Equisetales. 111 
lobes; but this obligation no longer exists and the theory suggested 
here seems more probable. 
Professor Lignier has also attempted to explain the difference 
in number between the bracts and sporangiophores of Calamostachys. 
He supposes that in a form in which, as in Palceostachya, the 
sporangiophores or ventral segments are equal in number to the 
dorsal segments or bracts, the former became associated and fused 
together (8). Dr. Jeffrey takes a different view, pointing out that 
the bundles in the axis of the cone are equal in number to the 
sporangiophores, and that this fact taken in conjunction with the 
position of the bracts leads to the conclusion that “the pairs of 
sterile leaves were really dichotomously divided segments of 
sporophylls, of which the sporangiophores were ventral segments 
.” (6). This seems a more likely theory, in view of the fact 
that the sporangiophores of C ala mo stachys only bear the same 
number of sporangia as those of Palceostachya ; if they were formed 
by the fusion of two of the latter we should expect them to bear 
more numerous sporangia. 
Since dioecism may have prevailed in the Calamariae it is 
impossible to prove that any given species is homosporous ; but 
there is very strong negative evidence of their primitive homospory ; 
indeed the heterospory of the Catamites is not of an extreme type. 
It is significant that neither in Archaeocatamites nor in Polhocites, 
probably the two most ancient types, has more than one kind of 
spore been found ; in some species of Calamostachys this is so too, 
though others have been proved to be heterosporous (12); further, 
the recent Equisetaceae are homosporous and are obviously descended 
from homosporous Palaeozoic Equisetales. Dr. Scott has shown 
that heterospory probably arose by the abortion of certain spores in 
order to facilitate the nutrition of the remainder (12). In that case 
it probably arose in more than one evolutionary series of the 
Calamariae. Dr. Scott’s suggestion that Palceostachya may have 
been heterosporous bears this out. 
As regards the affinity of the two orders, they are evidently 
very closely related. Many botanists regard the Equisetaceae as 
the direct descendants of the Calamariae (12) (16). For as we pass 
upwards from the Palaeozoic tree-like Catamites to the older of the 
Mesozoic Equisetites, which, though still very large were smaller than 
Catamites, and to the more recent species of Equisetites and finally 
to the living Equisetum we trace a steady diminution in size. 
Presumably some of the Mesozoic Equisetaceae possessed secondary 
growth in thickness, for Professor Seward states that their rhizomes 
