HISTORY OF THE PLAXT KINGDOM 



301 



generation being smaller and wholly dependent for its food 

 supply on the other generation to which it is attached. 



382. Development of the Plant from 

 the Spore in Pteridophytes. — In the f 



pteridophytes there is an alternation of 

 generations, but here the proportions 

 are reversed, the prothallium, or sexual 

 generation, or gametophyte, being usu- 

 ally short-lived and small (sometimes 

 microscopic), and the non-sexual gen- 

 eration, the sporophyte, often being 

 of large size. The ferns (non-sexual 

 generation), for instance, are peren- 

 nial plants, some of them tree-like. 

 Some pteridophytes, as the Salvinia, a 

 small, floating aquatic plant sometimes 

 known as a water-fern (Fig. 216), pro- 

 duce two kinds of spores, the large ones 

 known as megaspores, and the small 

 ones as microspores (Fig. 217). Both /, floating leaves ; r, sub- 

 kinds produce '"^'^sf '''^^'^^' f 1"^ 



•■^ _ as roots ; s, spore-fruits. 



microscopic 



prothallia, those of the former bear- 

 ing only archegonia, those of the 

 latter only antheridia. From the 

 prothallia of the megaspores a plant 

 Fig. 217. Two Indusia of (non-sexual generation) of consid- 

 erable complexity of structure is 

 formed. 



383. Parts of the Flower which 

 correspond to Spores. — In seed-plants the spore-formation 

 of spore-plants is represented, though in a way not at all 



Fig. 216. A AVater- 

 Fern (Salvinia). 



Salvinia. 



mi, microspores; ma, mega- 

 spores. 



