198 The Ohio Naturalist. [Vol. XIV, No. 1, 



THE CLASSIFICATION OF PLANTS, X.* 



John H. Schaffner. 



Our knowledge of the anatomy, cytology, and life history of 

 many of the groups of Pteridophytes is still far from satisfactory 

 and only a tentative arrangement is at present possible. How- 

 ever, three great lines of development are clearly marked giving 

 three great phyla with which to begin. There may be some dis- 

 pute as to the true relationship of a few isolated groups but in the 

 great majority of living forms the connection is quite evident. 

 Some of the recent speculations in respect to the Pteridophytes 

 have very little morphological evidence for their support. The 

 writer believes that it is best not to disturb the aiTangements of the 

 various groups as accepted in the past until there is more than a 

 mere foundation of assumptions based on doubtful evolutionary 

 hypotheses, many of which are all but disproven at the present 

 time. 



There is a notion that external characters are less stable than 

 internal anatomy. But there is really no evidence that this is so. 

 We should first find out whether there is any ecological response 

 and if so whether one set of structures responds more readily than 

 another. Even if it could be shown that there is ecological adap- 

 tion by direct response to environment or by natural selection this 

 would still be inconclusive, for the internal structure would neces- 

 sarily have to be co-ordinated with the external. A given type of 

 vascular system may be found in a group and thus indicate rela- 

 tionship, but the same is sometimes tine of unimportant external 

 structures like the ligule in Selaginella. The vascular system of 

 the Ptenophyta, for example, shows a remarkable diversity and it 

 is probably because of this very plascity that some of the groups 

 related to this phylum have evolved into the higher forms of seed 

 plants. 



The evolution of the Pteridophytes, in general, has been from the 

 homosporous condition to the heterosporous ; from the independ- 

 ent gametophytes to minute semidependent gametophytes ; from 

 low erect perennials to tree forms with little or no branching to 

 branched forms and from these to geophilous perennials and 

 occasionally to annuals. Several types of leaf venation appear 

 to have developed independently and also several types of vascu- 

 lar system. What the true relationshi]) between the several 

 types is, is at present largely conjecture. There is no definite 

 evidence as to which type of stele is the oldest, nor has there yet 

 been much progress made as to the probable evolution and dcrix^a- 

 tion of the several types. The hiatus between the primordial 

 vascular systems of living Bryophytcs and the highly specialized 



* Contribution from the Botanical Laboratory of Ohio State Univer- 

 sity, No. 77. 



