354 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 rap- 

 idly, 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. 



507. 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 genera- 

 tion, 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, 

 Rhabdonia, etc.) the tetraspore plant (paragraph 402) represents 

 a second generation. It is, however, independent, of the same 

 form as the sexual plant and grows under the same conditions. 

 There are reasons for believing that the tetraspore plant is of 

 the same nature as the first generation and that it is part of the 

 gametophyte, while the fertilized egg to the carpospores is the 

 sporophyte. In plants showing an alternation 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 (sporo- 

 phyte), because it bears the spores. The capsule (and stalk 

 when present) of the liverworts and mosses is, therefore, the spore 

 plant stage (sporophyte) of these plants, while the thallus with the 



