Biology of Higher Plants — Anatomy and Physiology 227 



round each stoma. The opening of each stoma is regulated by the guard 

 cell and this is influenced by the amount of humidity in the surrounding 

 atmosphere. Transpiration results in a constant current of water through 

 the plant. This circulation transports foods and wastes from one part of 

 the plant to another. 



VII. CONDUCTION OF LIQUIDS 



The constant absorption of water by roots and the loss of water by 

 transpiration in the leaves result in a constant flow of water and its 

 absorbed materials through the plant. Water absorbed by a root hair 

 is passed into the cortex of the root, then to the endodermis, to the peri- 

 cycle, and finally into the xylem of the root. From the root xylem it 

 passes into the xylem of the stem which is connected with the veins of 

 the leaves. From the veins the water passes to the mesophyll cells of 

 the leaf. From these cells it escapes into the intercellular air spaces and 

 hence outside through the stomata. 



The three factors which explain the ascent of water and dissolved ma- 

 terials in a plant are (1) root pressure, (2) tensile strength of water, 

 (3) transpiration. The differences in osmotic pressures, in the proto- 

 plasm of the root hair cells and the soil water, result in root pressure, 

 which causes an absorption of water by the roots and a tendency to force 

 it upward. A thin, continuous column of water is found in the small 

 xylem vessels. Under such conditions, the tensile strength of water is 

 great enough to resist successfully forces tending to pull the column apart. 

 Hence, the column of water when once started will not be easily broken. 

 A loss of water by transpiration in the leaf results in its replacement from 

 deeper tissues. This is accomplished by the upward pull on the water 

 column in the xylem tissues. The xylem seems to conduct materials 

 upward, while the phloem transports food materials from the point of 

 their manufacture to their places of use and storage. This is often down- 

 ward through the stem. The medullary rays of the stem conduct foods 

 and water radially (Figs. 53 and 57). 



VIII. MANUFACTURE, DISTRIBUTION, AND 

 STORAGE OF FOODS BY PLANTS 



Plants without chlorophyll, such as bacteria, yeasts, and molds, select 

 and absorb their foods from the materials on which they grow. Chloro- 

 phyll-bearing plants combine carbon dioxide and water to form carbo- 

 hydrates in the presence of light through the process of photosynthesis. 



