Vascular System of the Genus Equisetum . 227 
There is no question that in certain species the xylem is less well 
developed than in others, and it is possible to arrange the species as 
Browne has done in a reduction series. Whatever factor may have been 
operative in bringing about this reduction of the xylem considered as 
a whorl, the present writer cannot admit that it has been influential in 
determining the course and distribution of the xylem strands and the 
disposition of the parenchymatous gaps. 
It is suggested that the determining factor in the relative development 
of the metaxylem, and hence of the meshes, is primarily a mechanical one. 
It is significant that the species with large and heavy cones have more 
abundant xylem and more regularly developed network. 
It has been shown above that the arrangement of the primary strands 
of xylem and the mode of origin of the sporangiophore traces do not lend 
themselves to an interpretation of the vascular system of the cone as built 
up, like that of the vegetative stem, of an alternation of node and internode. 
If, then, the vascular structure of the cone is so strikingly different from 
that of the leafy shoot, the contention that the sporangiophore is a foliar 
structure — whether whole or part of a leaf — loses much support. In 
C 'alamo st achy s, as Williamson and Scott (18) have shown, there is a remark- 
able difference in the structure of the axis at the region of insertion of the 
sterile bracts and at the level of attachment of the sporangiophores. The 
nodes which bear the bracts show essentially the same character (e, g. the 
short nodal tracheide) as the vegetative nodes of Equisetum ; the traces of 
the sporangiophores, on the other hand, are inserted directly on the axial 
strands without interrupting them. In this respect they resemble those of 
the cone of Equisetum . 
Hickling (19) points out that the so-called ‘sporangiophore’ or ‘ fertile 
nodes ’ should not be regarded as nodes in the same sense as the true or 
‘ tract nodes ’. With that conclusion the present writer must agree, and in 
extending it to the cone of Equisetum concludes that in the latter genus the 
fertile axis is entirely undifferentiated into node and internode. 
Bower (17), after a careful consideration of the available data, has 
pointed out that the balance of evidence is strongly in favour of the non- 
phylome theory of the sporangiophore in Equisetum , and the facts brought 
out in the present investigation lend .it still further support. 
The annulus has generally been considered to be a foliar structure. Its 
morphology has been fully discussed by Goebel (20), and abnormal forms 
approaching foliage leaves on the one hand and sporangiophores on the 
other have been described by Milde (13), Gluck (21), and others. 
It is of some interest to consider to what extent this view is supported 
by the vascular anatomy of the cone. 
Just above the level of the insertion of the annulus the characteristic 
vascular structure of the cone begins and the transition from the typical 
