Equisetales. 105 
are connected with the distribution of the endodermis. These 
differences in the distribution of the endodermis have been clearly 
summarized by Sadebeck (11). Without entering into the morpho¬ 
logical value of this layer it seems probable that the most primitive 
condition is that in which it forms a single ring surrounding the 
bundles. This is rendered probable by the fact that this condition 
is more widely spread than the others, and that no separate or 
internal endodermes are known in the Calamarise, from which, as 
will be shown presently, the Equisetaceae are probably descended. 
It also seems likely that the forms with an endodermis round each 
bundle (the so-called schizostelic or astelic types), and those with 
an internal endodermis, arose independently of one another from the 
normal type. 
Conflicting theories have been brought forward concerning the 
nature of the vascular bundle of Equisetum ; this consists of a 
median and two lateral portion's. Long ago Strasburger put 
forward the view that these three strands represented the coalescent 
remains of three bundles. Dr. Scott, however, in a Course 
of Lectures delivered at University College in 1904, stated that he 
considered it clear that this was not so, because the protoxylem of 
the “ bundle ” was single and continuous throughout the longitudinal 
course of each bundle. Mr. Gwynne-Vaughan pointed out, in 1901, 
that only the median portion of the bundle, which contains the 
protoxylem passed out at the node as a leaf-trace ; he also showed 
that the lateral strands had a different course from the median 
one ; in the internode they lie “ on adjacent sides of two different 
bundles ” (4) ; they again approach one another at the node above, 
and in the next internode they occur in the same bundle. The same 
botanist remarks that the lateral strands do not appear to be a 
continuation of the centrifugal development of the median strands, 
but appear to be developed centripetally. This, however, could not 
be definitely proved, as no incompletely differentiated portions of 
the stem were available. Mr. Gwynne-Vaughan suggests that they 
are the remnants of a primitive mass of xylem, pointing out that 
this would be in complete agreement with their apparently centri¬ 
petal development and cauline course. Another argument in favour 
of their representing primary centripetal xylem is that of exclusion;. 
If they are not the vestiges of primary centripetal xylem, they must 
be the remains of two other endarch bundles, as suggested by 
Strasburger; or the remains of secondary xylem ; or fresh develop¬ 
ments of the individual bundles. The first of these possibilities is 
to a great extent barred by the fact that these lateral strands not 
