i8.— ON SOME ASPECTS IN THE VEGETATION OF 

 SOUTH AFRICA WHICH ARE DUE TO THE PRE- 

 VAILING WINDS. 



By R. Marloth, Ph.D., M.A. 



In tracing the relations between the climate of a country and its 

 vegetation, it is often thought sufficient to discuss the temperature, 

 i.e., the mean for the year or the months with the extremes of heat 

 and cold, and the rainfall, viz., its total amount and the distribution 

 over the different seasons. One calls the climate of Eastern South 

 Africa the reverse of that of the West simply because their 

 rainy seasons are opposite to each other. 



It is, consequently, often overlooked, that climate with regard to 

 vegetation includes several other factors, which have a considerable 

 influence on the structure and aspect of plants, and that some of 

 these are capable of modifying the effect of the two principal con- 

 stituents of the climate to a large extent. One of these important 

 factors is the relative humidity of the air, which does not necessarily 

 go parallel to the rainfall. Further, the annual and daily amount 

 of sunshine, for light is as necessary to the living plant as heat and 

 water. One of the reasons why many Cape plants do not thrive in 

 English conservatories is the curtailing of the supply of light to which 

 they are accustomed. 



Quite as important, however, as these factors, is the wind, if it 

 does not even exceed their influence considerably. 



The various ways in which the wind affects the vegetation of 

 South Africa may be grouped under three heads : — 



1. Mechanical effects. 



2. The exhausting action, which it exerts on the leaves and 

 growing points of plants. 



3. The supplying of moisture to the vegetation of some of our 

 mountains by means of the clouds which accompany certain 

 kinds of wind. 



Everybody is familiar with the shorn shrubs and dwarf trees 

 along the seashore. The wind, often charged with salt spray or 

 sand, destroys every leaf or twig, which projects above the sheltering 

 rocks, and gives a slanting face to the top of the shrubs on the wind- 

 ward side. On the shores of False Bay, and even a mile or two 

 inland, such wedge-shaped bushes are very common ; but, of course, 

 also everywhere else along the coast right up to Algoa Bay and East 

 London. Some of them are as sharply defined as if they had been 

 kept constantly under the gardener's scissors. 



In other cases the pressure of the wind has forced the trees 

 over to leeward. One may see whole rows of pines or eucalypts at 

 Salt River and other equally windy places, which are leaning over to 

 a considerable extent. Even groves of silver trees exhibit occasionally 

 the same phenomenon, and in the coastbelt near East London stand 

 thousands of dwarfed mimosa trees leaning over in the same direction, 

 with a crown that is as flat as a table, all branches having been shorn 

 down to the same plane by the sea wind. 



