1386 A NATIONAL PLAN FOE AMERICAN FORESTRY 



WOOD ITS STRUCTURE, COMPOSITION, AND PROPERTIES 



Wood is both a finished natural product and a storehouse of raw 

 materials. It is a fibrous aggregate containing cellulose and other 

 carbohydrates, lignin, and extractives, combined in variable quantities 

 and arranged in a complicated and variable microscopic structure. 

 There are 150 important species of wood in American forests, each 

 differing from the others in structure and properties, and each 

 varying within itself to a considerable degree. The chemical com- 

 position of wood substance, the arrangement of the constitutent 

 parts in the wood cells, the size and spacing of the cells, and the 

 variation of all such characteristics according to species and growth 

 conditions determine the usefulness of wood as such and its poten- 

 tialities of conversion into other products. A scientific understanding 

 of these matters opens the way to success in the silyicultural control 

 of the material and its properties, in its selection, its seasoning and 

 handling, its impregnation with preservatives, its use in construction, 

 and its conversion into pulp and other products. 



To visualize and emphasize the complexities and importance of 

 research that lies ahead in these fields something of the facts at 



E resent known and the main lines of further study required will be 

 riefly reviewed. 



STRUCTURE OF WOOD 



The structure of wood is so complex and variable that an adequate 

 conception of it cannot be conveyed in a few words. Essentially it 

 is a cellular structure, but there may be several different kinds of 

 cells with different arrangement and different means of intercom- 

 munication. Most of the cells are arranged longitudinally, parallel 

 to the tree trunk, but some extend radially. Beyond this cellular 

 structure, plainly visible under the misroscope, are smaller structural 

 units. The cell walls are made up of concentric layers, which in 

 turn are composed of fibrils arranged spirally. The fibrils are the 

 smallest units that become evident through any simple mechanical 

 disintegration, but by careful chemical treatment they themselves 

 may be subdivided into spindle-shaped "fusiform bodies" and the 

 latter into minute spherical units. The spherical unit the ultimate 

 visible component of the cell wall is about one hundred-thousandth 

 of an inch in diameter, and beyond it the microscope cannot penetrate. 

 It is possible, however, by indirect methods using the ultracentrifuge 

 and the X-ray, to determine the approximate size and arrangement 

 of submicroscopic units. 



The arrangement of these various parts to form the cell wall, the 

 shape and size of the various cellular structures, and their arrange- 

 ment and mode of joining to form the wood determine completely 

 the gross mechanical properties of the material. The submicroscopic 

 units and the peculiar attractive forces between them give wood its 

 colloidal properties, such as hygroscopicity. This absolutely basic 

 field of wood research is largely unexplored. It abounds in hypoth- 

 eses of colloidal behavior which await experimental verification and 

 correction. Other more specific structural research is concerned with 

 the means of communication between the cells. The cell cavities 

 are separated by thin pit membranes through which there are open- 

 ings of submicroscopic size that can be measured only by indirect 

 methods. The number, size, and location of these openings, together 



