THE EVOLUTIONARY HISTORY OF PLANTS 301 
372. Development of the Plant from the Spore in Green 
Alge, 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 alge 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. 
anew 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 alge 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 
