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- 
vineing 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. Thestomatal 
openings may be compared with the relative transpiration when the readings 
are taken simultaneously from the same tree, but a reference to experiment 
JX 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. 
