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THE AMERICAN NATURALIST. [Vol. XXXVIIL 



strongest reticulations are of the highest type, the two being 

 united by a transitional form characterized by the presence of 

 simple teeth. The evidence at hand does not appear to justify 

 the idea that the various genera have been segregated into small 

 groups representing side lines of development, but it rather 

 favors the thought that each genus is in itself a complete, short 

 line of descent, and that among these a prominent parallelism 

 has arisen in the tendency toward the development of tracheids 

 — a tendency which has been carried to completion in the case 

 of only five of the series, and in such 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 Pinus, and their value for phylo- 

 genetic 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. cdnlis. It is to be 

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

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

 minous with them. No further evidences of such structural 

 alterations are to be noted until we reach the more highly 

 developed representatives of the hard pines. Among these 

 definite transition forms occur in P. murmyana, P. coultcri, P. 

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

 arhonica, P. ponderosa, P. sabimana, P. pungcns, etc., the orig- 

 inal relations are exactly reversed and the thick-walled cells show 

 a diminishing frequency, until in P. glabra and P. tceda they 

 are rarely met with. Such facts give effective proof to the 

 belief that structural alterations of this nature are not only evi- 

 dences of the highest type of development among the pines,, 

 but in the Coniferales as a whole. 



The invariable absence of the fusiform ray 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 



