36 THE WANDERINGS OF ANIMALS [CH. 



grey branches of wiry tufts. There are no broad-leaved 

 plants : most of the leaves are lance-shaped, spiky, 

 thorny, e.g. Yuccas : or the leaves are large, thick, 

 full of juice, but covered with an air- and water-tight 

 epidermis, again spiky like the African aloes and the 

 American agaves ; or the plants have dispensed with 

 leaves altogether, having turned them into thousands 

 of hooks and spikes, and the whole, often large, stem 

 has a green rind. The chlorophyll is spread over the 

 stem instead of in the leaves ; for instance the cactus- 

 tribe of America, and exactly the same transformation 

 occurs in the Euphorbias of Africa where there are 

 no cacti. A large globular cactus stores water 

 sufficient to last it for years, or the plants adopt the 

 principle of the bulb. These produce a short-lived 

 glorious bloom in the short wet season, and during 

 the greater part of the year, or occasionally for several 

 years on end, the bulb sleeps unnoticed underground. 

 A very peculiar feature, not easily explained, is 

 that in arid districts the perennial vegetation always 

 grows in patches, with bare spaces between. This 

 patchiness is carried even further. Some sprawling 

 shrub gives, so to speak, shelter to other plants and 

 thus a little colony is formed, but it remains an 

 isolated patch and they never join. Sagebush, arte- 

 misias, hard wiry tussocks of grass, cactus, broom - 

 shaped leafless euphorbias, here and there a big tree- 

 like Yucca, are characteristic of desert vegetation. 



