THE COMPOSITION AND STRUCTURE OF WOOD' ARTHUR B. ANDERSON University of California, Forest Products Laboratory, Richmond
Lu~mR-moDccmGtrees and trees which are chemically utilized consist of two broad groups, namely: (1) gymnosperms or softwoods, largely needle-bearing, and (2) angiosperms or hardwoods, the broadleaved tree. The softwoods or coniferous woods embrace 47 genera and about 500 species in comparison with at least 150,000 angiosperms. Of these t,wo classes, the conifer occupies a unique position eommercially, which is out of all proportion t o its botanical size. Coniferous trees produce wood of a type entirely different from that obtained from the angiosperms or so-called hardwoods of commerce. Of the 800 or so species in the United States, about 100 are used commercially. Unlike man-made materials with relatively uniform properties such as metals and ceramics, wood is a natural product with considerable variability. I n using wood, therefore, it is necessary to uuderstand its many properties and something of its natural variability so that difficulties in pesformance as a raw material will be minimized.
rings." By counting the annual rings at the ground line, it is possible to determine the age of a tree. The width of the annual rings indicates growth conditions. Years of drought and growth under cover of other trees are characterized by narrow annual rings. The study of the annual rings of trees in retracing weather conditions before t,he existeuce of written records and in k i n g dates when certain trees were cut has become quite a specialized science known as dendrochronology. Speaking of dendrochronology, until a few months ago the oldest living things on earth known t o mall were said to be some of the California redwoods or sequoias which are about 3500 years old. Recently the Xational Geographic Society reported that the oldest living thing on earth is a battered wreck of a pine tree in California's Inyo National Forest ((2). "It has more than 4600 years under the bark," the Society reported, adding that this particular tree and others surrounding
PHYSICAL STRUCTURE OF WOOD
When examining the cross section of a mature tree, certain structural characteristics are usually perceptible to the naked eye. Starting from the outer area of the cross section, the distinguishable characteristics are: (1) outer bark, (2) inner hark, (3) sapwood, and (4) heartwood (Fig. 1). The layered structure of wood indigenous t o temperate zones is generally distinguishable throughout the cross section from the largest outermost growth ring t o the smallest circle of the center marking the first year of growth (1). The fact that each of these distinct layers generally consists of the wood added during one year has given them the name "annual Figure 1.
Presented as part of the Symposium on Wood as a. Chemical Raw Materid before the Divisions of Chemical Education and Cellulose Chemistry at the 133rd Meeting of the American Chemical Society, San Francisco, April, 1958.
VOLUME 35, NO. 10, OCTOBER, 1958
A Wedge-shaped Block from Trunk of a Hardwood Tlse (Ring-Porous)
Fissured outer, dead bark. B. Inner living, light-colored bark. sapwood composed of four annual rings. D. ~ ~ ~ of seven annual rings. . pith. ~ r o m ~ r o w n . an shin, and ~oreaith.) A.
