THE VASCULAR CRYPTOGAMS 77 



the two generations is not absolute, but that the one may 

 sometimes pass directly into the other. 



In apogamy, which has been sometimes observed in 

 our type, the Male Fern, and in many other species, the 

 vegetative tissue of the prothallus grows out into the 

 various organs (leaf, stem, and root) of the new Fern- 

 plant, the origin of which cannot be traced to any single 

 cell, or even necessarily to any definite initial group. 

 At the same time vascular bundles appear in the tissue 

 of the prothallus. Archegonia may be absent altogether, 

 or, if present, have nothing to do with the production of 

 the new plant, which arises altogether as a vegetative 

 outgrowth on the prothallus. Every stage of transition 

 between prothallus and plant may be found. In a Fern, 

 nearly related to our type, and in at least one other 

 species, sporangia are sometimes produced on the prothallus 

 itself, among the archegonia and antheridia. Eecent in- 

 vestigations have shown that a fusion of nuclei a kind 

 of false-fertilisation takes place in the prothallus-cells 

 concerned in the apogamous development. 



In the converse case, that of apospory, which has 

 been observed in several native Ferns, especially garden 

 varieties, either an abortive sporangium grows out into 

 a prothallus, without first forming spores ; or else the 

 sporangia are altogether undeveloped, and the prothallus 

 arises simply as a vegetative growth from the tissues of 

 the leaf itself. In both these cases the sexual generation is 

 formed from the asexual directly, without the intervention 

 of spores. Frequently apospory and apogamy occur together 

 in the same plant, in which case there is no nuclear fusion. 



We thus see that we must regard the regular alterna- 

 tion of sexual and asexual reproduction as the normal 

 course of life-history in Ferns and their allies, but not 

 as a cast-iron scheme which can never be departed from. 



