20 
Isabel Browne . 
in others the very poor development of the lateral metaxylem at 
this level. In this species, however, the lateral metaxylem groups 
may be well-developed over the whole of the region between the 
last whorl of leaves and the point, just below the insertion of the 
annulus, at which the xylem of the bundles assumes the more 
compact form characteristic of the axis of the cone. 
We may now ask which among the types of internodal bundle 
found in Equisetum is the most primitive. These bundles belong to 
five principal types. The first, that with a solid mass of xylem, is 
found, so far as we know, only in some of the tubers of E. arvense ; 
the second, in which xylem and parenchyma are irregularly inter¬ 
mingled in an oval or circular mass, is characteristic of some of the 
tubers of E. arvense and of all the tubers of other species the internal 
structure of which is known to us. Thirdly, there is the well known 
type of bundle consisting of a central protoxylem group and two 
postero-lateral groups of metaxylem. Then there is the form of 
bundle in which these two lateral groups are absent or very poorly 
developed. Lastly there is the type of internodal bundle in which 
the xylem is more or less continuous, though its outline varies. 
Often, especially in the aerial axis, the xylem-strand is U-shaped, 
with protoxylem at the innermost point. In other cases, e.g., 
usually in the cone, the xylem represents the segment of a circle, 
often very narrow radially, and with one or more groups of small 
elements at or near its inner edge. 
It seems clear that the tubers are modified rhizomatous 
branches, adapted for purposes of storage, and it can hardly be 
doubted that their bundles are ccenogenetic in character. They are 
interesting in that they show that the bundle consisting of three 
separate groups of xylem is not universal throughout the internode, 
even of the better known European species of the genus. It seems 
natural to regard the tubers as modified storage branches of the 
rhizome and their bundlesas phylogenetic derivatives from the bundles 
of the; rhizome. Duval-Jouve, indeed, has induced the production of 
branches below the whorl of leaves that forms a crown to the 
tubers. He found that if the tubers were grown in very wet earth 
they produce branches that grow into rhizomes; but that if grown 
in water and exposed to the light, the branches developed as aerial 
axes (6, p. 6—7). It is probably significant that though Fritel and 
Viguier give a list of eleven extinct, tuber-bearing fossil species of 
Equisetum (or Equisetites ), the oldest being the Triassic E. Mougeoti 
Brgnt. and E. arenaceum (Jaeger) Bonn, yet no definite tubers have 
