THE WATER SUPPLY OF DESERT PLANTS 



lOOi 



203 



Osmotic pressure 



30 Atm. 



98% 



Relative water 

 vapour pressunz 



100-" 



Fig. 3. Growth of shoots and roots (pea seedlings) in relation to the osmotic pressure 

 or relative water vapour pressure (from H. Walter). 



the water capacity) and in dry soil (40% of the water capacity). During the 

 first week the total increase in dry weight of the plants was the same in 

 both cases (16 g), although the ratio of the shoots to the roots decreased 

 appreciably in the dry soil (Table 3). 



Table 3 



Trifolium ituarnatum. Percentage of total dry weight of plants in 



moist and dry soil 



Plants in 



I. Shoots 2. Roots 



Ratio 1 : 2 



Moist soil 

 Dry soil 



66-0% 

 41-4% 



34-0% 

 58-6% 



1-94 

 0-71 



