EFFECT OF STOMATAL MOVEMENT UPON TRANSPIRATION. 95 



age, and were grown in large earthenware pots, an average of 10 

 plants to a pot. Great care was exercised to keep the soil tempera- 

 ture of the pots and containers the same throughout the day, and 

 the plants under the same conditions in other respects. This was 

 done by embedding the metal containers in the soil of the pots, and 

 keeping the soil from the container by a metal cylinder. A small 

 piece of moist flannel was kept over the top of the container, except 

 when weighed, in order to duplicate as closely as possible the moist 

 soil-surface at the base of the other plants. The containers were 

 removed for about 5 minutes every 2 hours for the purpose of weigh- 

 ing. They were weighed on the half hour, in order that the interval 

 would coincide better with the stomatal observations. Strips were 



140 



130 



10 



7 8 9 10 II MT. I 2 3 4- 5 6 7 8 9 10 II NOON I Z 3 4 567 



FIG. 50. Series 32, showing stomatal movement averaged for 2-hour periods 

 (A) and transpiration in milligrams per minute for the same 

 periods (B) in heavily watered potted plants of alfalfa. 



collected at hour intervals, but, for purposes of comparison, the two 

 sets of strips collected during each interval between weighings was 

 averaged. This method, of course, produced smoothed curves, but as 

 it was not possible to weigh at shorter intervals, the smaller fluctua- 

 tions could not be taken into account. 



From 7 to 8 h 30 m p. in. the water-loss of the watered plants was 

 44.7 ing. per minute per square decimeter of leaf-surface. During 

 this period the stomatal opening was 16 per cent. From 8 h 30 m a. m. 

 to 10 h 30 m p. m. the loss was 11.4 mg. per minute and the average 

 stomatal opening 3 per cent. The same rate of loss occurred during 

 the next 2-hour period, while the stomatal opening was 4 per cent. 

 The lack of a corresponding increase is accounted for by the rise of 

 relative humidity and the fall in the rate of evaporation shown by 

 the two types of atmometer. During the next period, from 12 h 30 m to 

 2 h 30 m a. m., the average opening was again 3 per cent, but as the 

 rate of evaporation rose again, the rate of water-loss remained at 



