Development in Equisetum . 75 
seen on focussing. While the nature of the isolated ring in Fig. 
1 1 is not beyond suspicion, no doubt can be entertained as to 
the contents of the lacuna in Fig. 10. Thus it may be inferred 
that these canals, like the carinal canals of Equisetum , orginate 
in the destruction of protoxylem ; and the accuracy of this 
conclusion is not affected by the fact that in neither of the two 
plants did the protoxylem ever occupy the whole length and 
breadth of a canal. To this extent it is true that the canal is 
due to the destruction of a tissue, but this tissue was lost, not 
after death, but during the development of the plant, and was 
xylem, not phloem. 
The projections of the xylem in the nodes, previously 
referred to, acquire significance in tracing the further develop- 
ment of the vascular system of Calamitae. 
Williamson 1 writes as follows regarding the structure of the 
wood as seen in a vertical section of a mature stem : — 
‘The first feature which arrests attention in the vertical 
section is the material transverse enlargement of the woody 
zone which takes place at the node. This enlargement is both 
internal and external. In the former case the woody layer 
encroaches on the pith, and in the latter upon the bark. The 
increment is due to the development of a considerable number 
of barred or reticulated vessels, but especially the former, 
which take their rise in contact with the outermost medullary 
cells above the node, and following an arching course across 
it, their concavities being directed towards the medulla, again 
terminate as they arose from the medullary cells above the 
node, in those below it. It follows from this arrangement 
that only the outermost of these nodal vessels are prolonged 
across the internodes to the adjacent nodes above and below. 
‘ In transverse section we find, as the vertical one would 
lead us to expect, that the woody wedges at the node are 
much longer from their medullary to their cortical surfaces 
than at the internodes, The canals from which they respec- 
tively take their rise are either wholly wanting here or are so 
reduced as to become quite inconspicuous.’ 
1 loc. cit. p. 483. 
