9 6 



ANATOMY OF THE GYMNOSPERMS 



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FlG. 30. PSEUDOTSUGA 



DOUGLASII. Tangen- 

 tial section of a fusi- 

 form ray showing (a) 

 the typical resin ca- 

 nal with thick-walled 

 epithelium, but devoid 

 of thyloses. x 280 



cavities of adjacent wood tracheids, thereby 

 giving to the ray a beaded appearance. As 

 an exceptional variation it possesses no ap- 

 parent significance with respect to questions 

 of descent. 



The second form of the ray is that which 

 has been designated as fusiform in refer- 

 ence to its characteristic outline (44, 39). 

 Such rays occur in relatively few of the 

 existing genera to the extent of 20 per 

 cent. They occur typically in Pseudotsuga, 

 Larix, Picea, and Pinus, and they are thus 

 seen to be characteristic of the most ad- 

 vanced types. Among extinct species they 

 are unknown except in the case of Sequoia 

 Burgessii (46, 42-46) and S. Penhallowii of 

 Jeffrey (25, 321), in which they present a 

 remarkable exception to the general course 

 of development and structure of that genus. 

 The fusiform rays are peculiar in their 

 structural features. They vary greatly in 

 height as between different genera, and 

 such variations also occur within a given 

 genus, the extremes being met with in the 

 genus Pinus, where P. palustris and P. pon- 

 derosa present the antithetic relations. 

 In most cases they are much higher than 

 the one-seriate rays with which they are 

 associated, but this rule is subject to sev- 

 eral exceptions. They are always distin- 

 guished by a broadening of the central tract 

 by from two to several times the original 

 dimensions, thereby becoming more or 

 less multiseriate. These variations depend 

 upon the nature of the included structure, 

 which exhibits modifications directly related 



