258 ELEMENTS OF BOTANY 
life have followed in their successive appearances on the 
earth may be traced by the application of the principle 
stated in Sect. 832. 
Such algee as the pond-scums produce spores which give 
rise directly to plants like the parent. 
A moss-spore in germination produces a thread-like pro- 
tonema which appears very similar to green alge of the 
pond-scuin sort. This at length develops into a plant with 
stem and leaves, — the sexual generation of the moss. The 
fertilized egg of this 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 the sexual generation is 
the larger and more complex of the 
two, the non-sexual generation 
being smaller and wholly depend- 
ent for its food supply on the 
other generation to which it is 
attached. 
335. Development of the Plant 
from the Spore in Pteridophytes. — 
In the pteridophytes there is an 
alternation of generations, but here 
the proportions are reversed, the 
prothallium, or sexual generation, 
Fic. 176. A Water-Fern oy gametophyte, being short-lived 
(Salvinia). ‘ a oH 
and small (sometimes microscopic), 
and the non-sexual generation, the sporophyte, often being 
of large size. The ferns (non-sexual generation), for 
instance, are perennial plants, some of them tree-like. 
Some pteridophytes, as the Salvinia, a small floating 
