136 THEORY OF THE STROBILUS 
higher. plants_have, been, derived inthe. course_of generations from_parts 
of an Algal thallus like that of /ywcus, or at least from Alga-lke plants, 
by means of the overtopping of dichotomous branches, and the develop- 
ment as leaves of the branches, which thus become lateral.” Dr. Hallier, 
who adopts Potonié’s position, prefers to draw the comparison with Liver- 
worts, which show a similar sympodial development of a dichotomous 
branch-system.! 
It seems not improbable that the condition of many branched Fern-leaves 
may have been derived through a process of “overtopping” in an indifferent 
branch-system of the leaf itself. But it lies with Potonié to show, on a 
basis of comparison of forms more nearly related to them than the Fucoids, 
that the relation of axis to leaf in the Ferns was so derived; and, further, 
that such an origin is in any way applicable to other foliar developments in 
Vascular Plants, especially in Pteridophytes such as the Lycopods, Equiseta 
and Sphenophylls. I am not aware that this has yet been done. But 
granting that this can be done, the question still remains whether similarity 
of method of branching is any criterion of comparison as to descent. 
For sympodial development of a dichotomous system (and this is all that 
such “overtopping” actually is) has occurred in cases where it cannot 
be held to have resulted in a branching which is foliar; and of this instances 
can be found without going so far afield as the Fucaceae. If this be so, 
then little value need be attached to the comparison of such branchings 
in plants not nearly allied to one another; these may be held to be quite 
distinct examples of a general phenomenon of branch-development, without. 
the one being in any sense the prototype of the other Such reflections 
as these indicate that the comparison in mode of branching between the 
leaves of Ferns and the thallus of Fucoids, which forms the groundwork of 
the view of Potonié (or between the Ferns and the Thalloid Liverworts, 
as may be preferred by others), are not to be held as more than distant 
analogies ; consequently they are no demonstration of the origin of the leaf 
by a process of “overtopping.” 
The view recently advanced by Professor Lignier (‘‘ Equisétales et 
Sphénophyllales: leur origine filicinéenne commune,” Bwd/. Soc. Linn. de 
Normandie, Caen, 1904, p. 93) is analogous to that of Potonié, though 
differing from it in detail. It involves ranking the Lycopod leaf as a. 
“phylloid,” of the nature of a flattened hair, and comparable to the 
amphigastrium of a Liverwort. The leaf of the Fern, however, is held 
to be a true leaf, or phyllome, derived by differentiation from an indifferent 
system of “cauloids,” on which the ‘phylloids” have become abortive. 
All such hypotheses have critical points in their application ; in the present 
case it lies in the comparison of the Psilotaceae and Sphenophylleae. For 
Lignier regards the leaf-lobes of Zmesipteris as only “phylloids,” whereas. 
the leaves of the Sphenophylls, and also of the Equisetales, are “‘ phyllomes,” 
reduced from the larger-leaved type of the Ferns. The argument is chiefly 
1 Bettrige 2. Morph. d. Sporophylle u. d. Trophophylls, Hamburg, 1902. 
