324 GENERAL MORPHOLOGY OF PLANTS 



amounts of water, and thus assist the leaves and humus in hold- 

 ing back the water after rains, so that it does not run off so rapidly, 

 thus lessening the danger of floods. On rocks they behave 

 much as the lichens do in holding decaying vegetation, and 

 adding to the humus from their own remains. They are thus 

 important as early soil builders. 



492. Alternation of generations. The alternation of gen- 

 erations in the mosses is similar to that in the liverworts. The 

 protonema, and the leafy-stemmed moss plant with the sexual 

 organs, make the first generation, while the capsule (with its 

 stalk) is the second generation. In a continuance of the com- 

 plete life cycle there is an alternation of these two generations. 

 The first generation is independent of the second when once 

 started, and can live from year to year, often multiplying Vege- 

 tatively and spreading by branching, by new protonema, and by 

 bulbils. The capsule, however, is dependent on the first gener- 

 ation, since it has no roots or rootlets by which it becomes free 

 and established as an independent plant. This is an important 

 biological principle in the life history and development of plants. 

 Very few of the algae show it. The fertilized egg usually at once 

 develops the first generation again. In Coleochaete there is a second 

 generation. In some of the red algae (as in Polysiphonia, Rhab- 

 donia, etc.) the tetraspore plant represents a second generation. It 

 is, however, independent, and of the same form as the sexual plant 

 and grows under the same conditions. In plants showing an alter- 

 nation of generations between a sexual stage and an asexual stage 

 developed from the fertilized egg, the first generation is often 

 spoken of as the gamete plant (gametophyte), because it bears the 

 gametes or sexual organs. The second generation is likewise called 

 the spore plant (sporophyte), because it bears the spores. The 

 capsule (and stalk when present) of the liver worts and mosses 

 is, therefore, the spore plant stage (sporophyte) of these plants, 

 while the thallus with the sexual organs, the leafy-stemmed plant 

 of the foliose liverworts, the protonema and leafy-stemmed moss 

 plant, is the gamete plant stage (gametophyte). Each one of 

 these stages begins and ends with a single cell. The gamete 



