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 aeneratioyis. That form ^- ^^^^^^^ ^^^^^g^ ^ 



•^ _ very youug Fern 



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



that bears g^ametes (q^^ cell, anther- '^^^\: ^. root ;p, the 



o V && ' ^ prothallmm ; a, a, 



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



Ferns tlie prothallium is the gameto- meTster." ^''^^' 



phyte. That form (stage or gen- 

 eration) which bears spores is the sioorophyte ; 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 Fei-n " 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. ^ 



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

 (a common species is ascending), with leaves dor siventr ally 

 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 Bryopliytes 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. 



