74 THE COTTON PLANT IN EGYPT CHAP. 



who cultivated plots of cotton in small cement tanks, with 

 the water-table fixed at definite depths in each tank 

 throughout the season; his data showed that even the. 

 absolute shedding, and much more the relative shedding, 

 was greatest in soil with a water-table only half a metre 

 below the surface, while it became proportionately smaller 

 in deeper soils, down to a two-metre water-table. In his 

 experiments there was no second mode, since no change 

 took place in the w r ater level. We shall appreciate the 

 bearing of these data better at a later stage. In general, 

 they imply that a sufficiently large volume of soil must be 

 occupied by the roots to ensure a uniform water-supply 

 for the stem. 



Since the " terraces " were open to agricultural criticism 

 on account of the soil-disturbance involved by their 

 construction, a set of six cylindrical iron tanks, each 

 80 cm. in diameter, and two metres in height, were sunk 

 in a pit in the ground, until their tops were flush with 

 the surface. Each was fitted with an inlet pipe at the 

 bottom, protected by a loose brick arch, rubble, and sand, 

 and was finally filled with good uniform soil. A rubber tube 

 leading from this inlet pipe, and connected to an adjustable 

 reservoir of water, allowed the level of the water-table in 

 each tank to be maintained or adjusted to any required 

 level. Various combinations have been employed with 

 these tanks, but the results have only substantiated those 

 already described. Fixed high water-tables cause undue 

 shedding, so that the early part of the boiling curve is 

 abnormally small ; changing water-tables cause temporary 

 shedding, so that a wedge is cut out of the boiling curve 

 about seven weeks after the change has been initiated. 29 



Such small tanks are, however, open to many objections, 

 and the ideal method of study would be to encase an 

 undisturbed mass of soil with armoured concrete by under- 

 pinning, so as to convert it into a large tank. A set of- at 

 least six such tanks should be arranged at intervals in a 



