254 PLANT PHYSIOLOGY 



in pairs on the two sides of an opening. These cells are generally 

 elongated, contain chlorophyll (unlike the other epidermal cells), 

 are fastened firmly to the adjacent cells, and are thicker walled 

 on the side nearest the opening. These characteristics are closely 



Fig. 15. — An artificial stoma, a, an ordinary air pump; b, glass connection; 



c, a rubber tube bearing on the inner surface an extra thickness of rubber; 



d, wire holding the end firmly in position. When air is forced into c, will the 

 opening between the two tubes become larger or smaller? 



connected with their function. As shown by Fig. 15, which repre- 

 sents an artificial stoma, anything which will increase the pressure 

 inside the guard cells will cause the opening to become larger. As 

 the light falls on the guard cells, sugars are made which increase 

 their osmotic concentration; in many cases this may reach 10-20 

 atmospheres and be 2-3 times as great as that of the adjacent cells. 

 Water then enters, the cells become turgid, and the opening in- 

 creases in size, permitting gases to enter to the cells of the meso- 

 phyll. This ability to take in water is also influenced by the hydro- 

 gen ion concentration which changes as the C0 2 is removed by 

 photosynthesis. The acidity of the guard cell contents is hence a 

 factor in their behavior, as shown by Scarth (1926). 



It was once thought that the stomata had an enormous regula- 

 tory effect upon transpiration. They were conceived of as " del- 

 icately balanced valves" which adjusted the transpiration of the 

 plant to a very fine degree. Then Lloyd (1908) published some 

 results which indicated that the stomata had practically no regu- 

 latory effect. The truth, as shown by Iljin and Loftfield, doubt- 

 less lies between these two extremes. It is light and not water 

 which determines directly the action of the stomata. Water is 

 conserved at night when photosynthesis cannot go on and the 

 stomata are shut. If there is enough light and water present, they 

 open in the daytime to admit carbon dioxide, but the light is the 

 deciding factor rather than the water supply. This agrees with 

 the conclusion previously reached, viz., that the leaf is constructed 

 to manufacture food, and that transpiration is, for the most part, 

 a necessary evil accompanying photosynthesis. The stomata 

 may not be as efficient as once thought, but they probably help 

 to a certain extent to regulate the water loss. Otherwise their 



