426 MISS L. S. GIBBS ON THE 
greenish film, as in European woods, is apparent and a few trees, 
like Pterocarpus angolensis with its crown of orange-coloured 
blossom, Afzelia cuanzensis and Securidaca longipedunculata, 
come into flower and leaf. 
Towards the end of October the natives systematically burn 
the veld to expose the young grass-shoots, on which the cattle 
can pasture. This means that all the standing dry grass over 
the whole country is burnt off, and with it the remains of last 
year’s herbaceous plants, which, in that dry climate, do net 
decompose, but remain as in a natural herbarium, simply dried 
up exactly as they grew. All the seed which has not fallen, 
and most of that fallen, must consequently be burnt up, 
and the bark of trees and the lower branches of shrubs singed 
and maimed; also herbaceous plants, which have been rash 
enough to send up early shoots, receive a very prompt set- 
back. Through the combined action of fires and white ants 
there can be no accumulation of humus, and the upper sur- 
face of this sandy soil is, perhaps, so dried by the long 
drought that the fires can have no physiological effect on it; in 
fact, this surface layer of loose sand probably effectively prevents 
the evaporation of water held by the subsoil, by interrupting 
capillary action. The prevalent grasses occur in tufts, always 
showing the surface soil between. Water is here the pre- 
dominant factor, for vegetation and roots go far to seek it. 
Therefore it is the subsoil which contains the vital elements, 
for these tropical latitudes, and not the upper layers, as with 
temperate surface-rooting trees. 
The practice of burning the veld obtains all through South 
Africa, South Central Tropical and German West Africa, and 
possibly over the rest of the dry tropical region of the continent, 
and must have an enormous effect on the distribution and 
survival of certain species. 
If we consider the other factors which prevail in this area, they 
all seem to make for the even distribution of a certain vegetative 
type: the summer rains, the long season of winter drought, and 
the extreme dryness of the atmosphere; even the physiological 
aspect of the country, with no great range of mountains, or 
large lake and river systems, which tend to segregate species ; 
also the habits of the natives with their internecine wars, whole 
tribes being continually on the move, cultivating the ground as 
they move on, and continually seeking fresh fields and pastures 
