CHAP. XXX TRANSPIRATION IN LAND-PLANTS 117 



Ficus religiosa, but also in the most diverse plants (ferns, Monocotyle- 

 dones, Dicotyledones), both in simple and compound leaves. The 

 drip-tip serves rapidly to conduct the rain off leaves that are capable 

 of being wet. Drip-tips are downwardly directed ; and the longer 

 the tip, the more rapidly docs the leaf rid its surface of water. The 

 sabre-like tip leads water away most rapidly, apparently at times in 

 an almost continuous jet. Drip-tips are found neither on leaves that 

 are incapable of being wet, nor among xerophytes. 



3. Furrowed nerves that conduct superficial water to the leaf-tip 

 are common. The arcuate course of the nerves in Melastomaceae and 

 others is thus of additional use. 



4. Velvety leaves are especially encountered among herbaceous species 

 growing on the ground in forest, also among species forming the lower 

 storey of the forest, where shade and moisture are at their greatest. The 

 epidermal cells project in the form of countless short papillae, which give 

 to the leaf a velvet-hke appearance, and produce a fine capillary system 

 m which the water spreads over the whole blade as a thin film. The 

 consequence is that water can evaporate much more rapidly than if 

 it were not spread out in this manner. But it has been suggested that 

 these papillae also serve to supply the leaf with an increased amount 

 of light, or act as light-perceiving organs.^ 



CHAPTER XXXI. ABSORPTION OF WATER BY LAND- 

 I PLANTS 



Submerged water-plants, or their overwhelming majority, exhibit 

 j no organs specially adapted for the absorption of water, whereas the 

 ! opposite is the case with land-plants. These possess adaptations that 

 are described in the succeeding paragraphs. 



I i. Hypogeous Organs that Absorb Water. 



Subterranean organs in the form of roots, rhizoids, and myceha are 

 designed for the absorption of water ; so likewise are some rhizomes 

 that have absorbing hairs, like those of Corallorrhiza, Epipogum, