274 R. Marloth, Ph.D., M.A. — The Acacias oj Southern Africa. 



epidermis consists of comparatively large cells with thin outer-walls^ 

 but in the latter the cells of the tissue close tightly together and the- 

 cells of the epidermis are provided with a very thick outer-wall. 

 This wall measures in the former case only 3 to 5 jw, but in the latter 

 15 to 18 ju. The Karroo-thorn like the ana-tree grows only along the 

 rivers, or at least on such spots where underground water exists, and 

 they can therefore well afford to lose a conssiderable amount of mois- 

 ture by transpiration, but the inhabitants of dry hills or plains, such 

 as the camelthorn and the hook-thorn for instance, must economize 

 the precious liquid and prevent its loss by all available means. 



But it is not only against the climate that the acacia-leaf must be 

 protected. 



What purpose, the inquisitive observer of nature will ask, is served 

 by all these nasty spines and prickles, which appear to be there only 

 to worry the traveller ? The answer to that question is not far ta 

 seek. Linnseus already called the thorns the weapons of the plants, 

 and it strikes us at once, that the spines and hooks of the acacias 

 protect their leaves against the attacks of grazing animals. In a 

 desert like the Kalahari where no blade of grass can exist for many 

 months of every year, whilst the country often teems with large 

 game, the delicate leaves of the acacias would be completely destroyed 

 year after year were they not so well protected. 



I have made an interesting observation bearing on this question. 

 Young bushes of the karroo-thorn as well as of the camelthorn 

 possess always the longest and most numerous spines, whilst those of 

 older and higher bushes are reduced in size and number. Young 

 twigs of the camelthorn for instance bear spines about two inches 

 long, but on older trees they seldom reach half an inch. As long 

 as the bush is low its leaves must be well protected, but once grown 

 to sufficient height it is no longer exposed to the attacks of game, and 

 the material which is not required for the construction of spines can be 

 employed for other purposes, as for instance, for the production of fruit. 



These relations between the acacias of South Africa and their 

 surroundings remind us that if we desire to understand the pecu- 

 liarities of the vegetation of a country we must study, not only the 

 floral characters of the plants, but also their biology, for the members 

 of the vegetable kingdom are as well adapt ed to the peculiar require- 

 ments of the position which they occupy as animals to theirs. I 

 think it is apparent even from the few observations which I have men- 

 tioned above, that this group of plants serves remarkably well to- 

 illustrate this natural law. 



