30 THE COTTON PLANT IN EGYPT chap. 



temperature operates during tlie night. On this latter 

 point, unfortunately, we are unable to generalise with 

 safety. The curve of growth — after sunset — leaps from 

 zero to a maximum, and thereafter follows the temperature 

 curve* till the small hours of the morning (Fig. 34). It 

 is not clear whether there is any limitation through 

 exhaustion of the photosynthetic food supply before sun- 

 rise. On the whole, the evidence is against this view, 

 except in rare cases, and in any case such limitation would 

 take place at the period of minimum growth-rate. 



Night-temperatures. — Combining our knowledge of 

 temperature control with the elimination of day- 

 temperatures provided by the " sunshine effect '' we are 

 enabled to interpret the growth -rates obtained under field- 

 conditions with fair exactitude. 



The curve shown in Fig. 30 was obtained by daily 

 measurements of the axial shoots of ten cotton plants, thus 

 summarising the mean growth of the twenty-four hours. 

 The correlation between minimum night-temperature and 

 the growth rate up to July 1st is very close (r = 0"7843± 

 0"0459) if those daily periods are excluded in which the 

 maximum temperature had risen above 35° C. The 

 correlation sinks to a value of 0'"5236±0"0755 when 

 these are included, owing to thermotoxic effects. 



At this stage of the plant's existence there can be no 

 question as to the economic interest of stem-growth. The 

 faster the growth of the stem and branches, the sooner 

 will the first flowers open, and the period which we are 

 now considering ends with the appearance of these. 



Thus, the date of "arrival" of the crop depends first 

 on the night-temperatures which have been experienced, 

 secondly, on the date of sowing, and thirdly, on the clouds. 

 We shall return to this subject later, in connection with 

 the date of the first flower. 



* Except after heat-poisoning, "when it remains constant, or rises slowly 

 until cut by the falling temperature. 



