Liverworts and Mosses 277 



chlorophyll-bearing cells stand up in short chains (Fig. 162). 

 Each chamber is connected with the air by a pore in the 

 center of its roof. In these rather simple plants, therefore, 

 protection against water loss is accomplished in much the 

 same way as in seed plants, but the epidermal pores in them 

 are chimney-like openings and are incapable of closing as do 

 the stomata of the higher plants. The development of an 

 epidermis and of pores is a third adjustment of plants to the 

 land environment. 



(4) The ability to withstand drying. When the cells of 

 water plants are dried, the protoplasm dies at once ; but a 

 few of the liverworts, like many mosses and like Protococcus 

 and a few other algae, do not die when water is lost from the 

 cells. Just what quahty the protoplasm possesses that en- 

 ables it to withstand drying, it is impossible to say ; but some 

 of the liverworts that grow on trees and rocks possess this 

 quality, and certain mosses have to a remarkable degree the 

 ability to withstand drying. A fourth adjustment of plants 

 to the land, environment is the development of the ability to with- 

 stand drying. 



The mosses. The mosses form a very large group found 

 in all parts of the world. They usually have upright stems, 

 though many live close to the substratum and have only 

 horizontal or inclined stems. They possess very simple 

 leaves, frequently only one cell layer in thickness, sometimes 

 thicker toward the midrib. 



The mosses, like the liverworts, are most abundant in moist, 

 partly shaded habitats. A few, however, grow on rocks and 

 trees where they are exposed to periodic drought. These 

 latter forms, like Protococcus, have the power to with- 

 stand complete drying. When dry, they are in a dormant 



