THE GENUS EQUISETUM. 183 



III the Introduction attention has been called to the characteristic arrano-ement of 

 the traces at the nodes in Archaeocalainites, which is diagramniatically represented in 

 PI. 26, fig. 10. It is to be noticed here that the leaf-traces are not opposite lacnnae 

 at all, but, on the contrary, the branches or their equivalents, the rhizophoric organs, are. 

 In this primitive type of Calatnite the leaf-traces were not subtended by any gaps 

 in the vascular tissues, but the internodal lacunae were ramular lacunae and appeared 

 ininiodiately above the brandies. In the Introduction the term cladosiphonic lias been 

 used to describe a tubular fibrovascular axis characterized by having ramular lacunae 

 but no foliar lacunae, and consequently Archaeocalainites, like Selaginella laevigata 

 and Lepldodendron harcourtii, is cladosiphonic. But Stur {oj). cit., p. 158) has shown 

 that in the Ostrau beds, passing from lower to higher strata, a series of forms, Galamltes 

 ramifer Stui", C. cisiiformls Star, C. approximat'iformis Stur, and C. ostravlensis Stur, 

 represent transitions from the bundle arrangement of Archaeocalamites represented in 

 PI. 1, fig. 15, to that of Equisetum represented in Pi. 1, fig. 16. It will consequently 

 be not unreasonable to infer with Stur, that the equisetal arrangement of the bundles 

 was derived in the coarse of geological time from archaeocalamital arrangement. The 

 final result of the shifting of the internodes upon each other at the nodes has been, 

 that the lacnnae primarily belonging to the branches no longer subtend the latter but 

 on the contrary the leaf-traces, which still, however, betray their true morphological 

 relations by the fact that their exit causes no break in the nodal wood. It may then 

 be assumed, if the reasoning based on these facts is correct, that the apparent foliar 

 lacunae of Equiseta are really ramular lacunae which have shifted from their orio-inal 

 position during the course of evolution of the Equisetales, and that this group is accord- 

 ingly cladosiphonic. The original state of affairs occasionally reappears even in modern 

 Equiseta, as is shown in photograph 4 (PI. 29, fig. 4) . A much more striking example 

 of the same ancestral phenomenon is shown by photograph .3 (PI. .30, fig. 3), which 

 reproduces the course of the vascular strands in the cone of Equisetum arvense. It 

 will be noticed that the bundles in this case for the most part do not alternate. This 

 feature is more or less marked in the cones of all the species of Equisetum which have 

 been examined by the writer. In this connection may be mentioned a striking cambium- 

 like arrangement of the cells in the young bundles of the cones of E. hiemale and E. 

 limositm. It disappears, however, almost entirely in the comparatively massive bundles 

 of the adult cone, and perhaps may also be regarded as an ancestral feature, since secondary 

 growth was frequently present in the vascular tissues of the stroliili of various Calamites. 



It has been pointed out in the Introduction that protostelic and siphonostelic axes 

 may be possessed by different species of the same genus e. r/., TA'pidodcndron selaginoides 



