THE PLANT SUCCESSION IN THE THORN VELD. 171 



will be seen that it is essentially similar. The pioneer species 

 are the same as those represented in the early stages, as de- 

 scribed above, their fruits being" such as are eaten by birds. 



Summary and Conclusions. 



Since Tree Veld is such an extensive and important type 

 in South Africa, as well as in the continent as a whole, the de- 

 tailed investigation of the plant succession in such a type is 

 of the greatest importance. The species of trees, shrubs and 

 lianes composing the Thorn Veld first establish themselves in 

 the ravines and near the stream-banks, where the birds and other 

 animals which serve chiefly as distributing agents first bring 

 the seeds. Such areas serve as bases of colonization for the 

 intervening areas. 



The acacias, especially/^ rac/a horrida and A. arabica, var. 

 kraussiana, are the chief pioneer species. They are shown to 

 be well adapted to establishing themselves with no shade, shelter 

 or protection against grass fires. 



After they have prepared the way, a great many other 

 species follow them, and after germinating in the seed-bed 

 prepared for them by the activity of earthworms, termites and 

 ants underneath the thorn-tree, they grow up in the shade. 



Various stages have been traced, and ultimately the subse- 

 quent species may kill the pioneer. As a rule, however, the 

 thorn-trees remain dominant in what is, at present, over the 

 largest areas, the final stage (Thorn Scrub). 



A list of over 230 species of trees, shrubs, and lianes be- 

 longing to the Thorn Veld is given with symbols denoting fre- 

 ({uency, and the undergrowth is also described. 



Three important sub-formations are distinguished, viz., 

 Thorn Veld towards the coast, the Thorn \''eld of Rocky Placci 

 and the Stream-bank type in the Thorn Veld area, but it is 

 pointed out that these might very well be considered as separate 

 formations. 



The question of seed-dispersal is discussed at length. Wind 

 distribution plays a relatively unimportant part. The chief 

 agents of dispersal are birds, but for species with capsular 

 fruits and small seeds, ants, which are very abundant in the 

 Thorn Veld area, are also important in connection with dis- 

 persal. The influence of white ants (Teniiites) on the plant 

 succession is also discussed. 



In conclusion, I must express my thanks to Mr. T. R. Sim, 

 who is always ready to put at my disposal his unrivalled know- 

 ledge of the distribution of species in South Africa, and also to 

 Mr. J. S. Henkel for many helpful suggestions. 



Explanation of Plates. 



Plate 6, a. — Acacia arabica var. kraussiana. growing near a nest of 

 Tcrmes trincrvius. Note the umbrella form. The undergrowth has 

 here been kept down by goat grazing. 



