22 Isabel Browne. 
brought about the separation of the xylem of the bundle into three 
groups. Unfortunately the aerial axes of this species are unknown ; 
but it seems probable that in them too the xylem of the bundle 
was continuous. A third argument in favour of the priority of the 
bundle with more or less continuous xylem over that in which the 
wood forms three separate groups, lies in the fact that the latter 
type of bundle is never found in the cone. Now, it is a well known 
and widely accepted generalisation, that reproductive are more 
conservative than vegetative axes. Further, bundles with continuous 
xylem are characteristic of the primary internodal axis of E. 
maximum , the only one with which we are acquainted except that 
of E. variegatum , the bifascicular stele of which is clearly reduced. 
Again, though from the phylogenetic point of view a knowledge 
of the internal structure of the Mesozoic Equisetales is much 
to be desired, we possess numerous descriptions of the axes 
of the palaeozoic Equisetales, the Calamariae, and, in no case, so 
far as I am aware, is there any evidence for the existence of 
bundles with three groups of xylem. 
It is suggested, then, that the type of bundle which is usually 
regarded as the typical internodal form has arisen in the phytogeny 
from a bundle in which the xylem was continuous, owing to the 
failure of some of the cells of the metaxylem to develop as tracheides. 
It would seem that in most cases, at least, the mass of xylem had 
become U-shaped before its fragmentation. It is quite probable, 
however, to judge from the varied position of the lateral metaxylem 
groups (these may be divergent, .parallel or even convergent) and 
from the differences in their outline, that the persistent tracheides 
have, in some cases, undergone a certain shifting in position since 
their separation from the protoxylem. Usually, apparently, the 
cells which ceased to develop as tracheides seem to have been 
those near the carinal group, i.e„ those at the bend of the U. But 
Queva has pointed out that when in the rhizome of E. maximum 
or E. limosum a lateral metaxylem-group is reduced to a single 
tracheide, this is often in contact with the carinal canal (14, p. 30). 
Here, then, the cells which fail to develop as tracheides are the 
outer elements of the metaxylem. I have noticed that in E. 
giganteum , in the part of the axis between the annulus and last 
whorl of leaves, in which, as already noted, lateral metaxylem may 
be wanting or much reduced, a single large tracheide not infrequently 
represents the only metaxylem present, and that it may abut on the 
carinal canal, or be situated at the periphery of the bundle. In 
fact, while reduction of the xylem of the bundle seems generally to 
