226 TEXTBOOK OF BOTANY 



During the daylight period both stomatal and cuticular transpiration 

 occur. At night there is usually only cuticular transpiration. This differ- 

 ence plus the effects of the higher day temperatures are shown in the 

 following table in which the relative amounts of the total transpiration 

 from certain plants during daylight and darkness under field conditions 

 are stated in percentages. 



Plant Daylight Darkness 



Wheat 96% 4% 



Oats 94% 6% 



Alfalfa 97% 3% 



Pigweed 97%, 3% 



Opening and closing of stomates. The opening and closing of stomates 

 are obviously matters of first importance in modifying transpiration. 

 The relation of stomates to the movement of guard cells was discussed in 

 Chapter XXIII. The stomates on a plant are not opened or closed simul- 

 taneously, because conditions are not identical in all parts of a leaf or 

 in the different leaves on the same stem. Consequently, when we say 

 that the stomates are gradually closed, we mean that some are closed 

 quickly, some are closed slowly, and others scarcely at all. When we say 

 that the stomates are closed, we mean that almost all are closed and the 

 remainder nearly so. 



We learned earlier that oxygen and carbon dioxide, gases important in 

 respiration and photosynthesis, diffuse into and out of the leaf largely 

 through the stomates. Here we are emphasizing water-vapor loss through 

 stomates. In this connection it is well to remember that stomates do not 

 open to promote photosynthesis and do not close to conserve water. 

 During droughts they may open most inopportunely and allow further 

 water losses. In late summer they may close early in the daytime and 

 restrict photosynthesis when all other conditions appear to be favorable. 

 The turgor movements of guard cells are conditioned neither by photo- 

 synthesis nor by transpiration, but by a series of changes in these cells. 

 These are slight changes in acidity which influence the activity of 

 enzymes and also the change from starch to sugar, or sugar to starch. 

 Increase in sugar content leads to diffusion of water into the guard cells, 

 and greater turgor. The stomates are opened. Change of sugar to starch 

 in the guard cells results in closing the stomates. 



Movement of water from veins to mesophyll cells. Let us, however, 

 again turn our attention to the mesophyll cells. During the morning the 

 sugar content of these cells increases. They also contain salts, acids, and 



