76 THE COTTON PLANT IN EGYPT CHAP. 



required before we can plot a curve to show this diurnal 

 change, yet the conception of a variable centre of gravity 

 for the water-absorption of the root seems so to clarify our 

 ideas on certain points that we will discuss it in more 

 detail than it probably deserves. 



As a basis for discussion we may take four series of soil- 

 water determinations made at intervals of 20 cm. down- 

 wards for a depth of two metres, in the soil occupied by 

 the plants on which the daily growth (Fig. 30) was 

 measured. The bores were made within a few feet of each 

 other, among these plants which it will be remembered 

 were growing under field-crop conditions. The soil 

 contained 25 per cent, of water when completely saturated, 

 and the water-determinations, made by drying in the 

 steam oven, are expressed in terms of the saturation 

 amount. 



The determinations w^re 'made under the following 

 circumstances : 



June 27th : Four days after irrigation, when the maximum growth-rate 

 of the whole year had just been recorded during the previous twenty-four 

 hours. The precedent watering had arrested the symptoms of water- 

 shortage yet shown by the plant. We may safely conclude that the 

 water-distribution shown on this date was nearly ideal. 



July llth : The day before the next subsequent watering, when severe 

 water-shortage was being exhibited by all plants, and the growth curve had 

 been falling fairly steadily since the last determination. The water- 

 distribution here was decidedly typical of insufficiency of water. 



July 15th : On the third day after watering, when the growth of the 

 main axis, having been accelerated for two days, had again begun to 

 decrease. The water conditions were probably very good, though no 

 growth-measurements were being taken on lateral branches by which this 

 statement could be corroborated, but comparison with the growth after the 

 previous watering indicates that the water-distribution in the soil had not 

 yet reached its optimum. 



July 30th : These determinations were made just before the next water- 

 ing. Water shortage was again apparent, and the period elapsing since the 

 last watering was the same as in the case of the determination of July llth. 



The actual changes in saturation-percentage under these 

 various conditions are shown in the following table. The 



