134 



weed stages are undesirable and some are particularly 

 "noxious," e.g., Xanthiu/n, Acanthospermum, Cnicus, Cen- 

 taurea, Tagetes, Schkuhria, Galinsoga, Rumex, Amarantus, 

 Cyaihula, Striga, Phytolacca, Argemone, Alternanthera, etc. 

 Others are eaten by stock, e.g., Bidens pilosa, the Blackjack, 

 one of the commonest. Many of the grasses are palatable and 

 nutritious, e.g., Digitaria ternata, Eleusine indica, Chloris 

 spp. What seems rather surprising is that farmers do not 

 more often try to control the succession. It is usually left to 

 chance to determine what seeds are present, and what crop 

 of ruderals will come up after the cereals, etc., are reaped. 

 Some farmers do encourage such species, as they have proved 

 capable of providing good pasturage, but a great deal more 

 might be done, by sowing the seeds of the better species or 

 of exotics if these are found to be more suitable, though some 

 farmers object to this method on the ground that it spoils the 

 main cereal crop. 



Other Subseres. The Plant Succession in Dongas. 



The study of this succession has a very important bearing 

 on the question of soil erosion. While soil erosion must 

 necessarily continue to take j)lace at a certain rate, there 

 is little doubt that since the laud has been taken up by white 

 settlers in South Africa, it has largely increased. The chief 

 causes have already been referred to. The burning- of the 

 grass lays bare the surface of the soil, and tends to destroy 

 the protective mat of vegetation. It tends to send back the 

 plant succession to the earlier stages, where the grasses grow 

 in more isolated tufts. The pasturing of large herds of cattle 

 and sheep tends to the hardening of the surface layers of soil, 

 which increases the run-off of water, and the formation of 

 numerous cattle tracks tends to guide the flow of water into 

 definite channels, which quickly deepen into Dongas. The 

 process is always to a certain extent checked by the vegeta- 

 tion itself, and incipient Dongas are sometimes filled with 

 rank growth, which prevents further erosion. 



The species, which occur, depend on the size which the 

 donga has attained. Pteris aquilina, the Bracken fern, some- 

 times fills shallow dongas. Some of the species of Cyperus, 

 Mariscus, and Carer act in a similar manner, and among the 

 grasses the more deep rooted species are of prime importance. 

 The Aristidas and Eragrosti.t species are much in evidence, 

 as well as the species described above as belonging to the 

 hydrosere, especially the later stages of it, with Pennisetums, 



