SECT. II 



PHYSIOLOGY 



199 



up through the scaly hairs which cover tlie leaf surfaces ; these are often termed 

 cistern plants. Again, many epipliytic Orchids and Aroids collect the rain-water 

 in a swollen sheath developed from the epidermis of the aerial root (velameu 

 radicum, p. 120). In the case of other exnphytic Orchids, Aroids, and Ferns 

 {Asplenium Nidus, for instance), tlie 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 species of Frullania 

 (a Liverwort common on Beech trees) possess, on the other hand, special water- 

 sacs on tlie under surface (Fig. 376). A particularly remarkable contrivance for 

 maintaining a constant supply of water is exhibited by the epiphytic Dischidia 

 Rafflesiana, some of whose leaves form a deep but small-mouthed urn. into whicli 



Fi(i. 1S5. — Veronica cupressoides, witli the 

 liabit of a Cypress, from New Zealand. 

 (From Schimper's Plant-Geography.) 





Fif!. 1S6. — Euphorbia globosa. The re- 

 duced leaves may be seen on the upper 

 globose shoots. 



the branching 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, the absorption of water is made more difficult, and, further, 

 it is necessary, as in other halophytes (Fig. 192), to reduce the amount of water 

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



In high latitudes, and also in alpine regions, w'here 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. 



In plants living in very damp situations, on the other hand, arrangements to 

 further transpiration are found. Of this nature are the poor development of the 

 cuticle, the placing of the stomata on exposed projections of the epidermis, and 

 the development of epidermal papillae, or of absorbent pigments, to catch the 

 rays of light. When the leaf is- able, either by absorption of heat from without 

 or by the production of heat within itself, to raise its temperature above that of 



