THE GENUS EQTJISETUM 161 



lary strands. In a photograph 1 (PI. 27, fig. 1), the stehir tube appears with a lacuna in 



* 

 its side, where the vascuhir strand of a branch has been given off, this gap is the ramular 



lacuna. The leaves originate from the stelar tube, however, without causing any break 



in its continuity. Photograph 2 (PL 27, fig. 2) shows the restoration of the integrity of 



the tube below the point of origin of a subsequent branch. 



In photograph 3 (PI. 27, fig. 3), we have a copy of Williamson's figure (Phil, 

 trans, roy. soc, 1881, PI. 52, fig. 9) of the tubular stele of Lepidodendron harcourtii. 

 The vascular tissues in this form are likewise arranged in a tul)ular fashion. The leaf- 

 traces are small and cause, according to his description, no interruptions in tlie walls of 

 the tul)e. Where a I)ranch is given off on the contrary, as in the figure, there occurs a 

 ramular lacuna. Both Renault (Cours do bot. foss., tom. 2, p. 71) and Gibson (Annals 

 of bot., vol. 8, p. 192) have been struck by this resemblance between Selaginella laevi- 

 gata var. lyallii and Lepidodendron harcourtii (other species of Lepidodendraceae had 

 similar peculiarities, e. (/., Halonia, Ulodendron, and species of Sigillaria, etc.), and 

 Bertrand {PhyUoglossum druvimondii, Arch. Ijot. Nord France, 1885, p. 71) considers 

 his Centradesmideae (in part) as belonging to the same type. 



In comparing these instances with the examples taken from the Filicales, it is to be 

 observed, that while both cases represent siphonostely, there is this marked diff"erence 

 between them: in the filicineous stems the siphonostely is characterized by the occur- 

 rence of foliar lacunae, while in the lycopodineous axes it is related to I'amular lacunae. 

 Without going further into the matter at the present time, the hypothesis i.s presented 

 that siphonostely has a mechanical significance, and that in the case of the Filicales, the 

 siplionostelic modification of the vaso-skeletal tissues arose in connection with the support 

 of large leaves. Among the Lycopodiales it made its appearance on the contrary, in 

 relation to the support of secondary axes. We may consequently call the siphonostely 

 of the Filicales phyllosiphonic, and that of the Lycopodiales cladosiphonic. 



At this stage we come naturally to the discussion of the bearing of these morphologi- 

 cal conceptions, assuming that they are well founded, on the phylogenetic position of the 

 Equisetaceae. In this connection it is necessary to examine with some care liotli the 

 structure and development of the genus Equisetum, and since certain of its features can 

 only be understood in the light of a knowledge of its ancestors, it will likewise be neces- 

 sary to refer as briefly as may be to the organization of the Calamites and Sphenophyl- 

 lales. It will conduce to clearness and b'^evity of exposition to begin with the extinct 

 groups and subsequently to describe their living representntives. 



