62 THE MORPHOLOGY OF PTERIDOPHYTES 



deeply buried. Surface living prothalli are green and photo- 

 synthetic, but subterranean ones are, of necessity, colourless 

 and are dependent on a mycorrhizal association for their 

 successful development. Indeed, a mycorrhizal association 

 appears to occur in all species growing under natural con- 

 ditions, whatever their habit. As a generaUzation, it may be 

 said that those species inhabiting damp tropical regions 

 germinate rapidly and have green prothalli, whereas those of 

 cooler regions tend to germinate slowly and produce sub- 

 terranean prothalli. Lycopodium selago is interesting in this 

 respect, for it shows variabihty. Fig. loE illustrates a surface 

 living prothallus with photosynthetic upper regions, in 

 addition to the fungal hyphae in the lower parts (and 

 rhizoids). Fig. loF, on the other hand, is of a subterranean 

 prothallus, with fungal hyphae in the lower regions but 

 covered all over with rhizoids. Archegonia and antheridia are 

 restricted to the upper parts in both cases. L. cernuum pro- 

 vides an example of a surface-living prothallus (Fig. loG). 

 It is roughly cyUndrical and the upper regions bear numer- 

 ous green photosynthetic lobes, among which are borne the 

 gametangia. In L. clavatum (Fig. loH) and L. annotinum the 

 prothallus is colourless and subterranean; it is an inverted 

 cone with an irregular fluted margin, growing by means of a 

 marginal meristem which remains active for many years, and 

 the gametangia are developed over the central part of the 

 upper surface. Epiphytic species, e.g. L. phlegmaria, also 

 have colourless prothalli, but they are very slender, they 

 branch and they exhibit pronounced apical growth. 



Archegonia and antheridia each arise from a single super- 

 ficial cell in which a periclinal division occurs. The subse- 

 quent cell divisions in the antheridial initials are similar to 

 those described for Tmesipteris (Fig. 7), but the mature 

 antheridium differs in being sunken into the tissues of the 

 prothallus. The archegonium diff'ers from that of Tmesipteris 

 in having several neck canal cells, which vary in number 

 according to whether the prothallus is subterranean or 



