HISTORY OF THE PLANT 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 py¢. 916. A Water. 
known as megaspores, and the small Fern (Salvinia). 
ones as microspores (Fig. 217). Both , floating Jeaves; r, sub- 
kinds produce merged leaves, acting 
rea Pp i 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 
Fic. 217. Two Indusia of (non-sexual generation) of consid- 
Salvinia. erable complexity of structure is 
mi, microspores; ma, mega- formed. 
ee 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 
