30 BOTANY 



concerns the vascular anatomist. The Mosses have but 

 little differentiation into true tissues, though the well- 

 known genus, Polytrichum, has something corresponding 

 to wood and phloem cells. 



The Algae have no differentiation into true tissues, 

 and only some of the largest of them, the Laminarias, 

 show anything approaching the vascular cells of the 

 vascular plants. In them there are zones of elongated 

 cells with sieve-like plates between which distinctly 

 resemble some of the bast cells in higher plants. The 

 thread-like algae and the fungi are simply composed 

 of slightly differentiated cells which are fundamentally 

 parenchymatous. For anatomical interest, then, we 

 must return to the Pteridophytes and the higher plants. 



From a study of the present-day ferns and the many 

 fossil genera of Pteridophytes and that extinct group, 

 the Pteridospermae, it appears that a great many varieties 

 of arrangement of the woody tissues have been attempted 

 by plants. Many of these were much more complex than 

 the simple hollow cylinder which is now found in the 

 most successful and highest types. It appears almost 

 as though the present simple type of structure were 

 the result of reduction from something more cumber- 

 some. The remnant of the endodermis, for example, 

 which is found in some Dicotyledon stems to-day, is 

 one of the clues that suggest this. Further, while 

 it is out of the question in the present state of our 

 knowledge to fill in the gaps in a direct series of descent, 

 it is yet possible among the fossils of different families 

 to show a conceivably parallel series in which the simple 

 hollow cylinder of wood is connected with foims which 

 had a solid central mass of wood, and, again, with others 

 in which the pith was beginning to be formed in the 

 middle of it. In the anatomy of all plants the rela- 