c,
487
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it "were venerable when King Solomon ruled Israel." The grove of bristlecone pines, Pinur aristata, in question includes 17 trees older than 4000 years. Thus, these trees were already 1000 years old when the famous General Sherman redwood was but a seedling. The pines are in the White Mountains between Bishop and the Nevada border. The late Dr. Edmund Schulman made the discovery. He began looking for old trees twenty years ago in a study of the climate of prehistoric West. Growth increments stand out in wood t o varying degree because the growth intensity and, consequently, the density of the wood produced, are not uniform throughout the growing period. Usually, increase in thickness is most rapid a t the beginning of the season and slows down appreciably as the season advances. The portion of the ring formed in the spring when growth is resumed is designed primarily for sap conduction; it is often quite porous and is frequently low in density. This tissue is designated as earlywood or springwood. Wood produced later in the season is called latewood or summerwood; it is usually denser and darker as viewed with the naked eye or a t low magnification. Summerwood is best adapted to insure strength t o the stem and probably does not participate in sap conduction to the same extent as springwood. The discrepancy in color between the denser and darker summerwood of a given increment and the more open, lighter-colored springwood of the succeeding zone is responsible for the delineation of growth zones. As is t o be expected, growth increments differ greatly in width and density in different tree species, in individual trees of the same species, and a t d i e r e n t heights in a given tree. The successive layers are deposited on the surface of the previously formed wood through the agency of a layer of living tissue, called the cambium, situated between the wood and the bark. The cambium envelops the wood of trunk and branches in a closefitting film. From the inner surface of the cambium is deposited nexv wood which increases the volume of the tree. Each year a layer of wood is deposited outside the previous wood of trunk and branch by this tissue which at the same time generates the bark from its outer surface. The layers of wood, when first formed, are light colored and, as sapwood, function as conductors of sap which is chiefly water derived through the roots from the soil. Sap is conducted upward t o the leaves where, from the gases of the air and the energy of the sun's rays, food is manufactured and conveyed downward through the inner bark t o the live tissue enveloping the wood, where the food is further elaborated into new layers of wood and bark. When, after a number of years, a wood layer has been separated from the outer living tissue by wood layers deposited subsequently, it usually undergoes slight chemical and physical changes and becomes heartwood, in which condition it ceases t o conduct sap and becomes dormant. The change from sapwood t o heartwood often is characterized by a darkening in color due t o infiltration of resins, gums, coloring substances, known collectively as extractives. The phenomenon of heartwood formation still remains unsolved. I n looking at a piece of split softwood, such as pine
or spruce, with the help of a good magnifying glass, a lot of fine hair-like cells, called tracheids, become evident. These tracheids, which run vertically, average about 3.5 mm. in length and 0.03 t o 0.05 rum. in diameter. Cell wall thickness is not greater than 0.01 mm. They are arranged in regular radial rows, from the center of the tree outward and are interspersed every few rows by flat plates of smaller very fine cells, called wood rays, running horizontally, instead of vertically. Interspersed among the vertical cells particularly, but also in many of the horizontal plates, are fine tubelike openings, the size of a pin or finer, more or less filled with resin. These are the resin ducts and are really not cells at all, but openings between the cells. The resin ducts are not present in the majority of woods, but are extremely plentiful in pines. On closer examination of an individual cell, little dots appear as chains along the walls of the vertical cells. These are bordered pits by which the living cells communicate one with another. The number of pits in a tracheid may vary from 50 t o 300. In comparison with coniferous woods, hardwoods present a surprising array of anatomical characteristics and departures. This situation has resulted, in part, from the presence in this class of timbers of the composite structure known as a vessel* tubular duct running with the grain which arises as a result of the fusion of cells in a longitudinal row through the breaking down, either wholly or in part, of their end walls. When examining a clear cut end of a hardwood board, the vessels appear as great numbers of small round holes or pores in the wood. These pores are transverse sections of the vessels, cells of much larger diameter than the fibrous elements. The fibrous cells in hardwoods, which compose the greater portion of the wood, are shorter, about 1 mm., than the tracheids of softwoods and are often so small in diameter that the cell cavities are undistinguishable when examined with a pocket magnifier. The vessel segments are comparatively short tubular cells with more or less open endings, which fit together like minute lengths of pipe and form long continuous channels especially adapted for conducting sap. Because of the porous appearance of wood exhibiting vessels when cut a t right angles to the grain, the hardwoods are sometimes known as the porous woods. For purposes of identification, the hardwoods may be divided into two general classes on the basis of pore arrangement. One class contains pores which are not only very much larger in the earlywood than in the latewood, but are also close together in a more or less continuous layer in the earlywood region of the annual ring. Woods with such distribution of pores are known as ring-porous woods. Other species with pores of more nearly uniform size distributed fairly uniformly throughout the annual layers are known as diffuseporous woods. Ash and oak are examples of ringporous woods, white birch, maple, and poplar are typical diiuse-porous species. I n the sapwood of all hardwoods, the vessels are usually open, but, as the growing tree ages, the cavities of the heartwood vessels of many specie8 become filled with ingrowths of small cells. These growths are known as tyloses. I n some woods, the cavities become filled with deposits of gummy substances. The effect JOURNAL OF CHEMICAL EDUCATION
I
Hemlccllviolc ~hFrnicOIIymmbined
Shorter chotnr
Mannose units Lhonic orid u n i t s Mcthoxyl graups
-
-glycopyronore u
motorioi of the
(m9ibly (I0lmtore. 01m. and orobinore units1
T
Border zone b t w e e n the hamicallvlore ond true cellulose
l-l(l"Pe 2.
