Theory of Antithetic Alternation of Generations. 119 
sporophyte, will find a good deal with which they must disagree. 
Professor Bower’s view is that in the “ strobilus ” or fertile 
sporangium-bearing spike characteristic of the small-leaved groups 
of Pteridophytes (the Club-mosses, Horse-tails and Sphenophylls) 
we have the prototype of the leafy sporophyte, and at the same 
time the point of contact with the Bryophytic sporangium. This 
is of course the crucial question, and goes to the root of our whole 
conception of the morphological construction of the leafy plant-body. 
After contending, quite properly, that “ we have no right to 
assume that the leaf was formed once for all in the descent of the 
sporophyte ” the author states three possible views of the origin of 
a foliar differentiation of the shoot in vascular sporophytes : “(1) 
That the prototype of the leaf was of prior existence, the axis being 
a part which gradually asserted itself as a basis for the insertion of 
those appendages .... (2) That the axis and leaf are the result of 
differentiation of an indifferent branch-system, of which the limbs 
were originally all alike ... (3) That the axis pre-existed, and 
the foliar appendages arose as outgrowths upon it.” 
The first of these views, the so-called “ phyton-theory ” proper, 
at least in Celakovsky’s most recent expression, is based on 
the assumption (not at all a necessary assumption, as Professor 
Bower remarks) that there must exist in the plant-body morphological 
entities intermediate between leaf-bearing shoots on the one hand, 
and cells on the other. These intermediate entities Celakovsky 
found in his Sprossglieder, each Sprossglied being a leaf together 
with its stem-segment. From the point of view of evolutionary 
morphology such a theory can only have a real meaning if we 
suppose the body of a vascular plant to have arisen by the super¬ 
position of successive leaves, or the prototypes of leaves, each of 
which springs from its predecessor. Such a supposition would 
carry with it the consequence that the axis or stem is entirely 
secondary in origin. But an origin of this kind is not supported 
by evidence from the lower plants. The assimilating thallus, 
whether dorsiventrally or radially organised, found in many Algae 
and Liverworts, represents the material, so to speak, out of which 
both leaf and axis can be differentiated. It is no more reasonable 
to regard it as the prototype of the leaf alone than of the axis alone. 
Thus the “ Sprossglied ” must be regarded as an entity which is 
ideally constructed and has no existence as an objective morpho¬ 
logical phenomenon. 
“ The second view, that the axis and leaf are the result of 
differentiation of an indifferent branch-system of which the limbs 
were originally all alike, has lately been brought into prominence 
by Potonie,” though it was suggested years ago by Professor Bower 
himself, and, as he remarks, both Hallier and Lignier have put 
forward similar theories, while the reviewer has recently expressed 
the opinion that “ a strong case has been made out ” for the 
hypothesis. 
Our author’s objections to this view are as follows. First he 
questions whether “ such an origin is in any way applicable to 
.the Lycopods, Equiseta and Sphenophylls,” i.e. to the 
typically microphyllous forms in which each individual leaf is at 
present at any rate, both by development and position, quite 
obviously a comparatively small lateral appendage to a radially 
organised axis. Secondly he points out that “ in their individual 
