MEDULLARY RAYS 



99 



serve to afford an idea of the rather limited specific value of this 

 character. In Taxus cuspidata the rays are numerous, while in 

 the two remaining species they are relatively few. The same 

 feature applies to the differentiation of Torreya nucifera from the 

 other species of that genus. In Pinus clausa, 

 P. serotina, P. Murrayana, etc., the same rule 

 applies, but in all such cases it cannot be ac- 

 cepted as final. 



The height of the ray is subject to such varia- 

 tions, even within the same species, that it can- 

 not be defined with sufficient accuracy to admit 

 of its application to classification in more than 

 a very general sense. It is true that the rays 

 of Gingko are always low, while those of Taxus 

 and Torreya are often high. In Juniperus 

 they are commonly low, while in Pinus they 

 range from low to very high. Such variations 

 do not possess sufficient constancy to admit 

 of either generic or specific application in the 

 strict sense, though they not infrequently serve 

 a useful purpose as controlling factors, and they 

 are therefore incorporated in all the diagno- 

 ses. Variations in breadth have a much more 

 definite value, since the element of constancy 

 is well defined. The genus Thuya (fig. 33) 

 may almost invariably be differentiated by this 

 feature. In Cupressus, C.thyoides may be dis- FIG. 33. THUYA 



. , , , -1 r U'l /- GIGANTEA. Tan- 



tmguished by a similar feature, while C. anzo- 



nica and C. Goveniana are equally well indicated 



by their great breadth. The same rule applies 



also to Juniperus, Sequoia (fig. 27), Pinus, and 



other genera, whence it appears that this feature 



is of specific value. It is always associated with and dependent 



upon the form of the component cells (tangential), which affords a 



means of distinguishing genera and species with much directness. 



The narrowly oblong cells of Thuya (fig. 33) serve to separate 



Q 

 Q 

 Q 

 



Q 

 ii 



Q 





 Q 

 







gential section of 

 a ray showing the 

 typically narrow 

 and oblong cells. 

 X 280 



