60 ELEMENTARY BIOLOGY. [VIT. 



ultimately form a plug-like, cellular, mass, which imbeds itself 

 firmly in the substance of the prothallus. Of the remaining 

 two cells, which also undergo subdivision, one gives rise to the 

 rhizome of the young fern, while the other becomes its first 

 rootlet. It appears probable that the plug-like mass absorbs 

 nutritive matter from the prothallus, and supplies the rhizome 

 of the young fern, until it is able to provide for itself. As 

 the rhizome grows, and developes its fronds, it rapidly attains 

 a size vastly superior to that of the prothallus, which at 

 length ceases to have any functional importance, and dis- 

 appears. 



Thus Pteris presents a remarkable case of the alternation 

 of generations. The large and complicated organism com- 

 monly known as the " Fern " is the product of the impreg- 

 nation of the embryo cell by the antherozooid. This " Fern," 

 when it attains its adult condition, developes sporangia ; and 

 the inner cells of these sporangia give rise, by a perfectly 

 asexual fissive process, to the spores. The spores when set 

 free germinate; the product of that germination is the incon- 

 spicuous and simply cellular prothallus; an independent 

 organism, which nourishes itself and grows, and on which, 

 eventually, the essential organs of the sexual process the 

 archegonia and antheridia are developed. 



Each impregnated embryo cell produces only a single 

 " fern," but each " fern " may give rise to innumerable pro- 

 thallia, seeing that every one of the numerous spores de- 

 veloped in the immense multitude of sporangia to which the 

 frond gives rise, may germinate. 



