95 
Sphenophyllales . 
for regarding the sporangiophores as the homologues of the leaves 
or bracts are the similarity of these organs at their first origin, the 
similarity of their position on the axis, and the occurrence, 
in the allied Equisetaceze, of forms of appendage intermediate 
between the leaf-teeth and the sporangiophores. Professor Bower 
clearly shows by his examples of the similarity at origin of 
such different parts as the sorus and leaf-margin of Angiopteris 
that though such similarity of origin generally obtains in 
homologous parts, it is not of itself proof of homology; his 
further examples that prickles and leaves, parts of different 
morphological nature, have a similar position relative to the axis, 
also shows that though similarity of position is frequent in 
homologous organs it cannot be held to prove homology. Professor 
Bower is probably correct in regarding the Equisetaceous annulus 
as a reduced leaf-sheath and in no way intermediate between the 
leaves and sporangiophores, for its teeth show no resemblance to 
the sporangiophores. So far Professor Bower’s arguments in 
support of the sui generis nature of the sporangiophore are founded 
on negative evidence, but he proceeds to attack the homology of 
the bracts and the sporangiophores on the ground that the numerical 
proportion and the relative position that the latter bear to the 
former are not constant, especially in the allied phylum of the 
Equisetales. As it seems to be generally admitted that the Equise- 
tales and Sphenophyllales are closely allied and that their sporangio¬ 
phores are homologous (6', (7), (9), (4), this is a legitimate objection. 
The numerical proportion of bracts and sporangiophores being 
inconstant does not seem a serious objection to the view that the 
latter represent leaf-lobes, for by analogy with the leaves we may 
well suppose that some and not all of the lobes of the bracts 
increased by the separation of ultimate lobes of a compound bract. 
Moreover Professor Bower himself supports the view of a fission of 
sporangiophores in the Sphenophyllales (3), (4). But he grounds 
one of his objections on an analogy with Calamostachys, a fossil 
cone belonging to the Equisetales, and consisting of equi-distant 
whorls of sporangiophores and bracts. Professor Bower points out 
that if the sporangiophores were leaves it might be expected that 
“ the alternate succession of the sterile leaves would be disturbed 
where the sporangia intervene between them ” (4), whereas each 
category of appendage, sporangiophore and bract, is inserted with 
reference to the whorls of its own category of organ and independent 
of the other whorls. This arrangement of the parts of the cone 
