466 General Botany 



grow in moist, shaded situations. During wet periods many in- 

 dividuals start in other places, only to be killed off later by the 

 light and its secondary temperature and drought effects. 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 plants to the aerial environment. In contrast 

 to the algae the land liverworts show several changes in structure 

 that are of advantage to plants in an aerial habitat. The more 

 important of them are : 



(i) Firmer, and in some cases thicker ^ cell walls and water - 

 storage tissue. The firmer cell walls are less permeable to water 

 and reduce the rate of water loss. Furthermore, the plants grow 

 flat on the soil in contact with the water supply, and some of the 

 forms develop layers of water-storage cells and mucilage pockets 

 on the side in contact with the soil . This enables them to with- 

 stand short dry periods better than do those forms that have 

 only the usual aquatic type of cell wall. 



(2) The development of rhizoids. Land plants are favored by 

 being anchored, and by having structures that will bring them 

 into contact with the soil-water supply. In the liverworts, 

 rhizoids anchor the plant and to some extent absorb water and 

 mineral salts from the soil. Rhizoids are elongated cells that 

 develop on the lower side of the plant body and penetrate the soil. 

 They resemble root hairs in form. 



(3) The development of an epidermis. The land liverworts 

 are covered by an epidermis which decreases the rate of water 

 loss. The liverworts with thicker bodies have pores in the 

 epidermis which afford a ready access to the carbon dioxide and 

 oxygen necessary for photosynthesis and respiration. In the 

 more complex liverworts the epidermis is raised like a transparent 

 roof on ridges of supporting tissues, leaving beneath it a series of 

 small air chambers in which the chlorophyll-bearing cells stand 



