198 



BOTANY 



PART I 



(Edelweiss), aud of some South African Proteaceae. Some of the Giamineae 

 {Stipa capillata, Festuca al2>estris, Sesleria tcnuifolia, S. pundoria, 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 bear- 

 ing the stomata (Fig. 184). Reduction of the leaves is illustrated by the desert 

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

 New Zealand Veronica, shown in Fig. 185, closely resembles these in habit. A 

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

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

 development of succulent swollen stems frequently occurs in the Euphorbiaceae 



Y/ 



Fio. 184. — Transverse sections of the leaf of Stlpa capUlata. The leaf above in the closed state, 

 the half leaf below expanded. U, lower surface, without stomata ; 0, ujiper surface with 

 stomata (S); C, ehlorophyllous mesopliyll. (x 30. After Keener von JIarilaun.) 



(Fig. 186), in the Compositae {Klcinia articuhita), Asclepiadaceae, and other plant 

 families found in arid regions. It has been estimated that the amount of water 

 evaporated by a Melon-Cactus is reduced by its succulent development to ^ttVu 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 Sempervivum, also in many species of Scdum, Aloe, and Agave. Both 

 stem and leaves are equally succulent in many species of Mesenihnjatitliemum. 

 In other plants, the parenchyma of their stem tubers (epiphytic Orchids) or of 

 their thickened roots (Oxalideae) serve as water-reservoirs. 



Epiphytes, i.e. i)lants which live upon others without obtaining food-materials 

 from them (cf. Parasites, p. 223), belong also to xerophytic plants. Some epiphytes 

 are characterised by their arrangements for water -storage, others by their 

 mode of absorbing water. Many epiphytic Brouieliaceae catch the rain-water in 

 reservoirs formed by their closely approximated leaves, and then eagerly take it 



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