THK I'LANT SUCCKSSION IN THE THORN VELD- 



157 



plant may be scorched right to the top, yet new growth soon 

 appears, and a year later it is little the worse. The root-system 

 may be seen exposed where new dongas form, close to thorn 

 trees. ( See Plate 6 b. ) The main tap-root is vigorous in its 

 growth, and descends as deeply as the soil permits, and often 

 reaches a depth of many feet. It makes its way even through 

 the shale as far as possible. The thorn-tree is thus rendered 

 independent of atmospheric precipitation during the long dry 

 season. In addition, however, to the main tap-root, there are 

 usually a number oi large side-roots, which spread out hori- 

 zontally for great distances near the surface of the soil. These 

 utilize such water as does not penetrate to any depth. The 

 general appearance of the root system is illustrated in Fig. i. 

 The most important thing with regard to a pioneer species such 

 as Acacia arabica is that it must be able to establish itself with 

 no further shade or protection from the beginning than what 

 is afforded by the surrounding grasses. Pioneer species can 

 nexer at any stage of their life-history be ombrophilous, or at 

 any rate, they are always less so than the species that follow 



3 



4 



'I .V- r 



Fig. 2. — Diagrammatic representation of different stages in the Thorn 



Veld succession. 



them. The critical period for the pioneer must be the earliest 

 stage of the seedling, before its root has penetrated down to 

 the deeper subterranean sources of water. Probably the later- 

 ally extended branch-roots which are near the surface help it 

 to tide over this period by utilizing surface water. It must 

 also be able to resist grass-fires 



For the first six years the tree is straggling in growth 

 and has little effect on the grass around it ; but as soon as it 

 begins to assume the umbrella form it throws enough shade to 

 change the nature of the undergrowth. (Fig. 2, Stage 4.) At 

 first the grasses grow taller, being to a certain extent etiolated. 

 The soil becomes moister and earthworms more active. Their 

 castings undoubtedly play an important part in changing the 

 soil conditions and preparing a suitable seed-bed. Very often 

 the white ant (termite) commences operations, if indeed it 

 was not present before the seedling thorn tree, for, as will be 

 pointed out later, seeds of all the species are apt to be dis- 



