PHYSIOLOGY 213 



,s'. jinnctoria, etc.) roll or fold their leaf-blades, in times of drought, by means of 

 special hinge-like devices, into narrow tubes, the protected inner surface being 

 formed by the side bearing the stomata. Reduction of the leaves is illustrated 

 by the desert forms of Genista and Sarothamnus, and by the Cypress-like Conifers. 

 The New Zealand Veronica, shown in Fig. 196, closely resembles these in habit. A 

 complete disappearance of the whole leaf surface takes place in most Cacti, in which 

 also the stems become swollen and converted into water-reservoirs. A similar de- 

 velopment of succulent swollen stems frequently occurs in the Euphorbiaceae (Fig. 

 197), in the Compositae (Kleinia articulata], Asclepiadaceae, and other plant families 

 found in arid regions. It has been estimated ( w ) that the amount of water evaporated 

 by a Melon-Cactus is reduced by its succulent development to g^^ of that given 

 off by an equally heavy climbing plant (Aristolochia). Instead of the stem the 

 leaves themselves may become succulent, as in the House-leek and other species of 

 Semper vivum, also in many species of 

 Sedum, Aloe, and Ayave. Both stem and 

 leaves are equally succulent in many 

 species of Mesembryanthemum. In other 

 plants, the parenchyma of their stem 

 tubers (epiphytic Orchids) or of their 

 thickened roots (Oxalideae) serve as water- 

 reservoirs. Epiphytic Bromeliaceae catch 

 the rain-water in reservoirs formed by their 

 closely approximated leaves, and then 

 eagerly take it up through the scaly hairs 

 which cover the leaf surfaces, as in species 

 of Tillandsia. Again, many epiphytic 

 Orchids and Aroids collect the rain-water 

 in a swollen sheath developed from the 



epidermis of the aerial root (velamen 



j. i/vnv r it. c xi FIG. 198. SaJ no?m. Full grown and bear- 



radicum, p. 109). In the case of other Jng fmits _ ' (Nat gjze; from SCHIMPER , S 



epiphytic Orchids, Aroids, and Ferns Plant-Geography.) 

 (Asplenium Nidus, for instance), the 



humus and other material caught in receptacles formed by the leaves or aerial 

 roots act like a sponge in taking up and retaining water, while the absorptive roots 

 penetrate into these moist, compost-like masses and absorb both water and nutrient 

 substances. Many pecies of Frullania (a Liverwort common on Beech trees) 

 possess, on the other hand, special water-sacs on the under side of their thallus 

 (Fig. 386). A particularly remarkable contrivance for maintaining a constant 

 supply of water is exhibited by the epiphytic Dischidia Mafflesiana, a number of 

 whose leaves form a deep but small-mouthed urn, into which the roots grow. It 

 would seem at first sight unnecessary that plants like the Mangrove tree, which 

 stand with their roots entirely in water, should require protection against too 

 rapid transpiration ; but, as this tree grows in salt or brackish water, it is necessary, 

 as in other halophytes (Fig. 184) to reduce the amount of water absorbed, in order 

 to prevent a too great accumulation of salt in the tissues. 



In high latitudes, where the soil remains frozen for months at a time, rendering 

 the absorption of water by the plant difficult, arrangements similar to those found 

 in desert plants are present to diminish transpiration. The dwarf habit which is 

 so frequently characteristic of the alpine and arctic plants (Fig. 198) is as much an 

 expression of this limitation of the transpiration as of the generally unfavourable 

 conditions of existence ( :H ). 



