THE LOSS OF WATER BY THE PLANT 



169 



quently about noon, or between 10 and 11 o'clock in the morning 

 the stomata may have closed completely. Under conditions of 

 maximum heat and dryness they remain closed the whole day long 

 and open only for a short time during the early morning hours. 



The behavior of stomata varies with different plants. In some, 

 as the potato, cabbage, beet, etc., they are usually open. Under 



Fig. 74. — Daily march of stomatal movements in the onion. Within the circle 

 are graphs showing diurnal changes in light intensity, temperature, and humidity 



(after Loft field). 



optimum conditions of water supply they tend to remain open 

 throughout the day and night. In other plants, for instance the 

 cereals, stomata are always closed at night. Moreover, they close 

 very early in the evening and at the slightest deficiency of moisture 

 may become shut in the morning hours. The majority of plants 

 occupy an intermediate position in this behavior (Fig. 74). It is 



