Vascular System of the Genus Equisetum . 2 1 9 
from specimens of E. arvense , owing to the more convenient size and shape 
of the buds. 
Text-figs. 13-16 show groups of nodal elements drawn from the nodes 
of an apical bud. From these it will be seen that the shape of the elements 
gradually changes from short cubical cells to slightly elongated elements 
tapering at both ends. The first change involves a slight lengthening of each 
element. Since little or no growth is taking place in the surrounding cells, the 
elongation of these xylem elements necessitates some special accommoda- 
tion. This is brought about by an overlapping of the ends of the cells and 
an inclination of their long axes. All the elements of a group have the 
same inclination, though the direction is reversed in contiguous bundles, as 
seen in tangential sections. As the elongation of these elements proceeds, 
it is accompanied by an increase in diameter, and finally the walls are 
thickened in a reticulate pattern and lignification completes the development. 
The cells which are destined to compose the nodal regions of the stem 
are sharply marked off from the internodal elements, which multiply rapidly 
later. The former are cubical parenchymatous cells arranged in very 
symmetrical rows, about four tiers of such cells being concerned in the 
formation of each node. 
When the tracheides are fully grown, but before they lose their proto- 
plasmic contents, the position of the nuclei still gives a rough indication of 
the original arrangement of the cells. If these groups of cells were 
sectioned in a transverse direction it would naturally appear as if the 
number of elements had increased as the node became older. This increase, 
however, is brought about by the overlapping of previously existing cells, 
and cannot in any way be referred to a process of secondary thickening. 
Each element in longitudinal view is seen to overlap one — two cells, which 
would result in an apparent multiplication by two or three, and on counting 
the number of elements present in a number of groups as seen in transverse 
section the average increase corresponded very closely, i.e. rather more 
than two. 
The examination of preparations of E. maximum lead to the same 
conclusions, and it would seem, therefore, that in these two species the facts 
on which the existence of a process of secondary growth has been based 
are susceptible of a simpler explanation, although one which perhaps will 
not be so acceptable to palaeontologists. 
A Forked Vegetative Axis. 
Several authors have already described cases of branching in cones of 
Equisetum, and Stiles ( 16 ) has investigated the anatomy of a branched 
strobilus of E. maximum. He accepts the view put forward by Bower 
( 17 , p. 682) that the branching of the strobilus is fundamentally dichotomous. 
It is interesting, therefore, to be able to record an example of an apparent 
