108 ANATOMY OF THE GYMNOSPERMS 



a way that in only a portion of one of these has that completion 

 reached its highest expression. 



The occurrence of two kinds of parenchyma ray cells is an 

 exclusive feature of the genus Finns, and its value for phyloge- 

 netic purposes is strictly confined to the relations of the various 

 species of pines. The first appearance of this differentiation is 

 among the soft pines in P. aristata and P. edulis. It is to be 

 observed, however, that the thick-walled cells are always domi- 

 nant, the thin-walled cells being interspersed among and con- 

 terminous with them. No further evidence of such structural 

 alterations is to be noted until we reach the more highly devel- 

 oped representatives of the hard pines. Among these definite 

 transition forms occur in P. Murrayana, P. Coulteri, P. Jeffreyi, 

 P. virginiana, P. insignis, and P. cubensis, while in P. arizonica, 

 P. ponderosa, P. Sabiniana, P. pungens, etc., the original relations 

 are exactly reversed and the thick-walled cells show a diminish- 

 ing frequency until in P. glabra and P. taeda they are rarely met 

 with. Such facts give effective proof of the belief that struc- 

 tural alterations of this nature are not only evidences of the 

 highest type of development among the pines but also among 

 the Coniferales as a whole. 



The invariable absence of the fusiform rays from all except 

 the four genera which attain the highest structural development, 

 and their constant occurrence in all the species of such genera, 

 presents an argument of great force as showing their relation 

 to the evolution of advanced types. There is here no evidence 

 of sporadic development, foreshadowing the general course of 

 evolution, but the fusiform rays with their resin canals appear 

 abruptly and permanently. Among fossil plants --except the 

 genus Pityoxylon, which, being essentially Pinus, falls under the 

 general rule - - there is no instance of such structures outside 

 of the four genera named, save in the case of the remarkable 

 Sequoia Burgessii, from the Lignite Tertiary (51, 42), and S. Pen- 

 hallowii of Jeffrey (25). As it will be necessary to further 

 discuss the essential structure of the fusiform ray, we need not 

 deal with it more in detail at the present moment. 



