42 



THE COTTON PLANT IN EGYPT chap. 



or three hours, while on hot, dry, windy days, with a 

 soil approaching to physiological dryness, the full aperture 

 has scarcely been attained before closure sets in. Such 

 facts indicate that the closure which follows must be due 

 to " water-supply shortage," and that its utility is to 

 check the excessive water-loss involved by free transpir- 

 ation. By noon, on most summer days, this closure is 



Mid- 

 night 



Fig. 39. — Stomatogbaph REceEDS. 



Curves show volume of air driven through an area of 80 sq. mm. of leaf per 



minute, under a pressure of 1 mm. of mercury. 

 Five-day record on the same leaf vf ithout alteration of apparatus. Fifth day 



is abnormal. 



almost complete, and we shall revert to its effects on 

 transpiration and photo- synthesis when considering these 

 functions. 



On cloudy days, or when a screen is placed across the 

 plane of the ecliptic so as to cut off all direct sun from 

 the whole plant, the maximum aperture is not attained, 



