II DEVELOPMENT AND ENVIRONMENT 27 



show 'that direct sunshine inhibits it completely.^" ^' 

 (Figs, 33 and 34). 



This simple discovery has had an almost revolutionary 

 effect on our study of plant physiology in Cotton. The 

 phenomenon, though almost unprecedented outside Egypt, 

 is exhibited by many other plants in Egypt, notably by 

 Helianthus. 



The " sunshine effect " was first recognised in May, 

 1910, and has since been found to be usual from the first 

 appearance of the seedling till the autumn, if not through- 

 out the season. The elongation of the stem is checked 

 immediately the sun strikes upon it, and a slight shrinkage 

 usually follows. A cloud passing across the sun is 

 immediately effective in permitting growth, which ceases 

 again directly the cloud has passed. 



The contraction of the stem in sunshine indicates that 

 loss of water is the direct cause of growth cessation. That 

 this explanation is the true one can be shown in many 

 ways ; thus plants covered by a glass bell-jar so as to 

 surround them with an atmosphere of high humidity, will 

 grow rapidly in the sun during May. Similar conditions 

 occur in the field for a day or two after watering, when 

 the plants are large. The most striking proof is provided, 

 however, by leaf-removal ; if the total leaf-area is reduced 

 about one quarter, by simply cutting off the lower leaves, 

 growth is almost instantly resumed, since the water-loss of 

 the stem is reduced. This last experiment shows in a 

 striking manner the delicacy of the water equilibrium 

 between root-supply and stem-loss. 



Disturbances of this equilibrium are normally effected 

 by variations in the weather, such as abnormally high sun 

 or shade-temperatures, hot dry winds, etc. It should 

 be noticed that the equilibrium-point is probably deter- 

 mined by the worst normal conditions, namely, at 1 p.m. to 

 2 p.m. 



Artificial disturbances of this equilibrium can be effected 



