50 THE COTTON PLANT IN EGYPT CHAP. 



varying nature of such controlling- factors as clouds and 

 sun-temperature and the age of the leaf, it is not very easy 

 to demonstrate the progressive effect produced by desicca- 

 tion of the soil over a period of many days. Mimicry of 

 this progressive change by sudden root-pruning may be re- 

 garded as tolerably just, and the effect of such pruning is 

 to cause all but the youngest leaves 'to behave like old 

 ones ; they are easily over-heated, and where they had 

 formerly held their temperature down below that of the air, 

 they now exceed the air temperature by several degrees. 

 Hence nearly all the plant tissues suffer from over-heating 

 during the day. The bearing of this fact on our interpreta- 

 tion of the fall in growth-rate which follows water-shortage 

 in July will shortly be mentioned. 



Another method of demonstrating this loss of thermo- 

 regulation is simply to examine the tissue-temperature 

 record of a young leaf during a day when the stomata 

 had closed at an early hour. This temperature-trace 

 starts below the air-temperature line and slowly rises up 

 to it, or above it. Differences of not less than 5 C. in 

 mean relative tissue-temperature may thus be produced 

 by stomatal closure. 



Photosynthesis. We can scarcely leave the general 

 physiology of the cotton plant without some reference to 

 this prime function, but a critical and exhaustive series of 

 data has yet to be obtained. 



The first attempts at obtaining such a series by the 

 Sachs-Thoday * dry- weight method were rendered fruitless 

 by the unexpected high error of a-symmetry in cotton 

 leaves. The probable error of difference between 

 nominally identical areas of 15 sq. cm., cut from the right 

 and left sides of the central segment, amounts to 

 4 per cent, of the mean weight of each pair. Hence 

 at least sixteen pairs have to be examined in each 



* Thoday, D. 



