WATER RELATIONS OF PLANTS 



309 



for instance, the cereals, stomata are always shut at night. 

 Moreover, they close very early in the evening and at the slightest 

 deficiency of moisture may be closed even in the morning hours. 

 In the majority of plants, the behavior of stomata can be said 

 to be between these extremes (Fig. 100). It is interesting to 



Fig. 100. — Daily changes in the stomatal movements in the onion. Within the 

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

 humidity {after Loftfield). 



note that in plants showing stomata on both sides of the leaf, 

 those on the upper surface open later and for a shorter time, 

 than those on the lower surface. Likewise, the stomata near the 

 tip of the leaf open earlier and close later than those at the base. 

 Since changes in the degree of opening of the stomata may be 

 observed directly, the stomatal movements until recently were 

 regarded as the only means of regulating transpiration. This 



