70 THE COTTON PLANT IN EGYPT chap. 



and do not create a humid " surface climate." In the 

 Sudan, however, spells of hot dry wind are generally 

 recognised as being the precursors of shedding epidemics. 



Before considering a few tjrpical shedding-curves it may 

 be well to insist once more on the economic importance of 

 these records. Plants whose flowering curves are identical 

 may give entirely different boiling curves ; in other words, 

 entirely different crops. 



The record of the behaviour of families sown on various 

 dates in the same year may be reconsidered in this 

 connection. We have already mentioned a decrease in the 

 growth-rate caused in the older families by water-shortage, 

 but by the time when boU-shedding is becoming notable, aU 

 the plants are suff'ering from root-interference, which in a 

 light soil results in periodic water-shortage. As the result ^. 

 of this we find successive modes in the shedding curves, 

 common to all the ftimilies. Following these modes we 

 have periods of less shedding, which immediately follow 

 irrigation. Some of these records show three such modes 

 in each family, followed by a fourth one. 



This fourth mode is apparently due to a distinct cause. 

 Firstly, it comes a week sooner than one would expect ; 

 secondly, it is followed by a general cessation of flowering ; 

 and lastly, it coincides with the intersection of the 

 root-depth line by the level of the infiltrated sub-soU 

 water (Fig. 30). 



Thus we have two ways of producing a mode in the 

 shedding- curve, relatively to the number of flowers 

 available for shedding. One acts through decrease of the 

 amount of available water in the soil, while the other way 

 is through increase of the soil-water-content to saturation- 

 point, thereby excluding air, causing root-asphyxiation, 

 and so reducing the size of the root-system ; this root- 

 system, having been developed to cope with a certain 

 mean maximum demand for water from the aerial portions, 

 or, more correctly, having controlled the development of 



