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. STOMATOGRAPH RECORDS. 



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 without alteration of apparatus. Fifth day 

 is abnormal. 



almost complete, and we shall revert to its effects on 

 transpiration and photosynthesis 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, 



