594 
E antes.— On the Occurrence of 
evident from the structure of the leaf-trace itself. The latter is nearly- 
divided in its proximal portion into three parts, in position and in structure 
corresponding to those of the complete, cauline, fibro-vascular bundle. A 
connexion with all parts of the stem-bundle is here indicated. For, if the 
protoxylem alone passed into the leaf, we should not expect to find this 
simple structure dividing in such a case into three new bundles, or taking on 
a form indicating such a condition ; especially is it difficult to conceive its 
forming two new bundles centripetal in development, when it, itself, is 
centrifugal. Whereas, if all parts of the internodal bundle unite in the 
formation of the leaf-trace, they would undoubtedly assume their normal 
position and relations, forming there a bundle of the same general structure 
as that of the internode. 
The internodal bundle of the Calamites has been briefly described 
above (p. 589). From this has come, by reduction of the metaxylem, the 
fibro-vascular bundle of Equisetum. In the reproductive axis of the latter 
genus the bundle-structure points to this relation. The cone-bundles of 
E.hyemale and of E.fluviatile , when seen in cross-section, show a condition 
intermediate between that of the internodal bundles of the Calamites and 
of the living genus. In many of them there is found the typical protoxylem- 
lacuna. Spreading externally from this is a considerable body of elements 
that are clearly seen to be metaxylem in radial sections. They are not 
solidly packed, but have many parenchyma-cells intermingled with them. 
There is a marked tendency for these metaxylem-tracheides to form groups 
outwardly flanking the protoxylem. In the cone of Equisetum we have 
accordingly repeated the Calamitean primary bundle well started on its 
way in reduction toward the condition found in the vegetative stem of the 
living descendants of the group. 
An analogy of some value can be drawn between aspects of the 
vascular system of two groups of plants which are believed to be much 
reduced, the Horse-tails and the Monocotyledons. The ancestors of both 
undoubtedly possessed secondary growth ; in the living members this is 
found only as a remnant. It is a striking fact that this growth, as observed 
by Dr. Chrysler 1 in the Gramineae, is in exactly the same situation as 
it appears in Equisetum — slightly above the node. The resemblance of 
the primary wood in the bundles of these two widely separated groups 
has been noted by many writers. It is, indeed, close ; in both are found 
a canal containing broken-down protoxylem and two externally flanking 
metaxylem-groups, to a greater or less extent separated therefrom. 
The internodal bundle, then, of Equisetum is, without doubt, centri- 
fugal throughout in its development, and is structurally a fibro-vascular 
unit, a much reduced Calamitean bundle. 
Paleobotanists have recognized a close relationship between the 
1 Chrysler, M. A. : The Nodes of Grasses. Bot. Gazette, vol. xli ; 1906. 
