THE PLANT ECOLOGY OF DRAKENSBEBG RANGE. 527 



which have been shown to give rise to them are as follows : 

 A barometric depression exists at the time somewhere oiT the 

 coast. The air at lower levels is drawn out towards this 

 region, and the air from above has to descend to replace it. 

 Their local variations depend on the complex topography 

 which interferes Avith the movement of the air. 



The south-eastern slopes are not affected by them to the 

 same extent as the opposite sides of the hills, which meet 

 their full force. Shelter and protection from the Foehn 

 winds is a most important point in connection with the 

 distribution of bush. The hot winds begin often from north, 

 gradually shift to north-west and west, and then the wind 

 veers round to the south and rain comes. Probably this is 

 due to a gradual shifting northwards of the main barometric 

 depression (cyclone). Hot winds are fairly general over 

 Natal, although sometimes they are more local in their nature, 

 caused by local barometric depressions. Though the Foehn 

 winds may occur at any time of the year, they are most 

 frequent and regular in their occurrence during the early 

 spring, August and September. The ascending and descend- 

 ing winds are of importance also in connection with the 

 distribution of seeds. 



III. THE PLANT FORMATIONS AND 

 ASSOCIATIONS. 

 The mountain region in Natal is of surpassing interest from 

 the ecological standpoint, because here we are able to observe 

 the birth and growth of the types of plant formation,^ which 

 have become more fixed and definite at the lower alti- 

 tudes. It is all carried out on a magnificent scale, and 

 the distances to be traversed in studying the different types 

 are very great ; but the succession and inter-relationships 

 of the different plant formations is, on the whole, simple. At 

 first it was thought advisable to divide the region into two 



' The term "formation" is here used in the sense indicated by 

 Warming, e.g. the "bush" (forest) of Natal is one formation, the 

 associations, such as yellow- wood bush, being local. 



VOL. 3, PART 3. 35 



