52 THE MORPHOLOGY OF PTERIDOPHYTES 



out through the cortex into the leaves. The leaves were 

 about I mm broad and up to 4 cm long and, in fertile shoots, 

 they were associated with reniform sporangia arranged in 

 zones. The preservation of the specimens is not good enough 

 to show whether the sporangia were borne on the leaves or 

 merely among them, but that they were indeed sporangia is 

 estabhshed by the extraction of cutinized spores from them. 

 Not much is known of the growth habit of the plant, but 

 there are suggestions that the aerial branches arose from a 

 creeping rhizome. Drepanophycus (=Arthrostigma) and 

 Protolepidodendron both occurred in Lower and Middle 

 Devonian times: the former in Germany, Canada and 

 Norway; the latter in Scotland and Germany. Of the two, 

 Drepanophycus (Fig. 8B) was the more robust. Its aerial 

 axes were up to 5 cm thick and forked occasionally in a 

 dichotomous manner. It is beheved that they arose from 

 horizontal branching rhizomes. The aerial axes were covered 

 with spine-like outgrowths (up to 2 cm long) in a manner 

 reminiscent of Psilophyton, but with the difference that these 

 outgrowths had a vascular strand and could therefore 

 properly be called leaves. Some of them bore a single 

 sporangium either on the adaxial surface (Fig. 8C) or in 

 their axils, but these 'sporophylls' were scattered at random 

 over the axes instead of being gathered together into a 

 fertile zone. 



Protolepidodendron (Fig. 8D) had dichotomous creeping 

 axes from which arose aerial axes up to 30 cm high and less 

 than I cm in diameter. All parts of the plant were clothed 

 (sometimes densely) with leaves having cushion-hke bases 

 and, in most species, bifurcated apices. Stems from which 

 the leaves had fallen showed a characteristic pattern of leaf- 

 bases (Fig. BE) arranged in a spiral manner. All the leaves 

 were provided with a single vascular strand and some of 

 them bore oval sporangia on their adaxial surfaces (Fig. 8F) 

 but, as in Drepanophycus, such sporophylls were not aggre- 

 gated into special fertile regions. Details of the stem ana- 