Schemetic Diemam Shoxeng th. component. of Ext..ctivofree Cell W a l k of Wood (Ah.. Wise)
of tyloses may be illustrated by the fact that one can blo\v through long sticks of oak sapwood or even through heartwood of the red oaks, where the vessels are mostly unobstructed. The heartwood of the white oaks in which vessels are closed with tyloses, is relatively impermeable except at very high pressures. Tyloses are visible in the large vessels of the white oaks and certain other hardwoods as shining structures frequently resembling tiny soap bubbles. The parenchyma rays in hardwoods are much more strongly developed than in softwoods. This has resulted largely from the increase in size, and in some instances, in the number of wood rays. In consequence, these structures are often relatively conspicuous. Porous wood consists of more kinds of cells than does coniferous woods. Add to this the fact that size and wall thickness of these also vary considerably, and the permutations that make possible the many kinds of hardwood are readily understandable.
CHEMICAL STRUCTURE OF WOOD Wood is a heterogeneous material chemically as well as anatomically. Every piece of wood contains three major chemical components: cellulose, hemicellulose, and lignin. The first two are also known collectively as holocellulose. These three cell wall constituents are complicated macromolecular or high polymer compounds, the chemical constitution of some of which is still incomplete (Fig. 2 ) . This is particularly true, for instance, of the hemicellulose and the lignin. Also present in wood are small amounts of mineral matter and significant amounts of a great variety of substances extractable with water or with neutral organic solvents, such as alcohol, benzene, and ether. These substances are the extraneous components of wood, commonly called extractives, and are not an integral part of the cellular structure. When the proximate analysis for cellulose, hemicellulose, and lignin is determined on softwoods and hardwoods, a certain trend becomes apparent. For instance, there is a striking difference between the pentosan content of the hardwood and the softwood, the former usually containing twice as much of this hemicellulose fraction. Another important difference between these two types of wood lies in the mannan VOLUME 35, NO. 10, OCTOBER, 1958
content. Here the softwood contains from 4%-10% mannans, while in hardwood it runs very much lower and at times contains but traces of this entity. The analytical results ohtained for specific species show considerable variations. Differences are found between different parts of the same tree as well as between different trees. Sapwood differs in many cases quite appreciably from heartwood and springwood from summerwood. These resul-ts are not surprising when one considers the anatomy of wood or the effect of chemical reagents on thin sections of wood. the.^ gross variations in chemical composition influence the conditions employed in the chemical pulping of various woods. Difficulties experienced in pulping, differences in yield as betmeen species, arid variations in the physical properties of the pulps can be traced, in part, t o lack of homogeneity in the chemical composit,ion of wood. Cellulose. Cellulose is the principal component of all moods and wood pulp, and is, by far, the most important chemical entity in ~ o o das far as chemical utilization of wood as a raw material is concerned (3).
Figure 3.
