154 THE PLANT SUCCESSION IN THE THORN VELD. 



rapidly, and excellent opportnnities for studying the succession 

 are afforded. 



Type of Grassland Invaded. 



The soil varies within certain limits. It is mostly of the 

 hard-baked type, badly aerated, but rich in chemical salts. The 

 iron it contains exists as the protoxide (FeO). About a foot 

 below the surface ironstone gravel (limonite) is frequent, and 

 nodules of an impure magnesian limestone. There is a certain 

 amount of organic matter in the soil, more than in the High-Veld 

 soils, consequently it is dark in colour. In places it is shallow, 

 but the numerous dongas (see Plate 6 b) show that it often 

 reaches a depth of several feet, and below the soil the shale is 

 often loose and broken. Where the veld is unchanged, the Low- 

 Veld variety of Anthistiria imberbis is completely dominant, but 

 there is usually a considerable admixture of other sjiccies of 

 grasses — Andropogon hiriiis, A. sclweiiantliits, var. versicolor, 

 A. aniplectens, A. perfusits, A. aiictiis, A. schireiisis, A. pUtrino- 

 dis, A. appendiculatiis, Aristida angustata, A. junciformis, Era- 

 grostis curvula, E. chalcantha, E. brizoides, Cynodon dactylon, 

 Tristachya lencothrix, Sporobolus iiidicits, Chloris petrcca, C. 

 pycnothrix, Imperata arnndinacca, Paiiiciiui spp., Digitaria spp.. 

 El en sine indica. 



Scattered through the grasses there are numerous associated 

 plants, of which the commonest are: — Dicoiiia argyrophylla, Cor- 

 chorus asplenifolius, Hypoxis laiifolia, Abittilou souneritianum, 

 Sida longipes, Hermannia sp.. Maker uia grandistipida, Lasio- 

 siphon linifolins, Crotalaria distaiis, C. globifera, and other 

 species, Indigofera spp., Leucas martinicensis, Ocimiim sp.,01den- 

 landia (Hedyotis) amatymbica. Acalypha sp., Laefiica capensis, 

 Hclichrysiim spp., Asclepias spp., Gasanea longiscapa, Alo^ 

 saponaria, Scilla spp. There are, of course, a great many others : 

 a complete list would run to several hundred. Gr,\ss fires and 

 grazing by stock have a great effect on this veld, the general 

 tendency being for the Anthistiria to give way to other species of 

 grasses, such as Sporobolus indicns. 



Bases of Colonization. 



The banks of the streams and the numerous deep ravines 

 serve as bases of colonization for the numerous species which, 

 given an opportunity, invade the veld. In such sheltered situa- 

 tions grass fires have little effect, nor, owing to their inaccessa- 

 bility, are they much disturbed by man. The various animals, 

 including birds, which act as agents of distribution, congregate in 

 such places, so that seeds are brought from a distance, and in 

 new areas of grassland tree-growth is first established in the 

 ravines. 



Good historical evidence from the residents near Bisley 

 proves that, 20 or 30 years ago, the thorn-trees in this neighbour- 

 hood were confined to the bottom of the valley near the stream. 



