168 



THE INDIVIDUAL ORGANISM 



chyma also contain chloroplasts, though not nearly so many as the 

 palisade cells. 



The stomata are openings enclosed by a pair of specialized epidermal 

 cells called guard cells. There are about 40 stomata per square millimeter 

 on the upper surface of the bean leaf, compared to about 250 per square 

 millimeter on the lower surface; this ratio is quite typical of leaves in 

 general. Under ordinary circumstances the stomata open when light 

 falls upon the leaf and close when the light is withdrawn. This is caused 

 by increase in turgor of the guard cells during the day and decrease at 

 night. Since light is associated with photosynthesis, it was formerly 

 believed that sugar manufacture within the guard cells was responsible 



Fig. 12.3. Leaf epidermis of a rock-garden plant, Sedum, showing stomata. (Courtesy 

 General Biological Supply House, Inc.) 



for the daytime increase in turgor; but this appears not to be the explana- 

 tion. Little photosynthesis goes on in these cells; most of their carbo- 

 hydrate comes from outside, and the total amount remains fairly con- 

 stant. At night, however, most of the carbohydrate in the guard cells is 

 in the form of starch, which is changed to sugar during the day, probably 

 as a result of acidity changes in the protoplasm. Whatever the explana- 

 tion for the turgor changes, their result is to open the leaf for free gaseous 

 interchange with the atmosphere during those periods when active photo- 

 synthesis requires it. 



Because of this relation to sunlight, however, the stomata are relatively 

 ineffective in controlling evaporation. Concentration of stomata upon 

 the lower surfaces of the leaves helps to reduce water loss, and devices to 

 reduce air movement past the stomatal openings and thus lessen evapora- 

 tion are common. Thus the stomata may open at the bottoms of deep 



