76 



PLANT LIFE. 



some groups of Cryptogams, as mosses and ferns, being only 

 vaguely indicated in the lower forms, while in the higher 

 ones it is indistinct through arrest ; whereas in the majority 

 of Phanerogams alternation of generations is obsolete. 



By alternation of generations is meant the alternation of 

 two diversely formed structures producing respectively 

 sexual and asexual reproductive bodies, the two conditions 

 constituting an individual. In a fern, for example, the spore 

 or asexual organ of reproduction, on germination gives 

 origin to a small plate of cells lying flat on the ground and 

 called a prothallus ; certain of the cells of this prothallus 

 become differentiated into the sexual organs, antheridia 

 and archegonia ; the whole structure constitutes the first or 

 sexual generation or oophore. The fertilized oosphere — the 



oospore — produced by the 

 sexual generation grows di- 

 rectly into the fern plant as 

 popularly understood, which 

 eventually produces reproduc- 

 tive bodies called spores, not 

 the direct result of fertilization, 

 and constitutes the second or 

 asexual generation, the sporo- 

 phore. The spores under 

 favourable conditions germin- 

 ate and give origin in turn to 

 the sexual generation or 

 oophore. 



(3) In Cryptogams the 

 asexual reproductive bodies 

 called spores are almost universal, whereas in Phanerogams 

 spores are entirely absent, although in the last-named sub- 



Fig. 13. — Prothallus of a fern, illus- 

 trating the first or sexual genera- 

 tion ; under surface showing, ar, 

 archegonia, an, antheridia ; h, 

 root-hairs ( x 10). (From Prantl.) 