Cellulose
We need but to refer t o the huge quantities of mood being processed annually for the production of pulp, paper, and chemical cellulose. This white fibrous substance is insoluble in water and organic solvents and has a high tensile strength. Cellulose is classed among the high polymers. The cellulose monomer is glucose and occurs as an anhydroglucose. The number of glucose units per cellulose molecule or degrees of polymerization, may vary considerably from several hundred t o several thousand units (Fig. 3). The fiber of wood contains aggregates of such chains. Upon examining these fibers by X-ray diagram, we
Fipu.r 4.
Alignment and Compc.sition of Fibrils of Alpha Cellulos. from Plant Cell Walk
A. Faggotlike bundle consisting of segments of se-en psrallel fibrils. inclined at a slight angle from t h e verticsl. B. One fibril in lateral sectional view showing nones in whioh portions of the long-chain moleoulea of cellulose have the perdlel arrangement (omtallites),separated by regions in whioh the chain moleoules are b u t phrtially parallel and disorganired (amorphous regional: there is no sharp bormdary Line between the crystal1ites and the a m a r ~ h o u sregions. (After Brorn, Penshin, and Forsaith.)
find that the fiher is not crystalline in its entirety; a portion of it appears amorphous (Fig. 4). Thus, here again, cellulose, the purest compound in wood, is not homogeneous and this duality in character explains why wood cellulose does not react chemically as a pure compound. Hemicelluloses. All woods contain an important group of polysaccharides termed the hemicelluloses (4). They are usually quite soluble in cold aqueous alkali and are hydrolyzed on treatment wiih hot mineral acids to simple sugars and/or closely related compounds, as well as some acetic acid. These characteristics, together with the fact that hemicellulose contains more than one type of monomeric sugar or sugar derivative, serve t o distinguish the hemicelluloses from cellulose. Further, the degree of polymerization of hemicellulose is between 100-200 and hence is much lower than that of cellulose. There appear to he two general classes of hemicelluloses present in wood: those linked with the lignin and those closely associated with cellulose. The former are often termed polyuronides because they cont,ain relatively large amounts of hexuronic acid uni%s. The latter are termed cellulosans, because of their relationship with cellulose. After their isolation, the properties of hemicellulose differ from those of the substances in the original wood. The acid hydrolysis of a hemicellulose isolated from a coniferous wood, such as spruce, yields the sugars, mannose, glucose, and xylose, and a hexuronic acid, together with galactose, arabiuose, and fructose. I n the case of the hemicellulose of hardwoods, the hydrolysis products include glucose, xylose, and a hexuronic acid, with little or no mnnose. More xylose is obtained from hardwood hemicellulose than from the hemicellulose derived from softwoods. Thus, we can see that the building blocks of the hemicelluloses contain a number of different types of monomers, as compared with cellulose, and add t o the complexity and nonuniformity of wood. The carbohydrate materials present in pulping effluent liquor are largely derived from the hemicelluloses. Lignin. With few exceptions lignin is second only t o cellulose as the main constituent of wood averaging about 28% in coniferous woods and around 24y0 in hardwoods. It makes up the greater portion of the middle lamella which surrounds the fibers and cements them together, adding strength t o the cell walls of the fibers and t o the fiber aggregates which together make up the tree. At the present time no clear-cut picture of the structure of the lignin molecule can be presented (6, 6). There is, however, substantial evidence that lignin is built up t o a large extent of phenylpropane building stones having either free or masked phenolic hydroxyl groups in the para position and methoxy groups in the meta position t o the side chain. Here again, the isolated ligniu has different properties from that existing in the wood. Since no way has been found t o reduce this polymer t o its simple building blocks, as is the case with the polysaccharides, it adds t o the confusion as t o the precise chemical constitution of the lignin molecule as it occurs in wood. One of the recent interesting aspects of lignin chemistry has to do with the theory of lignification. One
of the theories is that a lignin precursor arises in the cambium; some diffuses centripetally and is incorporated into the differentiating cells of the xylem and phloem. One possible lignin precursor is coniferin, the glycoside of coniferyl alcohol. The idea was specifically formulated by Freudenberg who has, in addition t o demonstrating the presence of coniferin in the sap of softwood, shown the analogous glycoside, syringin, t o he present in the cambial sap of some hardwoods (7). The work of Freudenberg and his school has shown further that glycosidases capable of hydrolyzing coniferin to coniferyl alcohol and peroxidases capable of converting the alcohol into ligninlike substances are present in the zone of differentiating cells (6). Furthermore, labeled coniferyl alcohol may be fed into the cambium zone and can be shown to be incorporated into the lignin of the mature tree. The results clearly support the hypothesis of an inwardly diffusing precursor from the cambium region.
