32 



THE DAILY MARCH OF STOMATAL MOVEMENT. 



ments (Russet Burbank) had three-twentieths as many stomata on the 

 upper surface as on the lower. Unlike alfalfa, the lower stomata were 

 more active than the upper. All the stomata of an alfalfa leaf become 

 functional at practically the same time. In potato, however, the 

 first stomata may begin to function long before the last have formed. 

 Two weeks have elapsed in most cases between the time the first 

 stoma on a leaf opened and the last stomata became functional. 



The curves made by the lower stomata of potato under three 

 conditions of water-content are shown in figure 15. The last curve 

 is much the same as that of alfalfa under similar conditions, as it 



B\ 



77 



10 II NOON I 



10 II MT. I 



Fio. 15. Movement in lower stomata of potato under high water-content 

 and moderate evaporation (A), low water-content and moderate 

 evaporation (B), low water-content and excessive evaporation (C). 



shows day closure and night opening. But where alfalfa under very 

 favorable conditions has stomata open all day and closed at night, 

 potato has stomata open continuously, save for the 3 hours following 

 sundown. Then, as the conditions for obtaining and retaining water 

 become less favorable, this period of closure begins earlier, until 

 finally it includes all of the day except an hour or two at sunrise. 

 There was in no case the mid-day closure and afternoon opening so 

 often found in alfalfa stomata. This may occur possibly in other 

 varieties of potato or under other conditions, but was not found in 

 this investigation. This would seem to indicate that a larger pro- 

 portion of the leaf-water was lost by the potato before its stomata 

 closed than is the case in alfalfa. The slightly wilted appearance 

 of the plants at the times when their stomata are closed would 

 confirm this, as there is no sign of such wilting of alfalfa plants during 

 the midday closure of their stomata. The more ready closing of 

 stomata in alfalfa may in part explain its greater drought-resistant 

 qualities as compared with potato. It is obvious that a plant which 

 waits until the last moment to cut down water-loss must waste 

 much more than the plant which is so adjusted as to cut down this 

 loss at the first signs of shortage. On the other hand, it seems 

 reasonable that when the water-supply is adequate, but evaporation 



