446 



INTRODUCTION TO PLANT GEOGRAPHY 



[chap. 



Acacias and other members of the Pea family (Leguminosae), and, 

 in Africa, the Baobab {Adansonia digitata), with its hugely swollen, 

 water-storing trunk. They often have thick and corky, fissured bark 

 and in favourable situations may form groves. In uplands the 

 Grasses tend to be lower and more mixed with forbs, though even 

 in the lowlands some forbs are to be found — both hardy perennials 

 and others having tubers or bulbs, which enable them to burst 

 forth into leaf and flower with the recurrence of the rainy season. 



Fig. 153. — Savanna in Australia under rainfall of 25-75 cm. annually. 

 (Phot. D. A. Herbert.) 



Although many present-day savannas and treeless grasslands have 

 probably resulted from clearance of closed forest, they usually 

 experience a longer period of drought each year than do forests. 

 However, they have more frequent rains and usually less permeable 

 soil than the relatively arid thorn-woodlands. Thus the rainwater 

 that falls is readily available for the shallow-rooting, tussocky 

 Grasses whose dead straw also accumulates to form a mulch, and 

 whose close ' felt ' of roots further helps in water-retention. On 

 the other hand, tree-growth is largely restricted to places where 

 the ground-water lies at no great distance from the surface, most 

 of the successful trees ha\ ing roots penetrating deeply enough to 