Phenyl propane unit
[Glyeosidase]
IPeroxidasel LIGNIN
I
OC,H,,Os Coniferin
Coniferyl alcohol
Since the pulping process largely involves the dissolution and removal of lignin from the wood chip, releasing the desired cellulose fiber, lignin, of necessity, is present in the pulping effluent liquor. The utilization of such lignin has been and still is one of the major unsolved problems confronting the pulping industries. Having briefly defined the chemical nature of the three major chemical components which make up the cell or fiher found in wood, at this point it would be well t o ascertain the distribution of each of these components in a single cell structure (8, 9). It has been recognized that the cell wall of the mature tracheids consists of two structures: the primary wall and the secondary wall. The primary wall is the envelope initially surrounding the cell following cell division. The secondary wall which is formed subsequently consists of three layers* thin outer, a thick middle, and a thin inner layer, which are distinguishable by the difference in their optical properties, due t o the orientation of cellulose which differs in each of the three layers (Fig. 5). The primary wall contains a high amount of lignin-from 60%-90%, the remainder being largely hemicellulose with a smaller amount of JOURNAL OF CHEMICAL EDUCATION
Figurm 5.
Diagrammatic Dravdng Shoving the Layere in the Wall of a Fibrous X y l w C.11
The solid black sane is the true middle lamella oompoaed of intercellular substance; the omsshatohed portions on either side indicate the position of the primary walls: these three lavers compose the oompound middle Lamella and are heavily lignified. a, b, and a, respectively, &re the lhvers of the secondary wall; the arrows a t the side indieate the orientation of the fibrils. (After Kerr and Bailey.)
cellulose. The main component of the secondary wall is cellulose, the least amount being present in the outer layer and gradually increasing toward the lumen in which the cellulose content of the innermost layer may run as high as SO'% or more. Here again, the remainder is largely hemicellulose together with very small amounts of lignin. Cellulose forms the structural framework material whereas lignin and noncellulosic polysaccharides constitute the encrusting substances. Thus, in general, the cell wall of wood is highly ordered around the lumen and becomes more amorphous as the middle lamella is reached, which consists largely of lignin and some hemicellulose.
