spates of water containing so much sediment that storage by means of dams was im- 

 practicable. Deposits caused a rise in the level of the river bed and this natural 

 rotation of the soil reduced weeds. The cultivator had the advantage of using the 

 equivalent of virgin land where the ground was watered only once in four years be- 

 cause pests were eliminated. Nomads noted the direction and duration of storms 

 before deciding where to cultivate. It was important to see that water was not con- 

 sumed by unwanted vegetation. Near Khartoum, where there were only a few inches 

 of rain annually, mosquito trees had been established on sand dunes by planting 

 them at shallow depth in moist sand and removing the inconspicuous weeds. Simi- 

 larly nomads guarded their lands from trespassing herds. Dr Williams then pointed 

 out that the frequency of the distribution of rainfall was on a logarithmic scale. 



Professor J. F.Danielli asked why alkalinity was so serious and Professor 

 Prescott replied that sodium carbonate made the soil impermeable and no crops 

 could tolerate an alkalinity above pH. 10.0. 



Session II 

 PLANT ECOLOGY 



Chairman Dr B. T. Dickson 



Professor F. W. Shotton asked whether the artesian water of the Bahrain Islands 

 originated from Central Arabia and Professor R. D. O'Good answered that such 

 was the local opinion. The water was believed to flow northwards towards the Per- 

 sian Gulf. Professor F. S. Bodenheimer said that between the times of aestivation 

 and hibernation there was a short, favourable period during which it would be fatal 

 for plants and animals to become active. Only in spring was the favourable period 

 long enough for development. Professor M. Zohary agreed, and added that the 

 Middle East Deserts belonged climatically to Africa rather than Asia. 



Dr C. B. Williams enquired about the possible hygroscopic value of the salt 

 crystals that encrust many desert plants, but Professor D. Thoday said that plants 

 could not absorb water from them. Professor G. E. Blackman said Professor 

 Zohary's measurements were all of dry weight and asked why he had given no mea- 

 surement of transpiration from unit areas. The latter replied that surface measure- 

 ment in desert plants in the spring was open to many errors. 



Session III 

 ENTOMOLOGY AND ECOLOGY 



Chairman Dr J.W.Evans 



Referring to Professor F. Bernard's paper Mr D. Wragge Morley said that the 

 more primitive ants were nearly always insectivorous while the more highly deve- 

 loped species were omnivorous and protected scale - insects, aphids and other harm- 

 ful plant- sucking insects. At the same time the more primitive ants, like Catagly - 

 phis could not compete with the social Monomorium. The latter and similar 'harm- 



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