THE EVOLUTIONARY HISTORY OF PLANTS 301 



372. Development of the Plant from the Spore in Green 

 Algae, Liverworts, and Mosses. The course which the 

 forms of plant life have followed in their successive ap- 

 pearance on the earth may be traced by the application 

 of the law above named. Such algse as the pond-scums 

 produce spores which give rise directly to plants like the 

 parent. 



In many liverworts the spore by its germination produces 

 a thallus which at length bears antheridia and archegonia. 

 The fertilized archegonium develops into a sporophyte 

 which remains. attached to the thallus, although it is really 

 a new organism. Liverworts, then, show an alternation of 

 generations, one a sexual thallus, the gametophyte, the 

 next a much smaller, non-sexual sporophyte, and so on. 



A moss-spore in germination produces a thread-like pro- 

 tonema which appears very similar to green algae of the 

 pond-scum sort. This at length develops into a plant with 

 stem and leaves, the sexual generation of the moss. The 

 fertilized archegonium matures into a sporophyte which is 

 the alternate, non-sexual generation. This is attached to 

 the moss-plant, or gametophyte, but is an important new 

 organism. In the moss, as in the liverwort, the sexual 

 generation is the larger and the more complex ; the non- 

 sexual generation being smaller and wholly dependent for 

 its food supply on the other generation, to which it is 

 attached. 



373. Development of the Plant from the Spore in Pterido- 

 phytes. In the pteridophytes there is an alternation of 

 generations, but here the proportions are reversed, the 

 prothallium, or sexual generation, or gametophyte, being 

 short-lived and small (sometimes microscopic), and the 



