STOMATAL MOVEMENT. 43 



DISCUSSION OF STOMATAL BEHAVIOR. 



In all of the seven experiments given above, the stomata of Parkinsonia 

 microphylla closed down and subsequently opened during daylight. Three 

 of the closures took place on adult trees and four on potted plants. In all 

 cases the closure began before the time of maximum evaporation for the day 

 had been reached. In the two cases (experiments XIV and XVI) where 

 transpiration and stomatal measurements were taken simultaneously from 

 different parts of the same tree, the curves of transpiration run parallel with 

 the curves of the linear dimensions of the stomatal openings during the fore- 

 noon, but in the afternoon depart and run in opposite directions. These 

 two experiments present some evidence that there is a connection between 

 stomatal opening and transpiration rate and that the connection is not 

 operative in the afternoon. The curves follow each other so closely during 

 the morning hours that it seems impossible to neglect their evidence, even 

 though the afternoon readings show evidence of the opposite sort. The 

 fact that the time of minimum transpiration corresponds with the time of 

 minimum stomatal opening in both experiments, and that this time is not the 

 same in the two experiments can not be ignored. It is well to note here that 

 between the dates of the two experiments the summer rains had come and 

 the soil moisture at a depth of 30 cm. had been raised from 14 to 32.1 

 per cent, and consequently the deeper layers of soil must have had their 

 moisture content raised proportionally. The trees had lost their leaves and 

 gained new ones between the two experiments. While there is no con- 

 vincing evidence of the direction of the cause and effect, a point might be 

 made against the theory that the closure of stomata is due to light effect 

 alone, for the difference in time of closure in the two experiments could 

 hardly have been brought about by the difference of position of the sun, 

 since the sun was about at the same distance from the summer solstice at 

 the two times. 



A reference to pages 19-29 shows that while the time of the morning mini- 

 mum relative transpiration is fairly constant for a given tree on successive 

 days, it is by no means constant in the case of potted plants. The stomatal 

 openings may be compared with the relative transpiration when the readings 

 are taken simultaneously from the same tree, but a reference to experiment 

 IX will show the impossibility of making such hourly comparisons in the 

 case of two potted plants unless the plants used can be shown to have parallel 

 hourly curves of transpiration. Several attempts were made to get stoma- 

 tal readings from one potted plant and transpiration from another which 

 had been found to give a similar relative transpiration curve, but various 

 interruptions of weather and accidents to the plants prevented good 

 readings. The curves given above show that there is some connection 

 between the relative transpiration and the size of the stomatal opening, but 

 there is no evidence of the relation of cause and effect nor even of priority 

 of time. 



