1 66 THE PLANT SUCCESSION JN THE THt)RN VELD. 



is another characteristic species, often dominant or sub- 

 dominant. Myrothamnus flahcllifolius is dominant in cer- 

 tain locahties. Over steep, rocky slopes the euphorbias 

 are dominant in many of the main river-valleys. Other typical 

 rocky species are: Acacia caffra, var. rupestris, Encephalarto^ 

 altensteinii. Olea verrucosa, Celasfriis albatus, Maesa ntfescciis, 

 Plectronia ciliata, Carissa arduina. Not only are the other 

 Thorn Veld species present as well as those species named, but 

 there is also much greater variation in the succession in rocky 

 places. This may be explained by the fact that in the open veld 

 the number of pioneers is restricted, because there is no shade 

 or shelter for the seedlings, and only a few are able to establish 

 themselves under such conditions. In rocky places, however, 

 there is a certain amount of shelter provided until the young 

 plants have reached a moi;e or less permanent water supply, and 

 they are also protected from grass fires. More mesophytie 

 species, therefore, in this case are able to act as pioneers. 



(3) Moist Spots and Stream-banks. — As mentioned above, 

 the ravines and stream-banks are areas where the Thorn Veld 

 species are first distributed and enabled to establish themselves, 

 and such areas serve as bases for the colonization of the rest 

 of the veld. Once more, any of the Thornveld species may be 

 found in such situations, but there are a number of species which 

 do not leave it or do not occur, except rarely, elsewhere. These, 

 named roughly in the order of their importance, are : Sali.v 

 capensis, S. woodii, Fiats capcnsis, F. natalensis, Eugenia cor- 

 data. Acacia caffra, Rauzvolfia uataloisis. Comhretmn salici- 

 foliiim, Trema bracteolata, Pittospormn viridiflonim, Rhus 

 mminalis, Royena pallens, Macaranga capensis, Greivia lasio- 

 carpa, Melianthus spp., Cliffortia strobilifera, Trimeria alnifolia. 

 T. trincrvis, BurchelUa capensis, Voacanga drcgcl, Psoralea 

 pinnata, Bowkeria triphylla, Gnidia ovalifoUa, Ocliiia atropur- 

 piirea, Excoecaria africana, Myrica ccthiopica, Hypericum 

 lanceolatum, Dissotis eximia. In many places, however, the 

 stream-banks are free irom bush, and are fringed by various 

 grasses, reeds and sedges, Phragmites, Sctaria, Eriantlius, Typha, 

 Cyperus, etc. 



The above types are all so distinct that they might very 

 well be considered separate formations. Further research will 

 doubtless bring to light other distinct types depending on local 

 variations in climate or in soil conditions. It was noted above 

 that other species besides Acacia arabica often acted as pion- 

 eers. Taking the Thorn Veld as a whole, the essentially similar 

 Acacia horrida is more frequently the chief pioneer and the 

 dominant species. It is not so easy to explain the differences 

 between types dominated by such closely allied species. They 

 do not seem to be determined by any clearly marked variations 

 in the habitat, though further research may demonstrate this to 

 be the case. It is probable that the factors controlling seed- 

 dispersal are of considerable importance in this connection. 



