128 FLESHY STEMS [CH. 



The best known are the leafless Cacti of Central 

 America and Northern South America, and the Cactus- 

 like Euphorbia of S. Africa and Asia, and the whole of 

 their organisation is adapted for the storage of water in 

 the tissues of the stem, and its safe-keeping and economy. 



It is, so to speak, an accident merely that these shoots 

 present such stumpy and curious shapes, because this is a 

 consequence of the reduction of their surfaces to minimise 

 transpiration ; and since this is brought about by the 

 suppression of their leaves, it entails a corresponding 

 diminution of their capacity for decomposing carbon 

 dioxide, and therefore of growing rapidly. Species of 

 Stapelid, South African Asclepiadeae, have similar swollen 

 Cactus-like shoots. 



The fleshy Cacti show all stages of reduction from 

 the leafy Pereskia, with thorns in the leaf-axils, and 

 Opuntia, with flattened fleshy stems still bearing temporary 

 leaves, to forms with flattened, or cylindrical, pyriform, 

 globoid and variously grooved stems entirely devoid of 

 leaves, but abundantly protected by thorns of the most 

 varied shapes straight spines, hooks, &c. 



In all such cases the hard cuticnlar covering and 

 reduced transpiring surface, the abundant stores of water 

 in the juicy fleshy internal tissues, and the protected 

 stomata in the furrows, are in accordance with the condi- 

 tions of extreme drought in the arid regions occupied by 

 these plants. They are adapted to existence in hot desert 

 climates, and to this end expose as little surface as possible 

 for evaporation ; and since they can also only expose a 

 small surface for assimilation, their growth is very slow. 

 Since they store relatively large quantities of water, how- 

 ever, it is of advantage to them to be protected by the 

 formidable armatures referred to against the marauding 

 animals which would otherwise soon exterminate them. 



