THE PLANT SUCCESSION IN THE THORN VELD. 1 55 



At the present time, the stream-bank and ravine vegetation at 

 Bisley is made up of the following species, named ronghly in the 

 order of their abundance: — Acacia arabica, var. kraussiana, 

 Celastriis bit.vifoliits, Hippobromiis alata, Ehretia Iwttcntotica, 

 Jasminnni muliipartituni, Jasniinum sp. {gerrardi?) Elccodendron 

 (cthiopicitm, Zizyphiis imtcrouata, Grewia caffra, Acacia caffra, 

 A. horrida, Randia rudis, Xanthuxylon capcnse, Coinbrctmn 

 krattssii, C. salicifolia, Nitxia oppositifolia, Royena pallcns, 

 Pavetta hinccolata, P. sp. [obovata?), Cussonia spicata, Helinns 

 ovata, Gymnosporia {Celastriis) spp., Brachylcrua discolor, 

 Dovyalis (Aberia) frisfis, Azinia tctracantha, Euclea undulata, 

 E. lauccolata, Calpurnia sp., Rhus, two or three species. 



Climbing over these we have various lianes, of which the 

 woody Dalbergia obovata may be mentioned first. Species of 

 Vitis (Cissus) are also common — (Vifis cnneifolia, V. cirrhosa). 

 Clematis bracchiata. Asparagus sp., Dalechanipia capensis, and 

 the succulent Sarcostcmma viminalis also occur. The under- 

 growth is rich in herbaceous forms — e.g., Sansexnera thyrsiflora, 

 Lippia asperifolia, Barleria obtusa, S cilia sp., Rubus rigidus, 

 I ernonia corynibosa, Melasiua sp., Kalanchoe rotiindifolia, Cya- 

 fhula sp. Near the stream-bed vlei grasses. Setaria intberbis, 

 Pemiisetuiu uiiisefum, Cy perns spp., and many others occur. 

 Reference will be made later to species which are more or less 

 confined to the stream-banks ( species of Salix, Ficus, etc.), but 

 at this j)oint emphasis must be laid on the fact that all the Thorn 

 Veld species are found in such situations. The chief imj>ortance 

 of the type lies in the fact that the species are close at hand, and 

 can easily be distributed through the neighbouring grass-land. 

 Careful attention has been paid to the means of distribution 

 and to the requirements of each species. Only very few appear 

 to be able to act as pioneers in the invasion. If we desire an 

 expressive South African term, we may call them " Voortrek- 

 kers." \\'hen such pioneers have established themselves, they 

 alier the conditions in the Grass Veld, and pre])are the way for 

 other species to follow them. The methods by which this is 

 brought about forms the most interesting part of our investiga- 

 tion. In the Thorn Veld every stage of it can be seen clearly, 

 but a similar succession of events can be seen in many other plant 

 communities in South Africa. 



The Invasion of Grass-Land by Trees — Pioneer Species. 

 — The chief and only important pioneer species in the Bisley 

 district is Acaeia arabica, var. kraussiana. Acacia horrida, 

 Celastrus buxifolius, Vangncria infausta and others also act as 

 " voortrekkers," but at Bisley these species are of little import- 

 ance, compared with Acacia arabica. It is obvious that as 

 full a knowledge as possible of this chief pioneer, as regards its 

 life-history, habits, and requirements, is desirable. It begins its 

 growth as a straggling, thorny, irregularly-branched young tree. 

 Gradually it assumes the umbrella form, and is usually more 

 densely leafy than the majority of acacias. Its stipular thorns 



