14] VEGETATIONAL TYPES OF TROPICAL LANDS 447 



tap this more lasting source. They also tend to have relatively low 

 and compact crowns, often of spreading, umbrella-like shape, 

 allegedly as a result of exposure to winds. But in many areas they 

 are kept at bay or ousted by burning, or by the intensive grazing 

 of herds of wild or domesticated Mammals ; of these, many of the 

 world's largest are inhabitants of the great grassy plains of tropical 

 and subtropical regions. 



Semi-Desert Scrubs 



The arid bushlands characterized by scrubby Acacias and other 

 xeromorphic shrubs are often included among deserts, though 

 actually it seems preferable to consider them as transitional between 

 true deserts and savannas or thorn-woodlands. In tropical and sub- 

 tropical regions they are found particularly on stony or rocky 

 hill-sides, in open rolling country, and on sandy or gravelly areas 

 exposed to the full glare of the sun. Examples may be seen in 

 the southwestern United States and adjacent Mexico, along the 

 foot of the Andes, and in Africa especially bordering on the Sahara. 

 In the East they occur in x\rabia and elsewhere near the northern 

 shores of the Indian Ocean, and also in Australia. The climatic 

 conditions supporting these warm-region scrubs lie between those 

 of the desert proper and of thorn- woodlands, the temperatures being 

 variable but high at all seasons, and the rainfalls occasional though 

 seasonal and commonly averaging from 20 to 50 or more cm. 

 annually. Important is the seasonal distribution and general 

 reliability of the rainfall. 



Many of the plants are veritable caricatures, such as those that 

 are grotesquely swollen to store water. 



The bushes in these semi-deserts are often of fair size and either 

 grow separated or aggregated into a more or less continuous scrub, 

 most often of thorny Acacias. Grasses if present are dwarfed and 

 wiry, and usually reduced to isolated bunches or ragged patches. 

 Other herbs tend to be leathery- or fleshy-leafed, or, in numerous 

 instances, geophytic, with underground storage of food and water. 

 Cacti in the New World and cactus-like Euphorbias in the Old 

 World frequently form a characteristic feature of the usually open 

 vegetation. Like some of the other plants, including the thorny 

 Acacias, they are often beset with prickly spines. Large-leafed 

 Agaves, Aloes, and Yuccas are also characteristic inhabitants, as are 

 smaller succulents. Although some of the shrubs may be fairly 



