THE GENUS EQUrSETUM. 177 



with that abnormally formed in the pith of certain Campanulaceae, etc. It is probably 

 equally justifiable to compare it with that occurring in certain cryptogams. Russow 

 (Vergleich. untersucli., p. 117), for example, has described the existence of periderm in 

 the Marattiaceae and the Ophioglossaceae and the writer (Trans. Giin. inst., 1898, p. 284) 

 has called attention to a similar formation at the bases of fallen leaves in Botrychmm 

 virginianxm. The frequent separation of the stem of equisetoid plants at the nodes, as 

 the result of normal or exceptional conditions, would make such a protection at these 

 points of considerable importance. In this connection it is interesting to notice that the 

 sclerified diaphragms of E. liniosum extend to the outer margin of the vallecnlar canals. 

 They thus surround the vascular bundles, just as is the case with the aljciss-periderm of 

 the leaf-stalks of Hippocastanum, etc. 



Some of the results described in the foregoing paragraphs seem to the writer not to 

 be without importance in connection with certain disputed points of calamitean anatomy 

 referred to in the Introduction. The two features wliich are most worthy of considera- 

 tion in this connection are the mode of insertion of the branches of the Calamites, and 

 the nature and position of the organs which gave rise to the infei'ior series of nodules on 

 calamitean medullary casts. 



The BiiAxciiES and Ixfkaxoual Canals of Calamites. 



Beginning with the first of these, the statement is commonly made by palaeobotani- 

 cal writers that the branches of the Calamites in contrast to those of Equiseta originated 

 above the node. Photograph 6 (PL 30, fig. 6) of the present memoir is a copy of a rare 

 and beautiful specimen of a calamitean stem showing extei'nal features, figured in Weiss's 

 admirable monograph (Steinkohlen-Calamarien, Heft 2, atlas, pi. 16, fig. 6) . Several 

 nodes are present, to which leaves are attached and the scars of a number of fallen 

 branches are to be seen in a singl-e horizontal row. On the lower margin of the branch- 

 scars are smaller scars, which Weiss interprets as belonging to fallen leaves, but when he 

 tells us that these fallen leaves are the appendages of a node which comes immediately 

 below the branches, it is probal)le that the botanical reader, remembering the state of 

 affairs in Equisetum, will hesitate to follow him. If a node is present in this position it is 

 certainly not very obvious and is irregulai-ly placed. Moreover, in similar specimens fig- 

 ured in an earlier monograph (Weiss, Steinkohlen-Calamarien, Heft 1, atlas, pi. 17, figs. 

 1 and 2), where the leaves were represented only by their scars, in accordance with the 

 conviction that the Ijranches arise above the node, he has considered the smaller scars, 

 related to scars of fallen branches, as dipping down underneath the branches and coming 

 up to the nodal line again in the intervals between the branches. In the example fig- 



