THIC PLANT ECOLOGY OF DKAKENSBEKG KANGE. 525 



summer rainy season. The rain-cloads are carried across 

 tliem without depositing moisture, and the conditions which 

 produce valley mists in winter (calm air, clear sky, intense 

 radiation) are not present. 



As regards the diu^rnal variation in deposition, the fore- 

 noons commonly i-emain clear. Clouds begin to gather in 

 the early afternoon, ending in a violent thianderstorm, which 

 afterwards leaves the sky clear towards night. In connec- 

 tion with the humidity of momitain regions, certain other 

 factors must be considered. Evaporation at high altitudes, 

 owing to the diminished pressure, is greatly increased. This 

 factor in itself has an important effect on the vegetation. 

 The abundance of xerophytic characters seen in the vegeta- 

 tion is one result. During the spells of fine weather the air 

 is not only rarefied, but also very dry. 



Tlie zones of maximum deposition naturally bear the 

 densest vegetation. If the soil conditions are suitable, the 

 type of plant formation is usually bush or dense scrub. 



By the greater amount of transpiration thus induced, a 

 local increase in humidity is caused as well as a cooling of 

 the surrounding air. The water thus evaporated cannot 

 ascend over the mountains, so it is again deposited. Moun- 

 tains thus tend to keep their own moist atmosphere. 



None of the mountain peaks of the Drakensberg rises 

 above the climatic snow-line, which, in the latitude of Natal, 

 would be about 4000 m. (13,000 ft.) or probably higher. In 

 winter, however, the main range of mountains is frequently 

 covered with snow for longer or shorter intervals. At ex- 

 ceptional times, all the foothills and the isolated peaks of the 

 Midlands also have snow. Such snowstorms follow on 

 atmospheric disturbances, the cause of which it is unneces- 

 sary to enter into here. They occur only rarely, at intervals 

 of several years. The reasons for the general absence of 

 much snow are to be found in the dryness of the atmosphere 

 and the lack of precipitation during the cold season. 



The Drakensberg, therefore, as already mentioned, 

 shows none of the retarding effects of the melting snow 