occurs in the protoplasm, and some trees contain alkaloids. Thus it is apparent that an extractive component may he a simple organic compound or it may be most complex. The types of components found may include such diversified compounds as sugars, acylic sugar alcohols, cyclitols, gums, mucilages, glycosides, fats, fatty acids, rosin acids, terpenes, sterols, quinones, tropolones, lignans, anthocyauins, flavones, pigments, tannins, and the like. Elucidation of the structure of many of these extractive constituents is beginning t o attract the attention of some of the foremost structural organic chemists throughout the world. The type of extractive components present in any one tree species often determines its characteristics. Certain heartwoods are known for their durability against decav. Included amone these are the redwoods. ced&s, and cypresses.-This particular ability to ward off wooddestroying fungi is attributable t o the presence of fungicides in the extractives. Western red cedar (Thuja plicata) contains tropolones, i.e.,. thujaplicins, which are extremely toxic t o wooddestroying fungi. Bioassays indicated that these particular, unique, seven-member carbon atom ring compound; are several times more toxic t o fungi than the well-known commercial wood preservative-creosote. contains a steamIncense cedar (Dibocedrus decu~~ens) volatile oil, which, in addition to tropolones, contains monomethyl ethers of hydrothymoquinone which were found to be highly fungicidal (Fig. 6). And
EXTRACTIVE COMPONENTS OF WOOD
Thus far we have been discussing the chemical composition of the integral components of the cell wall or wood fiber. An understanding of the peculiarities of wood would not be complete without referring t o the extractive components. For a matter of fact, if trees were extractive free, many species would no longer retain their distinctive or individual characteristics (10). Extractives are those substances in wood which are soluble in neutral organic solvents and water. When wood is extract-free, its anatomical characteristics remain intact. The quantity of extractive components found in wood may vary from as little as 1% t o as high as 40%-50%. And this latitude of extractive content may be found in a single tree, contingent upon the portion of the tree from which the sample was obtained. What types of substances make up the extractive components in trees? They may embrace practically all classes of compounds known t o the organic chemist. Among them may be included hydrocarbons, alcohols, aldehydes, ketones, phenols, ethers, esters, lactones, oxides, and acids. One or more of these functional groups may appear in any one of the compounds in a single extract. The elementary composition of these compounds involves carbon and hydrogen with or without oxygen. Nitrogen in the form of protein VOLUME 35, NO. 10, OCTOBER, 1958
The thuiaplioins are the decay-retarding extractive components in weatern red cedar; hydrothymoquinone and its monomethyl ethers, p-methoxythymol and pmethowcarvaorol. are largely reaponaible for the durability of ineenee cedar heartwood.
redwood (Sequoia sempervirens) contains tannins of the type and quantity responsible for the outstanding durability of this species. Extractives likewise play a role in the pulping processes. For instance, in some of the pine heartwoods, the presence of a small amount of a dihydrophenol, i.e., pinosylvin,
was found t o act as an antipulping agent. Similarly, dihydroquercitin is reported t o he responsible for the ant,ipulping properties of Douglas fir heartwood.
1
8
OH Dihydroquercitin
Pulping southern pines by the sulfite process as used on spruce and hemlock would produce disastrous results. Why? It is the relatively high resin content, consisting of rosin and fatty acids, in these pines, which necessitated the use of the kraft process before these pines could he pulped satisfactorily. So it is, when a particular wood species does not lend itself to a conventional pulping method, one may look t o the extractives for the solution t o the problem. Since the chemical nature of some of the extractive components in various species and subgenera is different, a thorough study of these components may reveal that one or more of these constituents may serve as a fingerprint for identification purposes. Some progress has been made in this direction. In a study of the volatile oils from oleoresin of various pines, Mirov has shown that species closely related from the hotanist's standpoint are quite different when their volatile oils are carefully examined (11). It was pointed out that, while it is difficult t o differentiate morphologically between the white pines (Pinus strobus and Pinus mmticola), the chemistry of each is quite different. The former contains n-nndecane, while this hydrocarbon is not present in the latter. Similarly, identification of the constituents in the volatile oils from the closely related pines Pinus ponderosa and Pinusjeffreyi offers a key t o the taxonomist for identification purposes. The volatile oil from ponderosa pine is terpenic in nature, while that obtained from Jeffrey pine is aliphatic in composition, the predominating component being n-heptane. Erdtman and and his co-workers in Sweden have carried out some extensive studies on the nature of extractives in pine heartwoods, which proved t o he of considerable toxonomic value (12). Of the seven species belonging t o the Haploxylon (mostly five needled) subgenera investigated, all contain the cyclitol, pinitol, while none has been found in the 11 Diploxylon species (two or three needled). Still other extractive components which distinguish these two subgenera are the flavones, chrysin and tectochrysin, that occur only in the Haploxylon series.
