ANIMALS— PHYSICAL ENVIRONMENT 109 



The effect of the wind upon the flora Indirectly 

 affects the fauna. To the plants it is harmful 

 both by reason of the physical damage done by 

 violent gusts and wind-blown sand and small 

 pebbles, and also because it rapidly dissipates the 

 sHghtly moist air which accumulates round the 

 plant owing to transpiration. Increased transpira- 

 tion results, that is to say, increased loss of water 

 which is already very scarce : it foUows that plants 

 may be able to exist on a specified amount of water 

 if they are protected from wind, and may be killed 

 if they are exposed to the wind. 



This is well seen wherever a wind of constant 

 direction blows through a narrow valley ; any 

 large rock which breaks the force of the wind, 

 and thereby reduces evaporation and transpiration 

 under its lee, shelters a richer flora than wiU be 

 found elsewhere. 



It is in fact true that places exist which are desert 

 primarily because of wind. For instance, through- 

 out great parts of the Libyan Desert the distribu- 

 tion of plants is determined rather by the presence 

 of shelter from wind than by any other influence. 



The effect of a strong wind loaded with sand upon 

 the very scanty spring vegetation on the Libyan 

 border of Lower Egypt is thus described by Thomas. 

 "On April 13, towards the close of the growing 

 period of most plants, a hot wind blew in from the 

 desert for about five hours, raising great clouds of 

 sand which drove along the surface of the groimd 

 like mist. The temperature went up considerably, 

 in spite of the sun being obscured, and was over 



