80 EFFECT OF STOMATAL MOVEMENT UPON TRANSPIRATION. 



was connected through a glass tee to the plant on one hand and a 

 flask used as a reservoir on the other. A pinch-cock permitted the 

 refilling of the burette from the reservoir after each reading; this 

 constantly maintained the water-pressure upon the cut end of the 

 stem at nearly the same level. Other precautions were observed; the 

 entire apparatus was sterilized with 4 per cent formalin, and the 

 water used was recently distilled and boiled just before using. The 

 stem of the plant used was sterilized at the point desired, and cut 

 under water with a sterile knife. The cut end of the stem was pro- 

 tected by a loose plug of absorbent cotton. These potometers were 

 run 28 hours and read at half-hour and hour intervals during the 

 two days and at the end of a 10-hour night interval. Since the read- 

 ings ran in parallel series, the test was considered satisfactory. The 

 stems used were then discarded and the apparatus resterilized. 



At 4 p. m. on August 8, the potometers were started with fresh 

 stems, which were gathered with all the precautions observed when 

 making the trial. Readings were made at half-hour intervals until 

 7 p. m. to insure that the potometers were working properly. As it 

 was found to be impossible to check the transpiration from the cut 

 stems against that of field plants, or even of potted plants, it was 

 decided to compare the stomatal movement of such cut stems with 

 the stomatal movement of field plants. For this purpose, 30 branches 

 of alfalfa were cut under water with the usual precautions. These 

 were placed in flasks of the same kind of water used in the burettes. 

 In order to find whether the head of water against the cut ends of the 

 stems in the potometers had some effect upon the stomatal movement 

 not found in those in the flasks, several stems were treated in the 

 same manner as those in the potometers, using plain glass tubing 

 instead of burettes. A sufficient number of these were set up so 

 that strips could be taken at 4-hour intervals during the series. As 

 may be expected, the increased water-pressure on the cut ends did 

 not have sufficient effect to make a discernible difference in the 

 behavior of the stomata. The leaves of one stem were stripped each 

 hour and the stem immediately discarded to prevent its being used 

 again. 



A slight and misty rain occurred between 10 and lip. m., but by 

 midnight the weather had cleared. At 1 a. m. the wind had risen 

 and was drying the vegetation. By 2 a. m., when the wind died 

 down, all effects of the rain had disappeared, and the night remained 

 clear and still from that time. The following day was warm, prac- 

 tically cloudless, and more humid than ordinarily, since the relative 

 humidity did not fall below 28 per cent. Although it was a sunny 

 day, haze prevented the light from reaching more than 75 per cent 

 of the maximum intensity for the region. As usual, the lowest 

 temperature, 55 F., was recorded at 5 a. m., just before dawn. The 

 highest was 78 F., occurring at 3 p. m. the same day (fig. 37). 



