48 THE COTTON PLANT IN EGYPT chap. 



— 6° (Fig. 41a). Old leaves, on the other hand, rarely fall 

 below air- temperature but frequently rise above it, varying 

 from -3° to +10° (Fig.41c). These oscillations in leaf- 

 temperature are extremely rapid, differences of 10° C. 

 being recorded often at intervals of only a minute. 



The thermo-regulation of the leaf seems to diminish 

 with age, and it is not inconceivable that the senescence of 

 a leaf may thus be hastened by thermotoxic effects. In 

 general terms, the old leaf follows the sun-temperature, 

 but the younger leaf follows the wet-bulb temperature. 

 Puffs of wind, whether natural or artificial, produce no 

 notable effect on the old leaf, but reduce the temperature 

 of young leaves. 



Thus it follows that in considering the effect of 

 maximum day-temperatures on growing tissues, we were 

 justified in taking the shade-temperatui-es as the 

 maximum tissue temperatures. 



That thermo-regulation is real may be seen by comparison 

 of Fig. 41a and Fig. 41b. The two records were taken from 

 the same tissue, at an interval of twelve days, during 

 which the vascular tissue had been destroyed by a " boll- 

 worm." The top of the stem wilted in the usual way 

 and became dry, and the recording couple inside showed 

 most erratic oscillations of temperature, due solely to the 

 intermittent cloudiness of the sky. 



The night-temperature of tissues is practically identical 

 with the air-temperature, excepting for variations of about 

 2° C, which have been shown to be caused by clouds, 

 checking radiation. The effect is not an important one. 



Fogs cause no noticeable alteration in leaf-temperature, 

 so that any chUling effect they may have is directly 

 recorded by the thermometer. 



The most important result obtained from these records 

 is one which relates to the effect of water-shortage on the 

 temperatures, through reduction of transpiration, and 

 consequent loss of thermo-regulation. On account of the 



