Development in Equisetum. 73 
Hence, if it be argued that the structure in question is 
primary and in no way comparable with a secondary 
thickening, it may be answered : that after the bundle has 
attained in the internode its full number of cells in radial 
thickness, and after tangential division in the corresponding 
tissues of the node has ceased, a plate of tissue has been 
intercalated between the protoxylem and protophloem of the 
node ; that the xylem thus formed is mostly reticulately 
thickened, whereas the thickening of the protoxylem is 
annular ; and that the intercalation has been accomplished 
by the activity of a meristem whose cells are cambiform. 
Further, there are recognized cases in which the transition 
from primary to secondary tissue is immediate and the 
distinction between them scarcely observable. Ranunculus 
repens is an example of this 1 , and still more striking in this 
respect is Ranunculus Jluitans where, in correlation with 
aquatic habit, the vascular system is weak alike in its primary 
and in its secondary development — so far as such secondary 
development exists. Indeed, were it not for its systematic 
position, such a plant as Ranunculus jluitans would scarcely 
be recognized as possessing secondary thickening ; and in 
the absence of preconceived ideas, Fig. 3 would be considered 
a much more typical illustration of such formations. 
The fact that in the node of Equisetum the thickening 
extends only a short distance longitudinally cannot be held 
to weaken the argument : it is a question of secondary 
growth in thickness. 
But whether the name, secondary thickening, is accorded to 
this process or not, matters very little for our present purpose, 
as it can be shown that a cambial activity exists, essen- 
tially similar to what is found in Calamitae, only less in 
extent. 
First, let it be noted that in the young internode (Fig. 7) 
the side-groups of xylem gg seen in Fig. 5 are not yet 
developed. They are differentiated during the basal interca- 
lation of the internode previously mentioned, and after the 
1 De Bary, Comp. Anat. Fig. 152 and Fig. 153. 
