210 THE KKFKCT OK SLOPE EXPOSURE. 



itself may, then, still further modify the climatic conditions, for 

 example, the presence of forest will increase the rainfall in its 

 particular area. 



The full effect of differences in the exposure to different winds 

 with the resulting differences in rainfall, relative humidity and 

 evaporation can only be gauged by a series of observations extend- 

 ing over at least a year, and the writer is endeavouring to obtain 

 experimental plots for the purpose of such continuous investigation. 

 The differences in sunlight intensity and other factors, which are 

 at least partly correlated with it, may, however, be measured in 

 the course of a single day. In fact, in the case of this particular 

 factor, measurements throughout the day are of considerable 

 interest, since the duration of high intensities is probably quite as 

 important as the total insolation in its effect upon the vegetation. 



For this preliminary work a typical hill — Signal Hill — was 

 chosen on account of its proximity to Maritzburg. This is a low 

 hill running east and west, that is, its slopes face north and south. 

 The northern slope is covered with grass land in which the 

 dominant grass is Aristida junclfor))iis, which has largely replaced 

 Therncda triandra. Scattered throughout the grass land are 

 numerous clumps of trees and bushes principally composed of 

 (Jussonia spicata, Combretum kraussii and Clerodendron gJabrum . 

 Scrambling over the clumps are climbers such as Ruhns pinnatiis. 

 Smodingium argututn, Cissiis cuneifolia- and others. The 

 succession in these clumps has already been analysed and 

 described by the writer in a previous communication to this 

 Association.^ 



The southern side presents a decided contrast in appearance 

 to the northern, but is less uniform in the character of its vegeta- 

 tion. On the less steep slopes and the flatter terraces the grass 

 land closely resembles that on the northern slope, Aristida again 

 being the dominant species. The characteristic clumps of the 

 northern side are entirely absent, but on the steep slopes facing 

 practically due south there occur unmistakably the early stages 

 of bush formation. There is little reason to doubt that these 

 slopes were at one time covered with dense bush which has been 

 destroyed by fire and is only slowly regenerating. Eecurring fires 

 retard the succession, and will probably effectually prevent its ever 

 progressing far beyond the early stages. Tall mesophytic grasses 

 and shrubs (principally belonging to the Compositae, for example. 

 Printzia spp.) form a dense covering of the slope, and scattered 

 amongst these are a few young trees of Cussonia spicata and 

 Macsa rufcsccns. Ferns also occur, such as Mohria caffrontvi 

 and Ptcris aquiliua (around the edges of the developing bush), 

 though they are entirely absent from the northern slopes. When 

 the trees of Ciissoriia spicata- are more closely examined, they are 

 found to have arisen as coppice shoots from the old stumps of 

 large trees, which have been destroyed apparently by fire. In 

 some cases quite young trees have been fovmd springing from very 

 large stumps, which are practically buried in the soil. 



