64 THE MORPHOLOGY OF PTERIDOPHYTES 



Spindle. L. selago (Fig. loP) is similar, except that the 

 hypobasal region does not swell up into a large foot. 



Lycopodium cernuum (Figs. loQ-T) is an example of a very 

 different kind of embryology. As in L. selago, the hypobasal 

 region remains relatively small, but the organizing of a stem 

 apex is considerably delayed. The epibasal portion breaks 

 through the prothalhal tissue and swells out into a tuberous 

 'protocorm', 't'. Roughly spherical at the start, it is provided 

 with rhizoidal hairs and mycorrhizal fungus. On its upper 

 surface a cyhndrical green leaf ('protophyll'), '1', appears and 

 then, as the protocorm slowly grows, further protophylls 

 appear in an irregular manner. This stage may persist for a 

 long time and secondary protocorms \' may be formed as 

 shown in Fig. loT. Finally, however, a stem apex V 

 becomes organized and a normal shoot grows out. This type 

 of development has led, in the past, to much speculation as 

 to its phylogenetic significance, for the protocorm was held 

 by some to represent an atavistic survival of an ancestral 

 condition. However, Wardlaw^^ has offered an alternative 

 explanation, based on the metaboUsm of the prothallus and 

 young sporophyte in the various species of Lycopodium. He 

 suggests that an abnormally high carbon /nitrogen ratio may 

 delay the organization of a stem apex and may lead, also, to 

 a swelhng of the tissues, such being expected where mycor- 

 rhizal nutrition is supplemented by photosynthesis. On this 

 basis, the protocorm might well be regarded as a derivative 

 and retrograde development, rather than as a sign of 

 primitiveness. 



When all facts are considered, it is Lycopodium selago 

 which is usually regarded as the most primitive species, in 

 lacking an organized strobilus, in having a relatively simple 

 vascular structure and in showing variabihty in behaviour in 

 its prothallus, but such conclusions can only be speculative 

 in the absence of clear fossil evidence. While there are fossil 

 remains, known as Lycopodites, they contribute Uttle to 

 these discussions. No petrified specimens have been found 



