DIFFICULTIES OF EXISTENCE 171 



primitive soils, and around them wind-blown dust 

 and debris of all sorts accumulate. 



Once a seed becomes established on a primitive 

 soil on some rocky ledge, free from snow perhaps for 

 less than two months in the height of summer, it 

 has to face certain other difficulties. The gap 

 between the extremes of temperature to which it will 

 be exposed is enormous. The temperature in the 

 sun at midday may rise to nearly 20 C. ( = 68 F.), and 

 sink several degrees below zero C. at night-time. 

 In winter-time it may fall to - 25 C. or more. Fur- 

 ther, the difference between sun and shade tempera- 

 ture in the daytime is much greater than at lower 

 elevations, so that whenever the sun is hidden by 

 a passing cloud, a sudden and considerable drop in 

 temperature takes place. 



Another difficulty often to be faced is the lack of 

 an adequate water-supply for the roots. This is not 

 due, as is sometimes supposed, to the coldness of the 

 soil, for it has been shown that, in summer at 

 any rate, the soil is several degrees warmer than the 

 atmosphere in the shade, and that the soil is generally 

 at a higher relative temperature in the Alps than in 

 the lowlands. But at high altitudes rain rarely falls, 

 and such water as penetrates to the roots is derived 

 from melting snow. Since the supply is precarious, 

 many High Alpine plants possess hairy coats or 

 other adaptations, designed, as much as possible, to 

 reduce transpiration or loss of water by evaporation. 



Other drawbacks to life in high altitudes are the 



