Lecture XXL 



177 



mother-cells is a single antheridium formed on this diminutive 

 prothallus. The whole gametophyte is thus practically reduced to 

 a single antheridium. Having no vegetative organs it is entirely 

 dependent on store materials laid up in the spore for its support. 



Although not quite so diminutive as the male, the female 

 gametophyte of Selaginella is very inconspicuous compared to that 

 of Aspidium. It often forms a few rhizoids from the exposed part 

 of the prothallus but neither these, nor the small amount of green 

 cells also formed in this region of the prothallus, are very effective 

 nutritive organs, and consequently the female gametophyte is 



FIG. 49. Selaginella, germinating megaspore, 

 longitudinal section, x about roo. a, repro- 

 ductive ; b, vegetative portion of female 

 gametophyte ; c, spore-coat ; d, embryo ; 

 e, ovum in archegonium. (After Lyon.) 



FIG. 50. Selaginella 

 martensii, young 

 sporophyte. a, 

 megaspore ; b, first 

 leaves (cotyledons). 



almost as completely dependent on the materials stored in the spore 

 by its parent sporophyte as is the male gametophyte. 



The differentiation of the gametophyte generation into male 

 and female individuals is a complication not found in Aspidium. 

 However, it should be mentioned that often small prothalli of 

 Aspidium are found which develop antheridia only. Probably 

 insufficient nutrition in these cases has precluded the formation of 

 the cushion on the prothallus and consequently there has not been 

 sufficient thickness to allow of the development of the archegonia. 

 Thus the sexual differentiation of Aspidium is probably due to 

 external conditions. But we have in Marchantia an example of 

 sexual differentiation which is as complete as that of Selaginella. 

 In Marchantia, however, the spores which produce the male and 



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