Liverworts and Mosses 



275 



drying effects of the air. In aquatic plants the cells are never 

 without an adequate supply of water, while in land plants 



Fig. 162. Cross section of Marchantia thallus, showing rhizoid (bebw), water- 

 storage tissue^ the air chambers containing the principal photosynthetic cells, and 

 the epidermis which forms a transparent roof over the air chambers. Starch 

 grains occur in many of the cells of the water-storage tissue. 



transpiration may reduce the water content of the cells to 

 such an extent that they may be injured or even die. 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, therefore, grow in shaded 

 and moist situations. During wet periods many individuals 

 start in intensely illuminated places, only to be killed off 

 by the light and its secondary temperature and drought ef- 

 fects. The shaded situation where the water is near the 

 surface of the soil is evidently the habitat where these plants 

 suffer the least, and this explains why liverworts persist in 

 moist situations and not in the open. 



Responses of the plant to the aerial environment. The 

 land liverworts show several changes in structure that are 

 of advantage to the plants in an aerial life. The more im- 

 portant of them are : 



