A NATIONAL PLAN FOR AMERICAN FORESTRY 1387 



with the colloidal properties of the membrane itself, are thought to 

 control all natural or artificial movement of liquids within the wood, 

 but no satisfactory theory of their action has as yet been worked out. 

 Even in regard to those types of structure that are readily visible 

 in wood sections under the microscope, quantitative statistical in- 

 formation that has a realistic bearing on the properties and utilization 

 of wood is very meager. Much more research is needed before the 

 picture of wood structure and substance as an industrial material can 

 compare, for instance, with that which the metallurgist has obtained 

 for such materials as steel and copper. 



CHEMISTRY 



As to the chemical composition of the various structural units of 

 wood, information is likewise far from complete. It is known that 

 the structural units of the cell wall are essentially cellulosic in com- 

 position. It is thought that a small amount of lignin is incorporated 

 with this cellulose structure, but the details of its distribution there 

 are not known. Lignin, however, comprising approximately one 

 third by weight of all wood substance, is the main constituent of the 

 cementing layer between the cells. There is present in the wood a 

 considerable percentage of carbohydrates other than the cellulose 

 whose location in the structure is unknown, and finally there are 

 extractives or infiltrated substances that are variously distributed, 

 either in specialized structures such as the resin ducts of certain species 

 or in the cell cavities, or more or less evenly disposed throughout the 

 cell wall. 



As distinguished from the chemistry of its minute structure, the 

 general chemistry of wood is fairly well known in terms of the gross 

 chemical groupings already stated. But even in these limited terms 

 there is little statistical information on the variations in composition 

 between species, within species, or within single trees. Moreover, 

 much additional information is needed as to the chemical composition 

 of the main groups, cellulose, lignin, etc. The term " cellulose" as 

 used here comprises a group of similar carbohydrates that make up 

 about 60 percent of the weight of dry wood. Only about three 

 fourths of this " cellulose " is true cellulose, however. The remainder 

 is made up of different sugar units put together in somewhat less 

 stable form, and our little knowledge concerning their constitution 

 and the nature of their combination is entangled with a mass of 

 speculation. Even less is known about the group of carbohydrates 

 not closely associated with the cellulose that make up about 5 to 10 

 percent of the wood. 



Lignin remains an unsolved mystery. Many isolated facts are 

 known in regard to its chemical characteristics, but they do not form 

 any clear or connected picture of its constitution. Probably it is not 

 a single chemical substance but rather a loose grouping of similar 

 substances in variable proportions. 



The extractives are really matter outside the mechanical structure 

 of wood, but they are important in connection with many of its 

 properties. Color, odor, and durability are basically dependent on 

 extractives, while pulping, painting, gluing, and even strength prop- 

 erties are considerably affected by them. 



They vary widely in both amount and composition, such different 

 classes of chemicals as resins, terpenes, tannins, gums, carbohydrates, 



