86 ORIGIN OF CORMOPHYTA [CH. 



In this group a peri caul ome was not always evolved. Bothro- 

 dendron, one of the earliest Devonian Lycopods (Fig. 37, p. (54), 

 does not possess this feature, which however occurs in Archaeo- 

 sigillaria (Fig. 38, p. 66) and Leptophloeum (Fig. 40, p. 67) of 

 the same period. Most of the later Palaeozoic Lycopods, such as 

 Lepidodendron, as well as many Sigillarias, possess a pericaulome, 

 but we see how in the later types of the latter genus (section 

 Clathraria) the pericaulome was abandoned. 



In the Lycopsida the leaves, arising from emergences, are 

 always unbranched and independent. The sporangia were 

 originally borne at the base of the upper (adaxial) surface of 

 the leaf, as we see them in Thursophyton (Fig. 11 5 p. 30) and as 

 indeed has been the rule among Lycopods ever since. Among* 

 Lycopods the Selago condition is thus primitive as regards the 

 place of origin of the sporangia. 



We are able to throw no light on the original morphology and 

 function of the ligule. Such an organ is universally present in 

 Palaeozoic Lycopods, but.it is there always in much the same 

 condition as in the living Selaginella. 



The pseudodichotomous branching of many Palaeozoic 

 Lycopods is another indication of algal ancestry. 



The Psilotales. 



There remains for consideration one further group, the 

 Psilotales, which may be dealt with here, since by some 

 authorities these plants are regarded as more nearly related to 

 the Lycopsida than to any other group. There are also further 

 grounds for a discussion of this race arising from the fact that 

 Kidston and Lang 1 have recently instituted a comparison 

 between Psilophyton and members of the Psilotales, and have 

 drawn the conclusion that some phylogenetic connexion may 

 exist between them. 



On the much debated question of the general affinity of the 

 Psilotales, we agree with Mrs Thoday 2 and others that they are 

 best regarded as an entirely distinct group from either the 

 Lycopods or the Sphenophylls. We should not include them in 

 either the Lycopsida or Sphenopsida. At the same time we do 

 1 Kidston and Lang (1917), p. 776. 2 Sykes (1908). 



