138 
as seen from above is fairly level. Much of the branching occurs 
at two-thirds to one-half the height from the ground, and many 
trees form only two, or three, primary branches; the branching is 
mostly at an angle of 30° to 40° so that the spread of the head is 
narrow in proportion to the size of the tree (see Plate II, fig. 1). 
A few large trees occur here and there which have a much more 
spreading habit, eg., the nGanza (Brachystegia siivivatiefolea) 
and the mooSaamba makooba (see Plate II, fig. 2). 
Soil Requirements.—The reproductive capacity of a tree, 
and its soil requirements, are important factors in its economic 
exploitation. Species which require very rich soil, or which are 
reproduced with difficulty, cannot be utilized with profit unless 
the products obtained are of high commercial value. But given 
the opposite conditions, even cheap products may be handled 
profitably. Several of the Brachystegias are found occupy- 
in supplies of plant food that many other forest trees of the same 
Phyto-geographical Region fail to grow there. This is so marked 
a feature of some species of this genus, that local residents of 
Rhodesia and the Katanga are able to say that where certain 
Brachystegias are gregarious, the soils are very poor and unfit 
for agriculture. This may not apply to all the species, but it 
is noteworthy that on o, strips of deeper-and richer soil along 
the stream banks, Brachystegias are largely replaced by trees 
of other genera. 
Seed Dispersal.—Brachystegia pods are woody and very 
rigid. When ripe, these pods open with explosive violence, under 
changes of temperature, each valve of the pod twisting corkscrew 
fashion with such force that the seeds are shot out. The twiste 
valves are so rigid that any force applied in an attempt to 
straighten them out results in the valve breaking into sections. 
The seeds are large, flat, hard and shining, and during the 
fruiting season can be picked up in quantity beneath the trees. 
The dense growth of seedling Brachystegias on abandoned 
agricultural land in Portuguese E. Africa, has already been 
referred to. Coppicing from the stumps of cut-over Brachystegia 
forest is very noticeable in the Katanga Province of the Belgian 
ongo. 
That Brachystegias will thrive on comparatively thin and 
poor soils, that seed is readily obtainable, and that the trees 
reproduce freely from seed and by coppicing, are important 
economic factors in the problem of afforestation of tropical soils 
of poor quality. 
VERNACULAR Nags. 
There is some confusion in the application of native names 
to the trees of the Katanga Province, due to the number of native 
tribes and languages there brought together from different 
indus of Central Africa for the development of the copper-mining 
industry. 
