xv NATURE OF THE ALTERNATION OF GENERATIONS 571 



sedes the sexually formed sporophyte, as in this species, appar- 

 ently, archegonia are never formed. (Sadebeck (8), p. 34.) 

 In other cases, both apogamous and normal sporophytes are 

 known. Lang (3) has found that exposure to strong sunlight 

 will sometimes induce apogamy. Apospory (Bower (6)) may 

 consist of the transformation of sporangia into prothallia, or in 

 some cases the latter may arise from sterile leaf-tissue, even 

 from leaves which bear no sporangia. 



Bower has pointed out that all known cases of apogamy 

 occur among the leptosporangiate Ferns, admittedly the most 

 recent and specialised members of the class. If apogamy is to 

 be looked upon as a reversion to a primitive condition, it is hard 

 to understand why it should be absent in the other more primi- 

 tive Pteridophytes. It must be admitted, of course, that these 

 forms have not received the same amount of study as the higher 

 Ferns, and it is quite possible that apogamy may be shown to 

 occur in some of them. 



Lang (1. c.) has suggested that the origin of the sporophyte, 

 assuming the homologous theory of alternation, may have been 

 something as follows: The primitive gametophyte of 'the 

 Pteridophytes was probably a flat thallus that under stress of 

 circumstances, owing to an insufficient water supply, may have 

 given rise to spores, the spore stage following the sexual stage, 

 but being an integral part of the gametophyte, and not produced 

 from the ovum. In connection with this special spore-produc- 

 ing function, the structure gradually assumed the character of 

 a leafy shoot, and later became replaced by a similar structure 

 which arose from the fertilised egg. 



It is not made clear, however, how the originally apogamous 

 sporophyte came to be transferred to the archegonium, nor why 

 the spores produced from it should so exactly resemble those 

 developed from the sexually produced sporophyte of the Bryo- 

 phytes, which according to the homologous theory of alterna- 

 tion has nothing to do with the sporophyte of the Ferns. 



Although many Bryophytes normally are subjected to all 

 the conditions which should, according to Lang's theory, induce 

 apogamy, no instances are known among them of such 

 apogamous production of spores, or anything resembling in 

 the remotest degree the normal sporophyte. Either the whole 

 gametophyte dries up and revives when water is applied, or 

 else special tubers are developed which survive the dry period. 



