DESERT PLANTS 553 



worts may hold a balance in mucilage cells, a capacity shared by an ex- 

 tremely large number of species. 



A review of the extensive data accumulated establishes the fact that 

 Echinocactus may live for nearly three years at the expense of its 

 water-balance, which may be depleted as much as 50 per cent, before 

 death results. Carnegiea loses nearly 30 per cent, before serious results 

 follow, although the extremely succulent seedlings of this plant may 

 shrink to one third the original weight and still live. 



Plants with a large water-balance are especially characteristic of 

 the arid regions of southwestern United States, Mexico, some parts of 

 South America and South Africa. It is notable that the great deserts 

 of central Asia and Asia Minor, as well as the whole of north Africa, 

 have but few native species of this habit. Some succulent Euphorbias 

 are reported from India, but information concerning the occurrence of 

 plants with a large accumulation of water in Australia is very meager. 

 The physical causes which might be operative in inducing this habit in 

 representatives of widely separated families are not known. 



vol. lxxix. — 38. 



