60 THE COTTON PLANT IN EGYPT CHAP. 



lag in the growth-rate behind the night-temperature 

 control. After June 26th this lag develops rapidly 

 into the dominant feature of the curve, and some 

 factor is obviously at work which leads to a general 

 and progressive falling off of the growth-rate. A part 

 of this degrading is probably of the chemical nature 

 indicated above, which results in a decreased range of 

 sensitivity in the growing protoplasm ; some such 

 inferiority would account for the slower growth of lateral 

 roots. To postulate such " deadening " of response is not 

 inadmissible, since it has often been noted in the fungus 

 studies, 9 where two hyphse in the same field, with the 

 same temperature relationships, were yet found to grow at 

 proportionally different rates, whatever their temperature 

 might be. In these same studies it was also found that 

 such decrease in the growth-rate sometimes appeared as 

 the result of short exposure to high temperatures, too 

 short to affect the stopping-point.* 



Such general slowing down of the growth-rate will not 

 cover all the observed facts, for if we smooth the growth- 

 records to the basis of such an assumption, we find that 

 the formerly preponderant influence of night-temperatures 

 on the daily growth is rapidy vanishing. On the other 

 hand we find that the actual records show a long-period 

 modality, corresponding to the intervals between the 

 waterings ; growth attains its maximum from two to five 

 days after watering. Obviously the root-absorption is 

 partially involved, probably indirectly through thermo- 

 toxy, or auto-toxy of some other kind, and also through a 

 partial and temporary suppression of the sunshine effect. 



These soil-water effects disappear in their turn after the 

 middle of September, when -in the particular series under 

 discussion the infiltration water from the river raised the 

 water-table and smothered the roots. By this time, how- 



* Loc. cit., 9 " Triple curve." 



