vi] LYCOPSIDA 85 



until we know something of the structure of the fern-like plants 

 of the Upper Devonian, it is impossible to feel confident as to 

 their precise affinity. As things stand at present we have no 

 certain evidence of the Pteridosperms before the Lower Carboni- 

 ferous period. They certainly appear to have occurred then and 

 in the case of the well-known Adiantites fertilis , (White) (Fig. 43, 

 p. 80), it would appear that the seeds took their origin in much 

 the same circumstances as the sporangia of the Primofilices, i.e. 

 by the metamorphosis of part of a fertile branch, as in the Algae. 

 No doubt the chief modifications in this case were the reduction 

 of the number of megaspores in the sporangium to one and the 

 development of an integument to the same. In Adiantites 

 f ertilis these features appear to have been arrived at very simply. 

 Cordaitales. There is one class of Pteropsida on the origin of 

 which the Devonian floras as yet have thrown no light, namely 

 the Cordaitales. A few obscure and fragmentary leaves, some- 

 thing like those of Cordaites, occur in Upper Devonian beds, but 

 in the earlier floras we meet with nothing, so far as we can see, 

 which appears to belong to this group. There is, it is true, the 

 Zosterophyllum of the Lower Devonian (Fig. 22, p. 41), with long 

 parallel-sided leaves or axes, but the fossil is so obscure at present 

 that no conclusion of any value can be drawn from it. Even 

 from Lower Carboniferous rocks evidence of the Cordaitales is 

 at present extremely scanty, and until we learn more about the 

 group at this period, we are not likely to trace it back to 

 Devonian times. 



The Evolution of the Lycopsida. 



The evolution of the Lycopsida along the lines first perceived 

 by Lignier, is particularly well seen among members of the 

 Psilophyton flora. From such a type asPsilophyton(Figs.3,p.l7, 

 and 5, p. 19) with its chaffy scattered scales, it is but a short step 

 to Thursophyton (Fig. 10, p. 28), which is to all intents and pur- 

 poses a Lycopod. Even if this type was not vascular, either in 

 axis or leaf, it is quite easy now, in the light of Psilophyton, to 

 understand how similar vascular types may have arisen. From 

 Thursophyton it is a short step to Protolepidodendron. 



