Vascular System of the Genus Equisetum . 231 
The reason for the extraordinary difference in character between the 
elements of the node and internode of the latter appears to be due to the 
difference in amount and rate of growth of those two regions. The inter- 
nodal condition represents the last stage in a reduction of xylem consistent 
with a continuous and effective supply of water to the leaves and branches. 
In the light of these facts it seems clear that the metaxylem found in 
the underground basal region of the sporeling, in the cone, the nodes and 
internodes of the vegetative branches, is all phylogenetically the same, i. e. 
it is the centrifugally developed wood of a monostelic system, differing 
only in the degree of reduction and the character of the elements. 
With regard to the question of the addition of secondary elements to 
the centrifugal wood of the node of certain species, the results of the present 
investigation negative the presence of any process of secondary thickening. 
The facts on which this idea was based allow of a simpler explanation, viz. 
that the increase in length of the nodal elements, accompanied by sliding 
growth, gives an appearance in transverse section of an apparent increase in 
the number of tracheides of the xylem. 
Having concluded that the existing structure in both vegetative and 
reproductive axes has been derived from a continuous siphonostele only 
interrupted by the insertion of the branches, we are faced with the problem 
of the determining factor associated with the distribution of the parenchy- 
matous tracts. Can it be met by an application of the conception of foliar 
gaps which is associated with the study of filicinean anatomy ? Browne has 
attempted such an explanation in her papers dealing with the cones of 
several species of Equisetum . She homologizes the parenchymatous meshes 
with foliar gaps, associating them with sporangiophores, and considers that 
these organs are modified whole foliar structures. 
As stated more fully above, the detailed anatomy of the cone does not 
support the view that the sporangiophores are really of this nature, and, 
judged in the light of the anatomy of a mixed strobilus such as that of 
Calamostachys , it cannot be doubted that nodes are completely absent in 
the cones of present-day Equiseta. 
Even if the sporangiophores were foliar structures, the anatomy does 
not favour the view that the meshes are foliar gaps. Jeffrey has pointed 
out, and it is abundantly confirmed in this investigation, that the gap does 
not, except in a few cases, occur immediately above the point of departure 
of the trace ; and most frequently shows no relation to it. 
The theory put forward by Jeffrey (5), that the gaps in the vegetative 
axis can be described as ramular gaps, is certainly not supported by the 
anatomical facts described in this paper. The gaps are not situated 
immediately above the point of insertion of the vascular supply of the 
branch ; they fail, in fact, to satisfy the criterion on which Jeffrey himself 
insists in the case of foliar gaps. 
