CRYPTOGAMS 207 



(zoospores) germinating without fusion, and gametes 



destined to conjugate. In Ferns it is plainly seen that 



the two sorts of reproductive cells 



(spores and gametes) are not borne 



at the same period, but at very dif- 



ferent stages of the life cycle. The 



two stages regularly alternate. This 



phenomenon is known as the Alter- 



nation of Generations. That form ^ Section trough a 



very young Fern 



(stage or generation) of the plant plant: s, stem; /, 



that bears gametes (egg cell, anther- leaf ; r, root ; p, the 



' prothallmm; a, a, 



ozoid) is called the gametopliyte ; in remains of arche- 



Ferns the prothallmm is the gameto- 



phyte. That form (stage or gen- 



eration) which bears spores is the sporophyte; in Ferns 



the leafy plant is the sporophyte. 



484. The Fern prothallium corresponds to the thallus 

 of a Liverwort and the protonema and shoot of a Moss ; 

 for these structures all bear archegonia and antheridia. 

 The final result of fertilization in Liverworts and Mosses 

 is a sporogonium, i.e. a spore-bearing body. The final 

 result of fertilization in Ferns is also a spore-bearing 

 body the Fern " plant." Sporogonium and Fern " plant " 

 have the same origin ; they are therefore of the same 

 nature : both are sporophytes. The sporophyte of Liver- 

 worts and Mosses (the sporogonium) has no root, but is, 

 so to speak, parasitic on the parent plant, or gametophyte. 

 The sporophyte of Ferns has a root, as well as leaves, and 

 after the very first is self-supporting. 1 



485. Selaginella (Fig. 353) is usually a creeping plant 

 (a common species is ascending), with leaves dorsiventrally 

 arranged ; i.e. so placed that the shoot shows an upper and 

 an under side. Special branches are often given off below, 

 from which roots are sent out. The sporangia spring from 



1 Alternation of generations is not confined to Bryophytes and Pterido- 

 phytes, though in the Pteridophytes it is easier to see than elsewhere in 

 the vegetable kingdom. It is foreshadowed in the Thallophytes and occurs 

 in all plants above them. 



