EFFECT OF STOMATAL MOVEMENT UPON TRANSPIRATION. 87 



time, the lower opened to 50 per cent at 2 a. m. and 70 per cent at 3. 

 At 4 a. m. both surfaces showed 50 per cent opening, and remained 

 in this condition for an hour. At 6 a. m. the lower were but 40 per 

 cent open, while the upper opened to 90 per cent. At 7 a. m. the 

 lower opened to 70 per cent, while the upper were wide open. At 

 8 a. m. the stomata of both surfaces were wide open and so remained 

 until 10 a. m., when the lower closed to 70 per cent. At 11 a. m. the 

 upper closed to 90 per cent and the lower to 30 per cent. At noon all 

 the stomata were closed and remained closed to the end of the series 

 (fig. 42). 



100 

 90 



7C 



ec 



50 



30 



\ 



\ 



NOON 



2. 34- 56 7 8 9 10 II MT 



23*56 789 10 II WON I 



FIG. 42. Series 20, showing movement in upper (A) and lower (B) stomata of 

 heavily watered potato plants, and in upper (C) and lower (D) 

 stomata of plants in very dry soil. 



Since the stomatal movement of the cut stems was very different 

 from that of the other plants, it is clear that the water-loss shown by 

 the potometers can not be considered representative of the trans- 

 piration from any field plants or watered potted plant. Moreover, 

 because of the difference found in the stomatal movement of heavily 

 watered potted and field plants, the transpiration from a plant in 

 a sealed pot can not, under these conditions, be considered to be 

 the same as that from a field plant. The plants, however, were under 

 conditions of high evaporation, high temperature, and very high 

 radiation. The potted plants were influenced by the increased 

 temperature of the soil-mass, due to the action of radiant heat. 

 The effect of extreme insolation is shown especially by the cut stems, 

 including those in the potometers, which did not recover turgor 

 during the night. Similar potometers kept in the shade functioned 

 a week before being seriously affected by bacterial action. This 

 experiment and the care taken to prevent bacterial infection of the 

 stems and the consequent occlusion of the vessels -tend to show that 

 wilting was not due to this cause. 



As in the case of alfalfa, the stomatal openings found on the upper 

 and lower surfaces of the leaves were averaged in proportion to the 

 number of stomata present per unit area for comparison with the 

 rate of transpiration. The changes found in one correspond fairly 



