122 ADAPTATIONS. OECOLOGICAL CLASSIFICATION SECT, in 



of two to three layers in Nerium ; and is very large in certain other plants 

 belonging to the Commelinaceae, Scitamineae, Bromeliaceae, and Rhizo- 

 phoraceae. 1 A collenchymatous hypoderma functioning as a water- 

 storing tissue is met with in a number of cacti ; it is traversed by 

 narrow intercellular spaces leading from the chlorenchyma to tlje stomata. 

 Mucilaginous cork may here be mentioned as a cork-tissue discovered 

 by Jonsson 2 in a number of Asiatic desert -plants. 



Internal Water-tissue. 



Water-tissue may occur among xerophytes in various other forms, 

 as is indicated in the succeeding paragraphs. 



(a) Longitudinal bands of water-tissue extending through the whole 

 thickness of the leaf from the upper to lower faces occur in some desert- 

 grasses, 3 and in Phormium tenax. Strips of chlorenchyma, in which 

 the veins are embedded, in this case alternate with the bands of aqueous 

 tissue. In the Velloziaceae similar longitudinal bands connect the 

 epidermis on the upper face of the leaf with the water-containing cells 

 that form a sheath round the vascular bundles. 4 



(b) Central water-tissue, occupying the centre of the leaf and surrounded 

 by a thin layer of chlorenchyma, is met with in many xerophytes, including 

 Aloe, Agave, Bulbine, Mesembryanthemum, Salsola, 5 Atriplex, Halogeton, 

 and Zygophyllum. In aphyllous stems the aqueous tissue may be 

 distributed in this same manner, as is the case in Salicornia and Haloxylon. 6 



Water-tissue and chlorenchyma may either be sharply delimited from 

 each other, or may gradually merge, owing to the cells in the interior of 

 the leaf containing but little chlorophyll, as in many Crassulaceae and 

 Cactaceae. Water-storing idioblasts appear in the chlorenchyma of 

 various desert-plants and halophytes. 7 



SUCCULENT PLANTS 



Succulent plants are thick and fleshy forms which are provided with 

 a water-tissue and parenchyma that contains abundant mucilage ; 

 they are xerophytes which have especially pronounced water-storing 

 tissue. They are commonly plump in form, and, like herbs, usually 

 possess green stems which exhibit but feeble production of cork and 

 of lignified tissue lignification and succulence are, in a sense, opposed 

 to one another. They are perennials, and often very long-lived. The 

 cell-sap is rich in mucilage, the epidermis strongly cutinized as a rule, 

 and the stomata are sunken. Succulent plants can store a large amount 

 of water, which they give up extremely slowly, and they therefore dry 

 only with great difficulty. The hottest and driest countries with a 

 regular periodicity of climate are generally their homes. 8 



We can distinguish two main types of succulent plants, succulent- 

 stemmed and succulent-leaved. 9 



Warming, 1883; O. G. Petersen, 1893; Areschoug, 1902. 



Jonsson, 1902 ; also see Haberlandt, 1904, p. 363. 



Volkens, 1887. 4 Warming, 1893. 5 See figure in Areschoug, 1878. 



Volkens, 1887 ; Warming, 18976. 7 See figure in Volkens, 1887. 



In regard to their adaptive features, consult Burgerstein, 1904, pp. 44, 205. 



Gobel, 1889-93. 



