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BOTANY: PRINCIPLES AND PROBLEMS 



three sections, and the characteristic appearance of the various 

 wood structures as seen in them, is shown in the segment of an 

 oak log (Fig. 61) and the magnified cube of pine wood (Fig. 62). 

 Woods of various species differ from one another markedly in 

 such gross characters as color, weight, hardness, chemical compo- 

 sition, width of annual rings, width of rays, and number, size 



Fit;. 62. — Cube of pino wood, much enlarged. ,4, transverse section. B, 

 radial section. C, tangential section. One entire annual ring and parts of two 

 others are shown. (Courtesy United States Forest Products Laboratory) . 



and arrangement of vessels; and in such microscopic features as 

 the size, shape, character, and location of the different classes of 

 wood-elements, the type and distribution of pits, and the various 

 markings on the cell-walls. The structure of two distinct and 

 important woods, those of pine and of oak, are well shown in their 

 transverse, radial and tangential sections in Figs. 63 and 64. 

 The various details of wood structure remain so constant that 



Fig. 6.3. — Pine wood as seen under the microscope. A, transverse section. 

 The wood cells (tracheids) are here cut across, at right angles to their length. 

 Note the thin-walled cells in the spring wood and the thick-walled ones in the 

 summer wood. The wood rays run at right angles to the annual rings. B, 

 radial section. C, tangential section. In both these, the tracheids are cut 

 lengthwise. Note the marked difference in appearance of the wood rays in the 

 three sections. The large openings are resin-canals. (Courtesy United States 

 Forest Products Laboratory) . 



