CHAPTER FORTY-THREE 



BRYOPHYTES: LIVERWORTS AND MOSSES 



The phylum Bryophyta includes two diversified kinds of 

 plants commonly known as the liverworts and mosses. Their 

 structures and life histories are somewhat more complicated than 

 those of the algae. All together they comprise some 16,000 

 species, three fourths of which are mosses. 



The largest of the mosses and liverworts never attain a height 

 or length of more than a few inches, and they are of very simple 

 structure in comparison with the flowering plants. In contrast 

 with the algas, which on the whole are water plants, mosses and 

 liverworts for the most part live on land. The passing of plants 

 from a water to a land habitat is one of the notable steps in the 

 evolution of the plant kingdom, and in connection with the study 

 of this group we shall contrast the environments of land and water 

 plants and consider the modifications in structure that accompany 

 the passing of simple plants from the water to a land habitat. 



Living conditions of land and water plants contrasted. In the 

 preceding chapter attention was called to the conditions under 

 which the algas grow. The water environment is most favorable 

 for the growth of simple plants, because of (i) the avoidance of 

 heating and drying effects of intense sunshine, (2) abundant 

 supply of carbon dioxide, oxygen, and mineral salts, (3) more 

 uniform temperature, and (4) longer growing season. 



The environment of the land plant, on the other hand, furnishes 

 through wet cell walls a supply of carbon dioxide and oxygen from 

 the atmosphere, and mineral salts may be secured only from the 

 soil water with which the plants are in contact. If the plant grows 

 in full sunlight, it is subjected to much more intense illumination 

 and heating than are water plants, and it must withstand the 

 drying effects of the air. A study of the amphibious liverworts 

 shows that they have become adjusted only to a medium light 

 and a moderate amount of drying. These plants, therefOxC, 



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