OH Pinitol
before the chemist can throw more light on the questionable aapects of taxonomy. Just as extractive components often serve to characterize a wood, the amount and type of extractive in various species more or less determine which species are suitable as raw material for the commercial production of noncellulose chemicals from wood (IS). Thus, the well-known commercial products, wood rosin and turpe~ltine, are obtained from southern pines, largely longleaf (Pinus palustsis) and slash (Pinus caribaea). Tannins are obtained from chestnut wood (Castanea datata) and from the barks of eastern hemlock (Tsuga canadasis), chestnut oak (Quercus montana), tanoak (Lithocarpus dasiflorus), and Douglas fir (Pseudotsuga menzessii). Rubber, a hydrocarbon, is obtained from the latex of the Para rubber tree (Heuea brasiliasis), carnauba wax from the leaves of the palm tree (Copernicea cerifera). Tung oil, known as "China wood oil" is expressed from the seeds of the tung oil tree (Aleurites fordii). Quinone is the important alkaloid from the hark of the cinchona tree (Cinchona ledgeriana). Some of the newer developments in processing wood for chemicals include wax and dihydroquercitin from Douglas fir,new sources of rosin from ponderosa pine and Jeffrey pine stumpwood, pinitol from sugar pine mill waste and stumps, palconate from redwood bark, and conidendrin, a lignan, from sulfite waste liquor of western hemlock. These are but a few of the noncellulosic chemicals derived from wood. We have seen that wood is complex in both its structure and composition. As a raw material, wood offers many challenges to the chemist t o utilize all of its components, rather than sacrifice one or more entities for single-product recovery. This, of necessity, would require integrated processing, wherein in addition t o the recovery of cellulose, as in the pulp industry, the lignin, the hemicellulose, and the extractives likewise would be recovered and utilized. Several pulp and paper mills are making progress in this direction. LITERATURE CITED (1) BROWN, H. P., A. J. PANSHIN, AND C. C. FORSAITH, "Text-
book of Wood Technology," Vol. I, McGraw-Hill Book Co., Ino., New York, 1949. (2) S c n a m ~ ,E., National Geographic Mag. CXIII, 1958, 155
(4) (5) (6)
(7)
(8) (9) (10)
R = H Chrysin R = CHI Tectoohrysin
While the chemist has been able t o offer some chemical data to aid the toxonomist, the extractives from many species must he thoroughly and carefully examined
E., in "Wood Chemistry" edited by L. E. Wise and E. C. Jahn, 2nd ed., Vol. I, Reinhold Publishing Corp., New York, 1952, pp. 117-31. WISE,L. E., op. n'l., pp. 369-408. BRAUNS, F. E., "The Chemistry of Lignin," Academic Press, Ino., New York 1952. ADLEE,E., Ind. Eng. Chem., 49, 1957, 1377. FREUDENBERG, K., R. KRAFT,AND W. HEIMBERGER, Chern. K., H. REZINK,H. Ber., 84. 472 (1951); FREUDENBERG, BOSENBERG, AND D. ROSENACH, Chem. Be?'., 85, 641 (1952); FREUDENBERO, K., H. REZINK,W. FUCHS,AND M. REICHERT, Natumi88., 42, 29 (1955); FREUDENBERG, K., Ind. Eng. Chem., 49, 1384 (1957). LANQE,P. W., Svensk Papperstidn., 57, 563 (1954). WARDROP, A. B., H ~ l z f ~ s c h u n8, g , 12 (1954). WISE, L. E., AND E. C. JAHN,"Wood Chemistry," 2nd ed., Vol. I, Reinhold Publishing Corp., New York, 1952, pp. 543-688. Mr~ov,N. T., Am. Rev. Biochem., 17, 521 (1948). ERDTMAN, H., in "Progress in Organic Chemistry" edited bv J. W. Cook, Academic Press Ino.. New York 1952. pp. 2S64. ANDERSON, A. B., Econ. Botany, 9, 108 (1955).
(3) WISE, L.
(11) (12) (13)
JOURNAL OF CHEMICAL EDUCATION
