of the secondary wall are extremely complex and variable, and are due (a) to varying porosities in different parts of the cellulosic matrix, (b) to varying orientations of aggregates of chain molecules in successively formed parts of the wall, (c) t o variations in the distribution of noncellulosic substances, and (d) in some cases to the presence of noncellulosic layers. Well-defined planes of structural weakness exist in the cellulosic matrix. Thus, the secondary wall may be dissected by various mechanical and chemical treatments into layers, fibrils, fusiform bodies, dermatosomes, and other fragments of varying shapes and sizes. Each of these fragments is heterogeneous and much larger than the finer visible structural details of the cellulosic matrix.
Cell Wall Structure of
Higher Plants I. W. BAILEY Harvard University, Cambridge, Mass.
I
N ANY discussion of the cell walls of the higher plants, it is essential to differentiate accurately and consistently between three distinct categories of structures:
1. Meristematic-i. e., embryonic-cells, and such of their derivatives as retain a capacity for growth and for increase in volume, are characterized by having a wall that is capable of expanding and of increasing in surface area. This wall is also characterized by its potentiality for undergoing various reversible changes-. g., in thickness. It is composed, in most cases at least, of cellulose and of true pectic substances (polygalacturonides containing galactose and arabinose), and may contain varying admixtures of hemicelluloses. It is optically anisotropic and exhibits a wide range of microscopically visible structural patterns. This type of wall should be designated the “primary wall” (IO). 2. Fibers and other highly dserentiated plant cells, which undergo various irreversible changes and thus lose their potentialities for growth and for increase in volume, retain their original primary wall in a more or less modified condition, and commonly form, in addition, a supplementary wall of a different physiological type. This wall has primarily a mechanical function and, once formed, is apparently incapable of growth or of increase in surface area. It is composed of cellulose or of varying mixtures of cellulose and hemicelluloses but, in most cases at least, does not contain any appreciable fraction of true pectic compounds. (Its uronides are usually composed of glucuronic acid and xylose.) It is intensely anisotropic and is usually more or less conspicuously laminated. This type of wall should be designated the “secondary wall.” 3. The adjacent cells of plant tissues are held together by a truly isotropic substance of a pectic nature. This isotropic intercellular layer should be called the “intercellular substance” or “middle lamella.”
or by the use of highly refined technics. The heavily lignified primary walls of adjoining cells and the intervening heavily lignified middle lamella (Figure 1)have similar indices of refraction and stain intensely with the same dyes. They tend, accordingly, to blend both in untreated and in stained sections (Figure 2) and when viewed either in ordinary light or in polarized light (Figure 3). Furthermore, where the firstformed layer of the secondary wall is heavily lignified, as is usually the case, its optical properties are such that it tends to blend with or to exaggerate the apparent thickness of the primary walls and the intercellular substance (Figure 2), except when extremely thin sections are viewed in polarized light between crossed Sicols (Figure 3). Owing to such technical difficulties as these, many investigators have failed to differentiatethe primary wall from the intercellular substance or from the first-formed layer of the secondary wall, and have adopted the extremely misleading procedure of referring to the intensely birefringent, firstformed part of the secondary wall as a primary wall. Under
Middle Lamella and Primary Wall in Lignified Tissues Each of these three categories of structures-i. e., the middle lamella, the primary wall, and the secondary wall-may become more or less heavily lignified and may a t times absorb a wide variety of other organic substances. The original physical properties and the chemical solubilities of the middle lamella and of the primary wall may, therefore, be modified or masked by the deposition of such substances during tissue differentiation. Thus, in wood and in other lignified tissues it is difficult to demonstrate the optical anisotropy of the tenuous, modified primary walls except in favorable material 40
JANUARY, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
41
this unfortunate terminology the so-called middle lamella is actually a three-layered structure composed of two modified, cellulose-containing primary walls and a truly isotropic intercellular layer. The problem of establishing an accurate and consistent terminology in dealing with different types of plant cells and tissues is by no meant? a trivial or a purely
academic one. The existing confusion in the literature leads to serious discrepancies not only in botanical but also in biochemical and biophysical investigations. In the case of woody tissues it is possible, by carefully controlled delignification and other suitable treatments, to nnmask the feeble anisotropy of the primary wall and to restore the original chemical sahib es of the middle lamella and of the primary wall (fff). In this connection, the pectic content of wood is low, because the lignin content of the middle lamella and of the primary wall is so high (18') and the volume of these tenuous layers is so small in comparison with that of the relatively massive secondary wall. In soft tissues, on the contrary, which are devoid of secondary walls, the middle lamella and the primary walls constitute a much 'IGrmE TRASShigher ratio of the dry weight of the tissue. It is eviddnt, mnsE SECTION OF ON%: E~~~~~plsERkliD OF accordingly, that a transformation of pectic compounds into P A R T S O F S E T E N hemicelluloses and lignin is not essential to account for the OTHERS; B. SECTION fact that soft tissues frequently give relatively high percentages of pectic substances and low yields of hemicelluloses, PIED (Io) whereas wood commonly gives comparatively higli percent(i. T ~ isotropio ~ I inte,.~ ages of hemicelluloses and lignin, and low yields of pectic oelltil~rsubstanco compounds,
&$ezg npg:
B
/
0. c,
Primary wall Outer layer o! seoondary wall
d. Centra1
I S Y B i
"i
Visible Structure of Primary Wall
seooodaiy wnli
e.
I
Inner layer * i Y ivaii of seoond-
f
TR.~SWERSF SECTION OF I A T E W O ~ U OF PINE, STAINED WITA HAIDENHAIS'S HEMATOXYLIN AND PHOTOGRAPHED WITH NoNPo-LaRIzED 1,Icu~( X 070); L n r E R s c OF FIGURE 1 ARE DARK AND TEND TO BLEYD WITH a AND b FIGURE2.
SAME AS 2 El:?. PoWl~OGRAPIZED WITH POLARIZED LIGHTBETTEEN CROSSED NICOLS( X 670); LAYERSc AXD e
FIQURE
3.
BIREFXINCENT
The structural patterns of the primary wall are most readily and clearly visible in cells where t.hey are not obscured by the uresence of a secondarv wall. I n the case of fibers and hairs, the critical stage for'observation is that immediately preceding the initiation of secondary thickening. At this stage the primary wall has ceased to increase in surface area and has attained its final morphological form.
1:IOORE 5. T R A N S m R S E SECTIOX OF WOOD FIBER SnolVIXG ALTEHXATING CELLULOSIC (LIGHT) A N D xoSCELL.;LOSIC (D.&RK)
L.
