Development in Equisetum. 77 
through the node, and the presence there of the greater mass 
of wood is of importance in transmitting water contained in 
the canals, for the number of the elements compensates for the 
absence of an open channel. Thus it is seen that the secondary 
thickening existent in the node is of physiological importance 
apart from questions as to whether it may be incipient or 
persistent. 
Returning now to the question of the affinity of the Cala- 
mitae, we have seen that the canal at the inner angle of each 
woody wedge does not represent lost phloem, but is the same 
in origin as the carinal canal of Equisetaceae. That these 
canals in Calamitae are sometimes continuous from one inter- 
node to another, is a difference unimportant in itself and 
rendered still less important by the fact that Calamitae differ 
among themselves in this respect. 
Further, we have shown that the formation of secondary 
wood in Calamitae is a point of resemblance rather than 
of mere contrast between this group and the Equisetaceae, 
the course of development being similar though unequal in 
extent. 
Certain distinctions, previously referred to, drawn by Wil- 
liamson 1 between Calamitae and Equisetaceae, amount to 
nothing more than amplified illustration of the fact already 
admitted, that cambial development has taken place to 
a greater extent in one case than in the other. Thus he 
points out in Calamitae the similarity between the wood of 
the node and that of the internode, and contrasts with this the 
abrupt transition in Equisetum from the annular elements of 
the internode to the reticulate elements of the node. But it is 
clear that more extensive thickening in Equisetum would have 
obliterated this difference. 
Similarly it is remarked that there is nothing corresponding 
to the ‘ muriform ’ tissue of the ‘ primary medullary rays ’ of 
Calamitae. Williamson’s drawings and description makes it 
certain that the bulk of the tissue to which he gives the name 
‘ primary medullary ray ’ is a structure produced by an inter- 
1 loc. cit. 
