304 H.WALTER 



In this case the decreasing shoot/root ratio is attributed to a functional 

 adaptation. Due to the severe competition in arid regions, we find only 

 species with a fast-growing, genetically fixed root system. 



In an area where the rainfall becomes less than lOO mm, the second 

 factor, the run-off, is more important. This is primarily due to the 

 low density of the vegetation, and consequently even in nearly flat deserts 

 the run-off is extremely high. The water penetrates into the soil more 

 readily in sandy runnels or depressions moistening the soil to a greater 

 depth. Following the rain the upper layer of the soil dries out, although in 

 the deeper layers the moisture may be conserved for years. 



The type of vegetation also changes under these conditions, The 'diffuse 

 type, that is a vegetation more or less evenly distributed, gives way to a 

 'restricted type', a vegetation confined only to rather restricted areas 

 (depressions, runnels, wadis). In this type of area, the water supply is 

 usually adequate. 



If the run-off in a region with 25 mm precipitation is 80%, and the area 

 in which the water accumulates constitutes 4% of the total area, that area 

 will receive an amount of water corresponding to a rainfall of approximately 

 500 mm. This is an appreciable amount of moisture in an arid region. 



The root systems of plants in this type of situation are quite deep and 

 correspond to the deeper water penetration. Recently Kausch (i960) 

 investigated the root systems of desert plants around Cairo and recorded 

 the following root penetration depths : 



Pituraiithus tortuosus 5 00 m 



Mohkia callosa 4-00 m 



Farsetia aegyptiaca 3 • 3 o m 



Zilla spinosa 2-80 m 



Convolvulus lanatus 2-50 m 



Euphorbia cornuta 2-30 m 



Centaurea aegyptiaca 2-10 m 



Fagonia arahica i-20 m 



These observations show that the water supply of the plants even under 

 extreme arid conditions is quite adequate. Therefore it is not surprising 

 that the osmotic pressure of the cell-sap of desert plants is not much higher 

 than of plants in humid regions and also the transpiration per unit leaf 

 surface may be quite high. 



The problem of halophytes is a special one. For them, the water economy 

 is usually not of primary importance but rather the salt economy. 



The adaptation of the perennial desert plants seems to be chiefly a 



