CBTPTOGAMS 207 



(zoospores) germinating without fusion, and gameten 

 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 ^^^^ '^'^^^'°" through a 



very young Fern 



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



that bears gametes fegg cell, anther- '"^^^ r root; j,,tiie 



. ^ . ' , prothalhum ; a, <i, 



OZoid) is called i\\Q gametoplli/te ; in remains of arche- 



Ferns the prothallium is the gameto- s"'"""' " ""*"- 



pliyte. That form (stage or gen- 

 eration) which bears spores is the sjyoropliyte ; in Ferns 

 the leafy plant is the sporophyte. 



484. The Fern prothallium corresponds to the thallns 

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

 ftn' these structures all bear archegonia and antlieridia. 

 The final result of fertilization in Liverworts and Mosses 

 is a sporogonium, i.e. a spore-bearing l)od3^ The final 

 residt of fertilization in Ferns is also a spore-bearing 

 body — the Fern " plant." Sporogonium and Vevn " 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. 35-S) is usually a creeping plant 

 (a common species is ascendijig), with leaves dor.nventraU// 

 arranged ; i.e. so placed that the shoot sIkiws 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 Brj'ophytes and Pterido- 

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

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

 in all plants above them. 



