84 EFFECT OF STOMATAL MOVEMENT UPON TRANSPIRATION. 



Another series was made with the Russet Burbank potato instead 

 of alfalfa, in order to determine the reliability of potometers in the 

 case of this species. The potometers were fitted up with the same 

 care observed in the earlier series in which alfalfa was used. In 

 this case the stomatal movement of the cut stems was compared 

 with that of potted plants, as well as with watered and unwatered 

 field plants, to discover if the cut stems would show movement more 

 nearly resembling that found in the potted plants than the field 

 plants. As no scales of sufficient capacity were available to weigh the 

 very large pots closely enough to measure their water-loss, only the 

 transpiration from the cut stems was measured. The experiment 

 (series 20) was started at noon on August 25 and ended at 1 p. m. 

 on August 26, 1916. The potometers were set up and the first readings 

 made at 10 a. m. on the 25th, in order to find whether each was 

 working properly, before the series was begun. As the readings 

 ran in a parallel series, they were considered as functioning properly, 

 and each potometer read to the end of the experiment the next 

 day. As in the case of the alfalfa plants, the leaves were then clipped 

 off and printed on Solio paper, and the total area determined for 

 each stem separately by means of a planimeter. An error enters 

 in at this point, because the stems also have functional stomata, 

 but their number is not great and they can not have much more 

 influence than cuticular water-loss. As the area of stem-surface 

 in proportion to leaf -area was nearly the same in each potometer, 

 the effect of the stem-surface could not be determined. Hence, it 

 would be more accurate to state that the water-loss given was that 

 from 1 sq. dm. of leaf -surf ace and 1.62 sq. cm. stem-surface. As, 

 however, no attempt is made to compare the transpiration of several 

 plants, the unit of area from which the water-loss occurs is immaterial, 

 providing it is not changed during the experiment. The real problem 

 is whether the changes in the rate of transpiration from some given 

 area, regardless of its size, shows any relationship to changes in the 

 stomatal openings. 



The temperature, relative humidity, and sunlight fluctuated a 

 great deal during the experiment. The two days were rather hot, 

 the highest temperatures being 87 F. at 3 p. m. on August 25 and 

 88 F. at 1 p. m. on the 26th. As usual, the lowest temperature, 

 57.5 F., was recorded at 5 a. m., just before dawn. The humidity 

 dropped to 19 per cent at 4 and 5 p. m. on the 25th, about one-third 

 lower than that recorded during the alfalfa series. Inconstant winds 

 caused irregular decreases of humidity during the night, and the 

 maximum of 71 per cent occurred several times. During the greater 

 part of the two days there was a gentle breeze, with occasional gusts 

 of strong winds, which increased transpiration during short intervals. 

 There were occasional periods of calm, which also affected water- 

 loss. Sunlight was very variable because of changing haziness. On 



