THE PLANT ECOLOGY OF DK'AKENSBERG RANGE. SIO" 



moimtain streams, which is clear as crystal, is very soft,, 

 showing absence of lime. 



On the top of the upper portion, from the Mont aux Sources 

 southwards, a distinct type of soil and plant formation 

 occurs — the mountain-top detritus. This will be described in 

 detail later, when the vegetation is dealt with. 



II. CLIMATE. 



A. Atmospheric Pressure and Insolation. 



The feature of mountain climate which shows the greatest 

 regularity is the decrease of pressure with increasing altitude. 

 It must naturally come first in our consideration, since so many 

 of the other features depend on it. It varies with tempera- 

 ture and therefore with latitude, and consequently the decrease 

 is not quite so great in Natal ns in colder regions. The 

 irregular changes in barometric pressure decrease with alti- 

 tude in the same ratio as the pressure itself decreases. On 

 the summit of the DraUensbeig the pressure is from 150- 

 250 mm. less than at sea-level, where it may be assumed to- 

 be 762 mm. (30 in.). 



As a result of the decrease of pressure there is increased 

 intensity of insolation, since the thickness of absorbing atmo- 

 sphere is less. Water- vapour, which is a better absorber of 

 solar radiation than dry air, decreases with altitude more 

 rapidly than pressure decreases (vide infra). Atmospheric 

 dust, which aifects chiefly the shorter wave-lengths, is com^ 

 paratively small in amount, consequently there is a marked 

 increase in the more chemically active rays. The great 

 intensity of the ultra-violet rays at high altitudes has been 

 demonstrated by many observers. Bunsen and Roscoe many 

 years ago gave the following figures regarding the chemical! 

 intensity of sunlight at different altitudes, expressed in per- 

 centages of the intensity just outside the earth's atmosphere,, 

 which they computed to be equal to 35*3 " light units." 



