THE GENUS EQUISETUM. 185 



position to the latter (Williamson and Scott, Phil, trans, roy. soc, 1894, B., p. 902, 906), 

 point to the conclusion that the pairs of sterile leaves were reality dichotomously divided 

 dorsal segments of sporophylls, of which the sporangiophores were the ventral segments, 

 and in this feature (i. e., dichotomy) resembled the foliage leaves of Archaeocalamites. 



The morphological nature of the sporangiophore of Equisetuni would appear in this 

 connection to be problematical. Is it to be regarded as the result of the fusion of dorsal 

 and ventral segments, such as has been shown to exist l)y Van Tieghem (Ann. sci. nat., 

 hot., ser. 5, toni. 10) and Strasburger (Coniferen u. Gnetaceen) in the interesting peltate 

 sporophylls of certain Cupressineae. or is it not rather to be considered as a ventral seg- 

 naent, the corresponding dorsal segment of which has Ix'come obsolete? If the former 

 supposition is correct, there is no indication in tlie form of vestigial vascular bundles, in 

 the axis of the sporangiophore, to indicate its morphological nature. We must await 

 further knowledge of the cones of Archaeocalamites before attempting to decide this point, 

 for these oldest known calamitean strobili had, according to the imperfect data at our dis- 

 posal, the same external organization as those of living Eciuiseta. If it ever becomes pos- 

 sible to examine their internal structure, the sporophylls of this genus may prove to be 

 analogous in organization to those of the Cupressineae referred to aljove, /. c, composed 

 of fused ventral and dorsal segments. In any case, there are many reasons for regarding 

 the primitive type of sporophyll in the equisetaceous sei'ies as composed of a dorsal and a 

 ventral segment, as Strasl)urger [op. cit.) considers to 1)e the case in the anahjgous series 

 furnished by the Couifei'ae. 



Although the Sphenophyllales and Eipiisetales resemble one another so closely in 

 their vegetative organization and in the structure of their strobili, a striking difference 

 exists between the two groups, as Seward (Fossil plants, p. 388) has pointed out, in the 

 structure of their vascular axes. In the former group, the central cylinder is protostelic, 

 while in the latter it is cladosiphonic ; but it has already been shown that these two stelar 

 types may coexist within the same order and even within the same genus. In view, con- 

 sequently, of numerous remarkable points of resemblance, the writer is of the opinion that 

 the sphenophyllaceous and equisetaceous forms should be regarded as Ijelonging to the 

 same natural group, the former series being only more primitive than the latter. If this 

 conclusion is correct, the ph^ylum Equisetales must be made to include a new order, the 

 Sphenophyllaceae, thus : — 



Sphenophy llaceae . 

 Equisetales Calamitaceae. 

 Equisetaceae. 



We may now turn to the questioii of the affinities of the Equisetales in the larger 

 sense above indicated. It has already been pointed out that the siphonostely of the Fili- 



