166 D. H. Scott. 



to render a union under the same name desirable. The most obvious 

 difference, of course, is the phyllotaxis, spiral or at least scattered 

 in the Psilotaceae but verticillate in the Sphenophyllales. From the 

 great constancj' of this character throughout the groups included 

 under Articulatae I am inclined to attach considerable importance 

 to it. Further, on present evidence, the mode of branching seems 

 also to mark a distinction between Psilotaceae and the Sphenophj'llales, 

 dichotomy of the stem occurring in the former, but not, so far as 

 we know, in the latter. For these reasons I prefer to treat Psilotum 

 and Tmesipteris as forming a class of their own. the Psilotales, having 

 most in common with the Sphenophyllales, though not wholly without 

 the Lycopodiaceous affinities which have hitherto been attributed 

 to them. 



IV. Lycopodiales. 



As is well known, the Lycopods of the Palaeozoic period formed 

 one of the dominant groups of plants, as shown by the great number 

 both of species and individuals, the lofty arboreal habit of most of 

 them, and the high organization which they attained. While the 

 best known representatives, the Lepidodendreae, were trees, reaching 

 a height of 30 metres or more, there is evidence for the contemporary 

 existence of small herbaceous plants, resembling the Club-mosses of 

 the recent flora. The extensive genus Leimlodendron, which we may 

 take as typical of the Class, ranges from the Devonian to the Permian. 

 The species were trees, with a tall upright shaft bearing numerous 

 dichotomous branches forming a dense crown, and clothed with 

 numerous long and narrow simple leaves, ranged in a complex spiral 

 or verticillate phyllotaxis. When the leaves were shed, their bases 

 remained on the stem, and the sculpturing which they present affords 

 the external characters by which the "species" are commonly dis- 

 tinguished. The markings on the leaf-cushion and scar are described 

 in all the text-books and need not detain us here. At the base 

 the stem branched out into the dichotomous subterranean organs, 

 bearing numerous rootlets, to which the name Sfigmaria is applied. 



Anatomicall}', the stem of the Lepidodendreae is in all cases 

 monostelic, with centripetal primary wood, which may extend to the 

 centre, or form a ring enclosing a medulla. While in Lepidodendron, 

 BotJirodendron ^) and some species of Sigillarm the primary wood is 

 continuous, in other Sigillariae {S. Menardi and S. spimdosa) the 



^) Mr. Lorn ax has shown that the stem called Lepidodendron nmndum by 

 Williamson, who described its structure, is in reality a BotJirodendron. 



