A Unit of Construction in the Ptevictophyta. 149 
the reduction and chorisis of the leaf during the evolution of 
microphylly. A leaf wholly composed of sporangiophores, as I in¬ 
terpret the case in Sphenophyllum fertile, assists one to realize this 
conception. 
It is, of course, yet possible that Palaeozoic ferns may be found 
with sporangiophores inserted directly upon the axis. I refer to 
this possibility because in the same way it is conceivable that such 
ancient seed-plants as Cordaites may have derived their spore¬ 
bearing organs directly from sporangiophores inserted on lateral 
axes. Such a hypothesis is of course unnecessary, as the commonly 
accepted leaf-reduction hypothesis could meet the case, but I think 
there is nothing incredible in the suggestion that throughout the 
Cordaitales and Taxoideae we are dealing with plants which have 
never had their sporangiophores taken up upon leaves, and that, in 
fact, they are constructed in this respect upon the Equisetal plan, 
where the sporangiophores are merely associated with bracts. 
The “stamens” of Cordaites and Taxns are radially constructed 
consisting of a pedicel bearing sporangia. We have in Torreya 
evidence of the evolution of the dorsiventral stamen from such a 
structure 1 . If the dorsiventral stamen of Torreya is derived from 
a radially symmetrical sporangiophore it is probable that that of 
other Coniferm has a similar origin. 
In these groups of Gymnosperms, and in the Pteridosperms 
and their descendants (where the sporangiophore is obviously 
taken up upon the leaf) it seems a reasonable corollary to look upon 
the evolution of the seed as a further elaboration of the mega- 
sporangiophore. Space does not allow of a discussion of this 
problem here. 
Conclusion. 
On the whole, a review of the great central phylum, Pterido- 
phyta, as a group of plants derived from a common ancestor whose 
sporophyte generation bore a special type of asexual spore-pro¬ 
ducing organ called a Sporangiophore, seems illuminating and 
possibly useful as tending to concentrate attention on the probable 
origin of the structure in question. 
1 Coulter and Land. Torreya taxifolia, Bot. Gazette, 1905, p. 159. 
