Between the arid and sandy northern wastes and the well-watered and arable Sudanese lands there is a transitional zone of level grassy steppes (partly covered with mimosas and acacias) with a mean breadth of about 60 m.
On the plains where grasses cannot find sufficient moisture their place is taken by " bush," composed mainly of stunted mimosas, acacias, euphorbia, wild pomegranate, bitter aloes and herbaceous plants.
We have wild olive, species of rock-rose, wild privet, acacias and mimosas, barberry and Zizyphus; and in the eastern ramifications of the chain, Chamaerops humilis (which is applied to a variety of useful purposes), Bignonia or trumpet flower, sissu, Salvadora persica, verbena, acanthus, varieties of Gesnerae.
The most common families on the eastern slopes, where the precipitation is heavy, are the magnolias, crotons, mimosas, acacias, myrtles, oaks, plane-trees and bamboos.
Timber trees are almost confined to the river valleys, where willows, yellow wood, iron wood, red wood, mimosas and, in deep gorges, the wild fig are found.