124 ADAPTATIONS. OECOLOGICAL CLASSIFICATION sect, hi 



Succulent plants owe their origin, according to Vesque/ to two direct 

 causes : 



1. Heating of the soil, which increases the osmotic power of the roots 

 (succulent plants can endure without injury very high temperatures, 

 and grow especially on warm rocks). 



2. The supply of nutriment in alternately strong and weak solutions. 

 Between succulent plants and xerophytes that are poor in water 



there are distinctions in appearance quite apart from those in thickness 

 and the like. The former are as a rule of a fresher green (because they 

 are glabrous) ; the latter, on the contrary, white-haired or grey-haired.^ 

 Still there are some hairy succulent plants, Sedum villosum for example. 

 In consequence of the production of wax, glaucous species occur in both 

 groups of xeroph3rtes. 



BULBOUS AND TUBEROUS PLANTS 



These must be considered in connexion with succulent plants. They 

 are adapted in a different fashion to endure prolonged periods of drought. 

 In many cases it is not only reserve food, such as starch, but also mucilage- 

 cells or mucilage-tissue which contribute to their fleshiness, and function 

 partly as food-materials for the production of new shoots^ and partly 

 as means of storage of water to provide against desiccation. Bulbous 

 and tuberous plants belonging to the Liliaceae, Iridaceae, Amaryllidaceae, 

 and other families, therefore occur especially in dry countries, and more 

 particularly in South Africa ; also on the steppes of Asia, where they are 

 among the species that develop rapidly after the commencement of spring 

 or of the rainy season. Poa bulbosa is, according to Aitchison,"* the 

 commonest grass on the great plains of Baluchistan, and is certainly 

 enabled to exist there by the aid of the thick leaf -sheaths that form a kind 

 of bulb. Marloth ^ suggests that South African bulbous plants are 

 designed to resist the enormous pressure to which they are subjected 

 by the drying soil ; for some of them, namely Cape species of Oxalis, are 

 protected by a hard coat, others by numerous, superimposed, soft, finely- 

 fibrous layers whose bundles of bast persist as rigid bristles. In South 

 Africa there are many remarkable, partly epigeous tubers (certain stem- 

 tubers) which in their leafless condition are distinguishable only with 

 difficulty from the stones among which they grow ; as an example may 

 be mentioned Dioscorea (Testudinaria) Elephantipes, which is protected 

 from desiccation by huge cork structures. Likewise belonging to the 

 category of epigeous tubers are the tuberous or at least swollen trunks 

 of certain South American trees, occurring in the Caa-tinga forests, and 

 including Chorisia crispiflora and Cavanillesia arborea (Bombaceae), 

 Jaracatia dodecaphylla (Caricaceae),^ also Jatropha podagrica (Euphor- 

 biaceae). 



Many tubers consist of root and stem combined, and thus lead the 

 way to those consisting of root alone ; such is the nature of the lignified 

 tuber (' xylopodium ' ') in many herbs and small shrubs in South American 

 savannahs.^ In Crocus and other Iridaceae one sometimes sees clear, 



^ Vesque, 1883. * See p. 114. 



^ Tubers of this kind occur even in aquatic plants such as Sagittariasagittaefolia. 



* Aitchison, 1887. * Marloth, 1887 ; see also Hildebrand, 1884. 



* See figure in Warming, 1892. ' Lindman, 1900. 



