259 
Cone and Fertile Stem of Equisetnm . 
the plant at this level. In other words, if the production of sporangia and 
the development of annular bundles as they occur in E . giganteum are 
primitive characters retained by this species, it is strange that just in it the 
axis above the annulus should have undergone more reduction than in the 
other species. If, however, the sporangiferous annulus is a phylogenetic 
innovation, we should expect it to lead to an increase of appendicular xylem, 
and should be by no means surprised to find that the axis above the annulus 
had become relatively poor in xylem and that no fresh meshes were formed 
here. 
This, then is the first argument in support of the view that the sporan- 
giferous annulus is not primitive for the genus. Secondly, there is the 
wider aspect of the question, one difficult to discuss without unduly enlarg- 
ing the scope of an ordinary anatomical paper. If the annulus was 
primitively sporangiferous, then it has become normally sterile in the species 
of Equisetnm other than E. xylochaetum and E. giganteum , l The annulus 
would, in fact, be a metamorphosed whorl of sporangiophores, and not 
a reduced whorl of leaves. On this view the sporangiferous annuli, common 
abnormalities in certain species, would be explained as reversions, while on 
the opposite view it would be the cases in which the annulus assumes the 
characters of a leaf-sheath which would be regarded as reversions (cf. Milde, 
p. 1 66). This leads us to the general question of whether leaves and 
sporangiophores are strictly homologous. In 1903 Professor Bower was led 
by his advocacy of the ‘ non-phyllome ’ theory of the sporangiophores 
(which he regarded as placental outgrowths) to look upon the annulus of 
Equisetnm as a reduced whorl of leaves rather than as a structure transi- 
tional between leaves and sporangiophores (Bower, pp. 221 and 241). There 
is, however, no need to enter here on the general question as to whether 
fertile and sterile ‘ leaves ’ are ultimately strictly homologous or not. For, 
even if they be referable to a common ancestral, presumably synthetic type 
of appendicular organ borne by the axis, yet we can safely assume that the 
highly peculiar form of the Equisetal leafy whorl (though itself an ancient 
character) is a later development in the phylum than the distinction between 
sporangiophores and leaves. Consequently, if the annulus represents 
a primitively sporangiferous whorl its similarity to a reduced whorl of 
leaves must be due to homoplasmy. Certainly as striking, if not more 
striking, cases of homoplasmy are known ; yet this explanation of the 
similarity hardly seems a probable one. 
On the view advocated here the annulus is throughout the genus Equi- 
setum a reduced whorl of leaves. It is believed that the vascula.r traces of 
1 Possibly a normally sporangiferous annulus may be found in other species, especially of the 
group Pleiosteichia of Milde, to which both E. xylochaetum and E. giganteum belong. In Milde’s 
figures of the cone of the former species no trace of the sporangia is observable from the exterior, and 
the same thing is true of my cones of E. giganteum. Still, on the evidence available, the sporangi- 
ferous annulus would not appear to be widely distributed in the genus. 
