CHAPTEE XXI 



PTERIDOPHYTES (FERN PLANTS) 



)^09. Summary from Bryophytes. — In introducing the Bryo- 

 phytes a summary from the Thallophytes was given (see § 

 60), indicating certain important things which that group 

 has contributed to the evolution of the plant kingdom. 

 In introducing the Pteridophytes it is well to notice certain 

 important additions made by the Bryophytes. 



(1) Alternation of generations. — The great fact of alter- 

 nating sexual (gametophyte) and sexless (sporophyte) gen- 

 erations is first clearly expressed by the Bryophytes, although 

 its beginnings are to be found among the Thallophytes. 

 Each generation produces one kind of spore, from which is 

 developed the other generation. 



(2) Gametophyte the chlorophyll ge7ieration. — On account 

 of this fact the food is chiefly manufactured by the gameto- 

 phyte, which is therefore the more conspicuous generation. 

 When a moss or a liverwort is spoken of, therefore, the 

 gametophyte is usually referred to. 



(3) Gametophyte and sporophyte 7iot independent. — The 

 sporophyte is mainly dependent upon the gametophyte for 

 its nutrition, and remains attached to it, being commonly 

 called the sporogonium, and its only function is to produce 

 spores. 



(4) Differentiation of thallus i^ito stem and leaves. — 

 This appears incompletely in the leafy Liverworts {Junger- 

 mannia forms) and much more clearly in the erect and 

 radial leafy branch (gametophore) of the Mosses. 



320 



