THE GENUS EQTJISETUM. 



185 



position to the Latter (Williamson and Scott, Phil, trans, roy. soc, L894, l>., p. 902, 906), 

 poinl to (In 1 conclusion that the pairs of sterile Leaves were really- 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 Equisetum would appear in this 

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

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

 hot., ser. 5, torn. 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- 

 ment, the corresponding dorsal segment of which has hecome obsolete? If the former 

 supposition is correct, there is no indication in the 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 Equiseta. 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 above, i. <?., composed 

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

 the primitive type of sporophyll in the equisetaceous series as composed of a dorsal and a 

 ventral segment, as Strasburger (op. cit.) considers to be the case in the analogous series 

 furnished by the Coniferae. 



Although the Sphenophyllales and Equisetales 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 belonging to the 

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

 conclusion is correct, the phylum Equisetales must be made to include a new order, the 

 Sphenophyllaceae, thus : — 



Sphenophy llaceae . 

 Equisetales Calamitaceae. 



Equisetaceae. 



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

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



