9 8 



ANATOMY OF THE GYMNOSPERMS 



presence of a resin canal in each case. Such resin canals traverse 



the ray continuously for its entire length. 

 They present the same details of structure 

 as the resin canals which lie within the 

 xylem. In Pseudotsuga, Larix, and Picea 

 the central canal is narrow, especially in 

 the first two genera, and the epithelium 

 consists of a single layer of thick-walled 

 cells. In Pseudotsuga and Larix (fig. 30) 

 thy loses are altogether wanting, but in 

 Picea they are of sporadic occurrence. 

 In Pinus (figs. 31 and 32), on the con- 

 trary, the canals are always distinguished 

 by their great breadth ; the epithelium is 

 composed of from one to several rows of 

 thin-walled cells, which are often resinous 

 and often much disorganized, while thy- 

 loses are an invariable feature of the 

 central canal. 



b \\- K""^ A comparison of different genera and 



species shows that there is a somewhat 

 striking variation in the number of one- 

 seriate rays (tangential) to a given area of 

 section. Such variations may arise within 

 narrow limits in the same species, accord- 

 ing to location and conditions of growth, 

 but apart from this there are somewhat 

 constant variations between different spe- 

 cies, which may be expressed by the use 

 of the relative terms "few" and "many." 

 No attempt has been made to define such 

 variations more exactly, but it is quite pos- 

 sible that a determination of the average 

 number to a square centimeter or other 

 convenient unit might disclose a somewhat greater differential 

 value than is at present apparent. A simple illustration will 



c. 



FIG. 32. PINUS CLAUSA. 

 Fusiform ray showing 

 (a) the resin canal with 

 thyloses; (/>) thin-walled 

 parenchyma cells; (c) 

 the terminals composed 

 of only one or two tra- 

 cheids. x 280 



