STORAGE IN STEMS AND LEAVES 75 



and great numbers of desert plants have bulbs or rootstocks 

 much exceeding in bulk the rest of the plant body, and con- 

 taining large quantities of water, protected from evaporation 

 by heavy exterior layers of cork. 



67. Water storage in leaves. Many of the most striking 

 examples of succulent or fleshy-leaved plants occur among 

 species which are natives of dry countries or of regions where 

 there are long rainless periods. The century plants (Agave) 

 (Fig. 62), ice plants (Mesembryanthemum), aloes (Alo'e), and 

 Echeveria are good instances of this kind of leaf. The leaves 

 are sometimes cylindrical or prismatic, thus offering little 

 surface for evaporation, and contain great quantities of water 

 in the form of a thin mucilage, not easily dried up. In many 

 such leaves the water is largely stored in special layers of the 

 epidermis, while in others the water-storage tissue is in the 

 interior of the leaf. During droughts fleshy leaves gradually 

 lose their firmness and become flabby in the same way as the 

 leaves of the purslane do when the plant is hoed up and left on 

 the surface of the ground. In this case the plant may live for 

 weeks and then take root and grow again after the first rain. 



Sometimes plants which grow in moist soil have leaves with 

 water-storage layers. The oleander, for instance, grows along 

 water courses but is exposed for months to very dry, hot air, 

 during the nearly rainless summers of the Mediterranean region. 

 The common rubber plant (Ficus), which in India grows to 

 be an immense tree, is one of the most familiar examples of 

 water storage in the leaves of a species growing in moist soil. 

 Such leaves are able to withstand the great changes of tem- 

 perature and moisture in the air of the tropics during every 

 twenty-four hours, the air for two thirds of the time being 

 almost saturated with moisture, while during the remaining 

 hours the moisture is relatively low and the temperature under 

 a nearly vertical sun extremely high. 



Plants with fleshy leaves are often found in cool, damp cli- 

 mates, but they usually grow on rocks or in other situations 

 where the water supply is at times nearly or quite cut off. 



