CHAP. XXXII STORAGE OF WATER BY LAND-PLANTS 125 



fusiform, juicy roots radiating from the tuber ^; these are also found on 

 the bulbs of certain species of Oxalis,- and in the cactaceous Cereus tube- 

 rosus, whose shoots cannot store much water but whose roots are tuberous, 

 juicy, and enveloped by a sheath of cork. South African xerophytes 

 have upon their long roots many fusiform or spherical tubers which are 

 water-reservoirs encased by cork ; Elephantorrhiza has close beneath 

 the surface of the soil a water-reservoir of this kind, which weighs up 

 to ten kilogrammes, although the stem is scarcely a foot in height ; while 

 a species of Bauhinia produces tubers weighing fifty kilogrammes.^ In 

 Egypt there are species of Erodium with root-tubers which, according 

 to Volkens,^ serve to store water. Spondias venulosa has gigantic sub- 

 terranean tubers. In temperate Europe, Sedum maximum possesses 

 thick fleshy roots. 



In some plants there have been discovered dwarf-roots that have 

 been, correctly or incorrectly, regarded as water-reservoirs ; among such 

 plants are Aesculus and some allies,^ some Australian conifers,^ and Sedum.' 



Dimensions of water-reservoirs vary greatly according to the part 

 they have to play in the life-history of different species ; in some cases 

 they necessarily function for months or even years without intermission, 

 but in others for instance, leaves of trees in tropical rain-forest only for 

 a few hours of the day ; some resign water rapidly, others slowly. The 

 structural features must necessarily harmonize with these differences. 



Combinations of xerophilous characters for instance, anatomical with 

 morphological are universal ; indeed, some characters demand the pre- 

 exist ence of others before they can arise. 



Correlations. One character often entails another. For example, 

 with peripheral water-tissue there appear accessory cells in connexion with 

 the stomata, so that the latter may be protected when the plant-member 

 shrivels as it dries .^ 



LATICIFEROUS PLANTS 



Up to the present we have dealt only with watery or slimy cell-sap 

 in which various salts may be dissolved. But mention must be made 

 of those plants that contain ' latex ', which is usually white and is con- 

 tained in tubular organs (laticiferous cells and syncytes). The functional 

 significance of latex is unknown, indeed it is probably multiple, and one 

 function may be the protection of the plant against desiccation. In 

 favour of this view^ is the fact that laticiferous organs are frequent in the 

 tropics, particularly in hot, dry districts, and often precisely in plants 

 which are thin-leaved and apparently lack any other means of replacing 

 the water lost by transpiration.^ The occurrence of latex in subterranean 

 bulbs, as in Crinum pratense,!^ harmonizes with this view when these 

 bulbs grow in clay which becomes fissured during the dry season. 



ISOLATED WATER-STORING CELLS. NERVE-ENDS 



The succulent plants hitherto discussed possess coherent water-tissue, 

 an arrangement that seems to be the most efficient ; laticiferous plants 



Raunkiiir, 1895. Illustrations, 1905, 1907. ' HUdcbrand, 1884. 



Schinz, 1893. * Volkens, 1887. ' J. Klein, 1880. 



" Berggren. 1887. ' Warming. 1891 : see illustrations, 1907-09. 



' W. Benecke, 1892. * Warming. 1892. '" Lagerheim, 1892. 



