32 THE MORPHOLOGY OF PTERIDOPHYTES 



pointed out, some well-preserved tubers showed no trace 

 whatever of fungus. This fact suggests that, instead of being 

 mycorrhizal, the fungus was a saprophyte which invaded the 

 tissues of the tuber after death. 



Another feature of interest, peculiar to Horneophyton, was 

 the presence of a sterile columella in the sporangium (Fig. 

 4J), a feature reminiscent of the mosses. One sporangium is 

 illustrated (Fig. 4K) which was bifid and it is interesting 

 that the columella was also bifid. This leads one to suppose 

 that the stem apex could be transformed into a sporangium 

 at any stage, even during the process of dichotomizing, and 

 rules out any idea of the sporangium being borne by a special 

 organ to which the name 'sporangiophore' might be given. 



The generic names Yarravia and Cooksonia are given to 

 certain reproductive bodies detached from the plants which 

 bore them ; indeed, we have no idea at all what such plants 

 might have looked like. Yarravia (Fig. 4C) has been interpre- 

 ted as a slender unbranched axis, terminating in a radially 

 symmetrical group of five or six sporangia, partly fused 

 into a synangium, about i cm long. Although Lang and 

 Cookson,^^ who first described this genus, were unable to 

 demonstrate the presence of spores within the sporangia this 

 interpretation is widely accepted and has been used as the 

 starting point for phylogenetic speculations as to the nature 

 of the pollen-bearing organs of fossil seed-plants, and even 

 of their seeds. Cooksonia (Fig. 4D) was much more Hke the 

 other members of the Rhyniaceae, in that the sporangia 

 were borne singly at the tips of tiny forking branches. Each 

 sporangium was broader than it was long (one species 

 being 2 mm x i mm) and contained large numbers of spores 

 in tetrads. 



The chief point of difference between the Zosterophyl- 

 laceae and the Rhyniaceae, described above, concerns the 

 manner in which the sporangia were borne, for instead of 

 terminating the main axes, they were in short terminal 

 spikes, each sporangium having a short stalk. The best 



