2i6 Report S.A.A. Advancement of Science. 



The exhausting and destructive effect, which strong winds exert 

 on the leaves of plants, is well seen after a South-East storm. The 

 oaks and other soft-leaved plants look as if they had been scorched 

 on the weather side. This is due to the enormous increase in the 

 transpiration of the leaves, produced by the air which rushes past 

 their surface with great velocity. As the supply of water from the 

 roots cannot replace the loss with sufficient speed, the cells of the 

 leaves are killed and the leaves dry up. Many introduced shrubs and 

 trees suffer in a similar way, and many kinds cannot be reared in 

 exposed situations, as e.g., chestnuts and horse-chestnuts, but do 

 thrive in sheltered nooks and valleys. 



Quite different is the appearance of the indigenous trees of the 

 Cape, even after the severest storm. While oaks are scorched, and 

 even eucalypts and pepper trees {Schinus molle) seriously damaged, 

 the olives and proteas show no sign of injury to their foliage. Much 

 less do this the hundreds of smaller shrubs and shrublets, which form 

 the plant covering of the South-Western districts. The explanation is 

 simple. Their leaves are of tough and leathery texture, they are 

 trained to such extreme conditions ; in fact, the wind has, to some 

 extent at least, been instrumental in gradually producing them bv a 

 kind of natural selection. Almost all shrubby plants of the South- 

 western districts possess such leathery leaves, hence this part of 

 South Africa is designated by ecologists as one of the typical regions 

 of sclerophyllous plants. 



It must not be thought that the wind is the only cause of this 

 special feature in our vegetation, but it has certainly had a consider- 

 able share in its production. 



The influence of this action of the wind is even more conspicuous 

 in other cases. In localities, which are regularly exposed to strong 

 winds from the same direction, one finds the trees often without 

 branches on the weather side. Near Cape Town there are many 

 pines on the slopes of the Devil's Peak with a perfectly straight and 

 vertical stem, but bare on the Southern and South-Eastern sides. 

 This flagstaff-like appearance is not caused by the removal of the 

 branches on this side, but by the destructive effect, which the wind 

 has on the soft tissue of the buds in spring. Only the terminal shoot 

 and those on the leeward side are allowed to develop, while the others 

 are destroyed as soon as they show themselves. 



While the effects of wind described so far are not specifically 

 South African, as many other countries exhibit the same or similar 

 phenomena, there is one feature of the wind that prevails over a large 

 part of South Africa during the summer months, which has no where 

 else such a preponderating influence on the vegetation. That is the 

 cloud covering of many of our mountains which generally accompanies 

 the South and South-East winds and which supplies a considerable 

 amount of moisture to the vegetation of the mountains during the 

 season, which is otherwise dry. It is this property of the South-East 

 wind which must be looked upon as one of the chief agents in the 

 delimitation of the area or areas of that famous vegetation of the 



