The Ohio Naturalist. 
[Vol. XIV, No. 1 
198 
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 arrangements 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 true of unimportant external 
structures like the ligule in Selaginella. The vascular system of 
the Ptenophvta, 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 relationship 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 deriva¬ 
tion of the several types. The hiatus between the primordial 
vascular systems of living Brvophytes and the highly specialized 
* Contribution from the Botanical Laboratory of Ohio State Univer¬ 
sity, No. 77. 
