42 



Mr. W. L. Balls. 



[Dec. 22, 



at 2 p.m., 35° C. Minimum night temperature at 5 a.m., 22° C. Humidity, 

 varying, from 20 per cent, of saturation by day to 100 per cent, by night. The soil 

 had been irrigated seven days before the record begins. Sunshine was direct and 

 continuous, between the limits indicated on the graph, during all five days. 



The leaf showed slight signs of injury on the last day, in the portion which 

 touched the wax, but this slight browning had disappeared within a week after its 

 removal from the chamber. The record for the fifth day is plainly abnormal. 



The general nature of these and of other similar records is as follows : — 

 At sunrise the stomata open slightly and continue to do so until the direct 

 sun strikes thern. They then increase their aperture very rapidly to a 

 maximum at about 9 a.m. After remaining wide open for a longer or 

 shorter time, which appears to depend on the development of the root- 

 system and on the humidity of the air and soil — provided, of course, that 

 sunshine is continuous — they begin to close more and more quickly till their 

 aperture is less . than it was before the direct sun reached them. The 

 explanation of this closure seems to be provided by an hypothesis of " water- 

 starvation " ; the root-absorption being insufficient to cope with the heavy 

 loss by transpiration, the latter is reduced in consequence. 



The closure continues until, on some days, the stomata are practically 

 shut by noon. Preliminary investigations indicate that, in consequence of this, 

 photo-synthesis only takes place during the early part of the day, and that 

 the plant is in a quiescent condition during the afternoon — neither growing* 

 nor feeding, but merely waiting for release from the tyranny of the sun. 



In the particular site where these records were taken, the sun passes behind trees 

 at 1.30 p.m., when the stomata close completely. A portion of this effect might be 

 due to the construction of the chamber (3a), and will have to be re-examined next 

 year with the new chamber (3b). 



The effect of shading the whole plant from direct sun, though not from 

 bright diffuse light, and so reducing the water loss, is shown in fig. 4 

 (June 8). The shade was applied before rapid closure had begun, and 

 provoked immediate partial closure, till the aperture was about the same as 

 during pre-sunshine period of the early morning. This aperture was main- 

 tained well into the afternoon, and was followed by slow closure until sunset, 

 when the final closure took place almost suddenly. 



The stomatograph has thus begun immediately to justify the purpose of 

 its invention by throwing a flood of light on the important problems of 

 water-relationships of the cotton-plant under the very severe conditions 

 of the Egyptian summer. The investigations suggested by considerations of 

 such records as those reproduced here are almost endless, and they have in 



* Balls, W. Lawrence, 'Cairo Sci. Journ.,' September, 1911. 



