86 ELEMENTARY STUDIES IN BOTANY 



tion) that results in spores ; but it works reasonably well for 

 plants living in moist situations. 



The Mosses use the upright gametophore for the display 

 of green tissue, and it becomes leafy ; but the larger and more 

 exposed the gametophores of Mosses become, the more un- 

 likely it is that fertilization can occur. It is evident that 

 still larger and more leafy plants would interfere with the 

 swimming of sperms still more. 



The three things that enter into this problem are food 

 manufacture (which means display of green tissue to light 

 and air), fertilization (which means water for swimming), 

 and spore-production (which means exposure for air-disper- 

 sal). In the Bryophytes, food manufacture and fertilization 

 belong to the gametophyte, and the condition that favors 

 one hinders the other. In other words, they are contradic- 

 tory in their demands. On the other hand, food manufac- 

 ture and spore-dispersal make the same demands for exposure, 

 and therefore they can be coupled together to advantage. 

 The further progress of plants, therefore, demanded that the 

 spore-producing generation (sporophyte) should also become 

 the food-manufacturing generation; and that the gameto- 

 phyte, with its peculiar need for free water, should be re- 

 stricted to fertilization. In the higher plants (Fern-plants 

 and Seed-plants), therefore, the sporophyte is the conspicuous, 

 leafy, independent generation, and the gametophyte is so 

 very inconspicuous that it is only seen by those who know 

 where and how to look. 



52. Summary. The contribution of the Bryophytes 

 to the progress of the plant kingdom is notable. Of first 

 importance is the establishment of the land habit by green 

 plants (Liverworts), which means exposure to air rather than 

 to water7~7This made possible the further development of 

 plants on the land surface. In consequence of this change 

 in conditions of living, the plant bodies are much more com- 

 pact, and develop protective structures against excessive 



