68 EFFECT OF PHYSICAL FACTORS AND PLANT CONDITIONS. 



The movement shown by the plants of plot 3 is characteristic 

 of the normal light-induced behavior of alfalfa stomata under favor- 

 able conditions, save in one very minor point ; the slight irregularity 

 in maintaining maximum opening, especially toward the last, shows 

 that evaporation was just on the verge of becoming critical. The 

 higher water-content of plot 2 caused the stomata to show partial 

 opening throughout the night. This is not characteristic of stomatal 

 movement of the alfalfa group of plants under normal conditions, as 

 found in the regions in which these experiments were conducted. 

 It may be characteristic of such plants, however, in regions where 

 there is a heavy rainfall each day. Maximum opening was reached 

 by the plants of this plot an hour earlier than those of plot 3. The 

 closure induced by irrigation may be attributed either to the cutting 

 off of air from the roots, which seems improbable, or to chilling or 

 shock inhibiting their functioning for a time. The wilting and con- 

 tinued closure of stomata of the plants of the first plot, however, 

 was undoubtedly due to the cutting off of air to the roots, causing them 

 to fail to function. 



The stomata of the plants in the fourth plot showed the effect of 

 very low water-content. The plants began to recover turgor after 

 sundown, and the first effect was observable at 11 p. m., when slight 

 opening of the stomata occurred. Not until after 1 a. m., however, 

 was the water lost during the day fully replaced in the leaves. With 

 the appearance of daylight the stomata of these plants began to 

 open, as well as those of the other two plots, but much more slowly. 

 At 7 a. m. the water-loss was becoming critical and the following 

 hour the stomata closed. The water-content of this plot was so low 

 that recovery of turgor was incomplete, as well as very much delayed. 

 As shown in figure 9, when evaporation is great and water-content 

 only moderately low, night opening occurs much earlier and is con- 

 siderably greater. 



LEAF TURGOR. 



The amount of water in leaves normally changes throughout 

 a 24-hour period. The changes are slight when water-content is 

 high and evaporation low, but they are usually great with mod- 

 erate water-content and high evaporation. The percentage of 

 water present in the leaf at any time is determined by the rate of 

 transpiration on the one hand and the rate of water-supply on the 

 other. When the stomata open at daylight and the factors con- 

 cerned in evaporation become more intense, the rate of water- 

 supply soon falls behind that of transpiration. In consequence, 

 the amount of water in the leaf begins to decrease, until the loss 

 often becomes critical. At this time the stomata close partially or 

 completely, and transpiration is diminished. If this reduces the 

 rate of water-loss sufficiently, the leaves begin to regain turgor, and 



