276 Science of Plant Life 



(1) 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 

 on the side in contact with the soil (Fig. 162). This enables 

 them to withstand short dry periods better than do those 

 forms that have only the ordinary green cells. The develop- 

 ment of firmer cell walls and water-storage tissue is the first ad- 

 justment to land conditions. 



(2) The development of rhizoids. Land plants are liable 

 to be washed away by rain and surface water, and on this 

 account they need some anchorage ; also, it is necessary for 

 them to have 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. The development of rhizoids, 

 therefore, represents a second important adjustment of plants to 

 the land environment. 



(3) The development of an epidermis. The land liver- 

 worts are covered by an epidermis which helps to protect 

 them against water losses. Since a ready access to carbon 

 dioxid and oxygen is necessary for photosynthesis and res- 

 piration, the liverworts with thicker bodies have openings or 

 pores in the epidermis. Through these pores the gas ex- 

 changes between the air and the cells within the body can 

 take place. 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 



