14] VEGETATIONAL TYPES OF TROPICAL LANDS 443 



trunk swells to form an almost barrel-shaped structure. There are 

 also sometimes tracts dominated by arborescent succulents, including 

 giant Spurges {Euphorbia spp.) in the Old World and characteristic 

 members of the Cactus family (Cactaceae) in the New World. A 

 few xeromorphic herbs may occur, such as terrestrial Bromeliads 

 with sharp-edged leaves. The accompanying shrubs are no less 

 xeromorphic, being often thorny Acacias or other members of the 

 Pea family, and sometimes forming a grey bushy jungle 3-5 metres 

 high, while a few thin woody climbers may also occur. Epiphytes 

 are usually absent, though Spanish-moss {Tillandsia sp.) may be 

 plentiful locally. With the onset of the rainy season the leaves and 

 flowers emerge and vast numbers of geophytic herbs may spring 

 from the soil, the rhythm of life being even more strongly marked 

 than in the monsoon forests. 



Tropical thorn-woodlands and -parklands, etc., are widely developed 

 in drv regions : for example, in northeastern Brazil and elsewhere 

 in tropical South America, in the islands of the Caribbean as well 

 as in Central America and IVIexico, and in and about the Sudan and 

 regions bordering on the southern Red Sea and the Gulf of Aden. 

 They also occur in southwestern Africa, in India, and in the west 

 and centre and northern hinterland of Australia. Thorn- woodlands 

 and allied types often occupy sandy or limestone soils that are very 

 permeable to water, and alternate with savanna which tends to 

 prevail on the stifi^er soils that retain rainwater near the surface. 

 In places where there is a local increase in humidity, as for example 

 in depressions or ravines, this savanna passes to savanna-woodland. 



Tropical and Subtropical Savannas and Other Grasslands 



As in other major regions, grasslands in the tropics and subtropics 

 constitute one of the main vegetational types and are usually, but 

 not always, dominated by members of the Grass family (Gramineae). 

 For sometimes grass-like plants of other affinity (particularly of the 

 Sedge family, Cyperaceae), may be the dominants over considerable 

 areas, or plants of quite different types may play a similar role. 

 Savannas are by some considered synonymous with treeless grass- 

 lands or steppes but in the present work the term is employed to 

 denote areas which ecologically speaking appear to be true grass- 

 lands, in that Grasses or similar herbs seem to be the real dominants, 

 but in which trees or tall bushes (in ' bush savanna ') occur in open 

 formation and give a particular character to the landscape. Nor- 



