36 THE COTTON PLANT IN EGYPT CHAP. 



during the night until the water content has sunk below 

 a certain limit. Since deficient water-absorption is the 

 real limiting factor of the sunshine effect, it follows that 

 the size of the root system is the true limiting factor of 

 water-absorption, and that the humidity of the soil may 



FIG. 36. SEEDLING. 

 Fourteen days old. 



vary between wide limits without affecting it. Much 

 more work needs to be conducted on this question, in 

 view of its importance in irrigation practice. 



The rate of growth of the roots under field conditions is 

 primarily controlled by temperature. Once the tap-root 



