Anatomy of the Cone and Fertile Stem- of Equisetum. 259 
a comparative study of the anatomy leads to the conclusion that in all 
probability the more regular type of stele, possessing more xylem relatively 
to its size, is the more primitive form, and in the following paragraphs 
I shall bring forward certain considerations that seem to me to detract 
greatly from the force of the suggestion that the larger cones are presumably 
anatomically the more primitive. At the same time I want frankly to 
admit that though, to my mind, these considerations weaken the force 
of this objection, they do not entirely remove it. 
Accepting the view that the living Equisetaceae have undergone 
reduction in size, it seems probable that their vascular system has Qs o 
undergone reduction. This assumption is supported by the remarkably 
small amount of xylem found in the internodes of the rhizome, aerial stems, 
and branches, and by the palustrine habit of a large number of the species 
of the genus. But we have no right to assume that all parts of the plant 
have been equally affected by reduction. For instance, we know from 
Halle’s study of the mesozoic impressions of the Equisetales of Sweden that 
in some species grouped by him under the genus Neocalamites , the bundles 
of the stem only gave off traces at alternate nodes or even less frequently 
(Halle, p. 6). If, as seems likely, we are here dealing with a case of reduction 
in the phylogeny of the number of leaves, it is clear that the reduction in the 
number of the bundles of the axial stele has proceeded at most only relatively 
half as fast as the reduction in number of traces and leaves. It does not follow 
that the leaves were twice as far apart as in the hypothetical ancestor in 
which the bundles gave off traces at every node, for along with this change 
a decrease in thickness of the cortex, and therefore in the circumference of the 
stem, may well have taken place. As regards the cones, it would seem that 
their greater or less height could have little or no influence on the closure or 
persistence of the meshes at the node. But if the reduction in width of 
the stele did not keep pace with the reduction of xylem at the neighbourhood of 
the node , the direct and immediate effect would be for some of the meshes to 
persist into the internode above. I think that this is the explanation of the 
greater reduction of the vascular system in the cones of the species of 
Equisetum that have relatively wide steles. Thus the cones of E. palustre 
and E. limosum are, on an average, much of the same height, but the reduced 
steles of the latter species are, on an average, about twice as wide as those 
of the former. Then, too, the most reduced axial vascular cylinders were 
found in the cones of the species that had the widest steles, namely 
E. maximum and E. limosum . Again, within the limits of single species, 
Cone C of E. arvense and Cones B of E. maximum and E. limosum have 
respectively wider steles and a more reduced system of xylem than the 
respective Cones A of these species. In E. palustre, excluding var. poly- 
stachion , in which all parts of the cone seem to have undergone an equal 
degree of reduction, the cones examined had steles of much the same width, 
S % 
