142 THE NATURE AND WORK OF PLANTS 



bearing a minute leaf which, gradually enlarges 

 until it resembles that of the plant from which the 

 spores were taken. A root may also be found. This 

 young plant is attached to the leaflike body and 

 apparently sprang from it. The young plant with a 

 stem will enlarge, and if placed in the soil it would 

 develop spores like those which were placed on the 

 brick, and the life history of the species would be 

 complete. 



197. Alternation of generations. — In the complete 

 life of the fern it is found to develop two indi- 

 viduals entirely difEerent in form, and each gives 

 rise to the other. This is known as the alternation 

 of generations. One generation is constructed with 

 a root, stem, and leaves, and it develops single 

 spores which are capable of giving rise to the other 

 generation. This large plant which is known to the 

 ordinary observer is the sporophyte. The other gen- 

 eration is a thallus, or prothallus, and the body con- 

 sists of a thin sheet of cells bearing green color, and 

 furnished with rhizoids for absorption and fixation. 

 This generation develops two kinds of reproductive 

 cells in little flasks on the lower side of the pro- 

 thallus. The flasks containing the male gametes 



