THE GENUS EQUISETUM. 165 



series of nodules on the medullary casts of the Calamites are really infranodal, and 

 agrees with Williamson in regarding them, as the result of the regular disappear- 

 ance of tissues in the lower medullary rays, but considers tliat the disintegration is 

 due rather to unec^ual maceration, than to the presence of any special organs or canals. 

 The opinion of Graf zu Solms in such matters rightly carried great weight, and after its 

 expression practically in favor of Williamson's view, the latter thought it no longer neces- 

 sary to defend his hypothesis (Williamson and Scott, Phil, trans, roy. soc, 1894, B., p. 888) . 



Seward in his recent admirable treatise on fossil plants (1898, p. 324) adopts Will- 

 iamson's explanation as quite proved, and does not even discuss the other views. 



Renault (Etudes des gites miueraux de la France, fasc. 4, atlas) has recentlj- pulj- 

 lished figures of tangential sections through the inner part of the secondary wood of 

 Calamites, in which are represented the usual more or less constantly alternating vascular 

 strands. Above the nodal anastomoses, and at the bottom of the upper medullary rays, 

 are indicated radiating traces, o, which are stated in the description of the plates to be 

 "aquiferous organs." Below them are I'epresented smaller traces,/, embedded in the vas- 

 cular strands, which are regarded by this author as leaf-traces. His " aquiferous organs " 

 are undoubtedly the homologues of the "leaf-traces" of our photograph 6 (PI. 27, fig. 6) 

 copied from the monograph of Williamson and Scott. In the full descriptive text, which 

 appeared three years after his plates Renault (Etudes des gites mineraux de la France, 

 fasc. 4, p. 93) informs us that his figures, one of which is copied in our photograph 1 

 (PI. 28, fig. 1) should be inverted, and that his " aquiferous organs" are really " organes 

 rhiziferes " which he regards as the equivalents of Williamson's infranodal canals. He 

 states also that he has found roots in connection with the organs in question. These 

 observations open up again the whole subject of the infranodal canals, and it is to be 

 regretted that Renault has not given us reasons for the subsequent inversion of his 

 figures. 



As will be shown in the sequel, a consideration of the structiu'e of Equisetum gives 

 us criteria for explaining all the foregoing conflicting accounts and apparently for set- 

 tling this much-disputed question. 



Sphenophyllales. 



There remains to be said something concerning the organization and structure of 

 the Sphenophyllales. Like the Calamites and Equisetaceae, they had a regularly jointed 

 and furrowed stem. The ridges of the stem did not alternate. The leaves somcwliat 

 resembled those of Archaeocalamites in being dichotomously divided, and were arranged 

 in superposed whorls. The branches, according to the account of Renault (o/>. cit., 



