THE EVOLUTIONARY HISTORY OE 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 algae 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 

 prothaUium, or sexual generation, or gametophyte, being 

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



