WATER RELATIONS OF PLANTS 305 



The diffusion capacity of the epidermis suppUed with innumer- 

 able stomata is so high that the plant seldom uses it to full 

 capacity. This is true at least for the process of transpiration. 

 The stomata play also an important part in assimilation, as they 

 are the openings that give access to carbon dioxide. Actually, 

 transpiration is always considerably lower than its calculated 

 value. From this, the conclusion may be drawn that besides 

 changes in the stomatal openings, other factors may exert a 

 checking influence on transpiration. 



71. Stomatal Apparatus of the Plant. Its Movements and 

 Methods of Its Study. Nonstomatal Regulation of Transpi- 

 ration. — One of the most important pecuharities of the stomata 

 is their ability to change the 

 diameter of their opening. 

 Details of the anatomical struc- 

 ture of the stomatal apparatus 

 are given in books on plant 

 anatomy and will not be dis- 

 cussed here. It will only be 

 indicated that the stomatal fig. 97. — Changes in the starch con- 

 opening is limited by two guard tent of guard cells during their move- 



ments {ajter ILjm). 



cells of a beanlike shape (l^ig. 



97). The internal walls of these cells Uning the opening are con- 

 siderably thickened, while the outer walls remain thin. Unequal 

 thickness of the outer and inner walls of the guard cells deter- 

 mines the possibihty of changes in the diameter of the opening. 

 When the guard cells imbibe water, their outer walls are much 

 more expanded than their inner ones; this leads to a curvature of 

 the cells, and the opening increases. Conversely, with loss of 

 water the cells decrease in volume, their walls straighten, and the 

 opening closes. 



Light and water content are the chief factors responsible for 

 the movements of the guard cells. With abundance of water in 

 the leaf, the guard cells are saturated with water, the cell walls 

 are extended, and the stomata are open. Deficiency of water 

 leads to a decrease of turgor and to a closing of the stomata. 

 In the majority of plants, the stomata are opened in light and 

 closed in darkness. 



The chemical processes causing the changes in turgor of the 

 guard cells and thus controlling stomatal movements are very 



2 3 



