ALTERNATION OF GENERATIONS. 



89 



Pteris possesses two modes of " vegetative" asexual repro- 

 duction, viz., the separation of entire branches from the rhizome, 

 as already mentioned (p. 69), and of " adventitious buds" from 

 the bases of the leaf-stalks (Fig. 33). But besides these, the fern 

 has a quite different method of agarnogenesis which regularly 

 alternates with a process of gamogenesis (alternation of gene- 

 rations). The following brief outline of this important process 

 may help to guide the student through the subsequent detailed 

 descriptions. 



Upon some of the leaves are formed organs called sporangia 

 (Figs. 45, 51, 52), which produce numerous reproductive cells called 

 spores. The spore becomes separated from the parent plant and 

 develops into an independent plant, the prothallium (Fig. 58), 

 which differs entirely in appearance from the fern, and ultimately 

 produces male and female germ-cells. The female cell is fertilized 

 by a male cell, and thereupon develops into an ordinary " fern," 

 which in turn produces spores. The formation and development 

 of the spores is evidently a process of agamogenesis, and the 

 fern proper is therefore neither male nor female i.e., it is 

 asexual. 



The formation and development of the germ-cells, on the 

 contrary, is a process of gamogenesis ; and the prothallium is 

 a distinct sexual plant, being both male and female (hermaphro- 

 dite or bisexual}. In general terms 

 this is expressed by calling the asex- 

 ual fern the spore-bearer, or sporo- 

 phore, and the prothallium the egg- 

 bearer, or oophore. The life-history 

 of the fern, broadly speaking, consists 

 therefore in the regular alternation of 

 the sporophore (asexual generation) 

 with the oophore (sexual generation), 

 that is, it consists of an alternation 

 of generations. An essentially simi- 

 lar alternation of sporophore with 

 oophore occurs in all higher plants, 

 though in most cases it is so dis- FlG . 51 . (After suminski.)-s P oran- 

 guised as to escape ordinary observa- gium of pteris serr "' at(t - P, P edi - 



. eel; c, capsule; a, annulus; s, spore 



tlOll. 



The Sporangia and Spores. The sporangia of ferns (Fig. 51) 



