LEAVES 



563 



as compared with other epidermal cells, are vigorous and active, living even for weeks 

 when removed from the leaf and placed in a nutrient medium ; they are unusually 

 resistant also to high temperatures and to other factors which generally are harmful 

 to plant cells. The opening of stomata in moist air is thought to be due to the dis- 

 tention of the guard cells when they absorb a large amount of water. The result- 

 ing approach toward sphericity in the guard cells and the stretching of their thin 

 dorsal walls, often supplemented by the action of hinge walls in the subsidiary cells, 

 cause the withdrawal of the median ridges and the consequent opening of the 

 central slit (fig. 806). Exposure to dry air is thought to lessen the distention, where- 

 upon the guard cells swing back to a position of closure. In grasses stomatal open- 

 ing is effected by the mutual pressure of the dumbbell-shaped ends of the guard 

 cells, supplemented by the buffer cells at each end, which resist longitudinal dis- 

 tention (fig. 796). 



The causes of the nocturnal closing and of the diurnal opening of stomata are 

 imperfectly understood, though it has been held that the sugar manufactured in 

 guard cells in daylight increases their turgor and hence produces the distention 

 necessary for opening. It has been shown that guard cells exhibit greater turgor 

 changes than do the adjoining cells, and that they differ sharply from palisade cells 

 in that the* maximum of starch accumulation takes place at night. In daylight the 

 starch is digested, and in the guard cells of Impatiens, at least, the presence of 

 sugar actually has been detected. Some stomata open and close periodically, even 

 in darkness, as though long exposure to alternating light and darkness had induced 

 an inherent rhythm. Stomata close at low temperatures, as in evergreen leaves in 

 winter; whether this is because of reduced synthesis or because of lessened water 

 supply is not known. That both light and moisture are factors in guard-cell move- 

 ment, whatever may be the exact mechanism involved, is shown (i) by the fact that 

 the stomata in most leaves close at night, whatever the moisture content of the air, 

 and (2) by the fact that wilted leaves have closed stomata, whatever the intensity of 

 the light. 



The role of stomata. Synthetic and respiratory exchanges. The 

 chief advantage of stomata to plants is the facilitation of gas exchange 

 in connection with carbohydrate synthesis. Experiments have shown 

 that synthetic activity is much reduced if the stomata are artificially 

 closed, as when the stoma-bearing under surface of a Ficus leaf is\. 

 coated with wax. When the upper surface of such a leaf is slit, 

 starch accumulates in the cells adjoining the incision, indicating the 

 resumption of vigorous synthetic activity, and doubtless betokening the 

 free entrance of carbon dioxid and the exit of oxygen. Small as are the 

 stomatal openings and slight as is their aggregate surface, the intake of 

 carbon dioxid by an ordinary leaf approaches the rapidity with which 

 this gas is absorbed by a free surface of caustic alkali. Indeed, stomata 

 might be considerably fewer or smaller than they are without appreciably 

 impairing synthesis. Respiratory gas exchanges are so slow and in- 



