44 THE COTTON PLANT IN EGYPT CHAP. 



boll-worm, while the .rest is either mere spoiling of the 

 seed-cotton in the open boll, or else a simple reduction of 

 temperature. 



Transpiration. We have examined the principal 

 machinery by which the water-loss of the plant is 

 controlled and it now remains to ascertain what the 

 amount of this loss may be under field conditions. 



On this subject we are profoundly ignorant, owing to 

 the experimental difficulties, and the present section is 

 intended rather to indicate the possible lines of attack 

 than to attempt a statement. 



The usual methods of investigation are divided into 

 gravimetric and volumetric determinations, which may be 

 used to summarise the net result of a long period, such as 

 a day, or to investigate the effects of a particular set of 

 factors at a given moment. Direct weighings of the plant 

 are feasible in the earliest stages, but we have seen that a 

 plant aged three weeks, growing in a six-litre pot, is 

 already abnormal. Further, we shall come later to 

 evidence which shows that two plants growing in an iron 

 cylinder 2 metres high and 80 cm. in diameter, are 

 suffering from root limitation by the middle of July ; 

 since similar signs are shown by close-sown plants in field 

 crop, the objection is not fatal, but the conditions cannot 

 be regarded as normal. Furthermore, such isolated plants 

 have room to develop all the branches they produce, and 

 their leaf area is consequently much larger than that of a 

 field plant. Lastly, being isolated, they are freely 

 exposed to wind, and this rapid removal of saturated air 

 must increase transpiration enormously. The only serious 

 attempt at determinations by weighing large tanks was 

 carried out by M. Audebeau. The obj ections j ust mentioned 

 apply to this excellent series of observations, especially 

 after June, when the field plants are beginning to produce 

 a " surface climate." The discrepancy between M. 

 Audebeau' s total transpiration and the actual state of 



