CHAP. IV.] REPRODUCTION OF PLANTS. 123 



sexual stage in the life-cycle of the fern. All the cells 

 of the prothallus contain chlorophyll, and after existing 

 for some time doing vegetative work the reproductive 

 stage is entered upon ; certain of the cells undergo a 

 series of changes, resulting in the formation of oospheres, 

 other cells giving origin to antherozoids. The oosphere 

 after fertilization is called an oospore, and lies perfectly 

 free in a special portion of the prothallus, but is not 

 organically attached to it in any way, secreting a new 

 cell-wall of its own after fertilization, and forms the 

 starting-point of the second (^asexual) stage or genera- 

 tion, which closes the life-cycle of the fern. The oospore 

 by cell-division develops directly into what is popularly 

 considered as the whole of the fern plant ; that is, the 

 portion bearing green fronds, these in turn producing 

 the spores on the. under surface, these spores are not 

 the result of sexual fertilization as in the case of the 

 oospore produced by the prothallus, but are purely 

 asexual or vegetative in origin, and on germination prO' 

 duce the sexual generation. Alternation of generations 

 first showed itself distinctly when plants succeeded in 

 establishing themselves on dry land, and in the mosses 

 and ferns the two generations are very sharply defined, 

 but in the last-named group this idea of having two 

 distinct phases in the life-cycle of the individual, con- 

 nected at one stage by a single cell — the oospore — 

 attained its maximum, and in the groups that followed, 

 as the club-mosses, selaginellas, quilworts, etc., the 

 sexual generation as a distinct and vegetative structure 

 became gradually suppressed, and in phanerogams has 

 altogether disappeared, the only remaining portion being 

 the indispensable sexual organs — pollen grains and 



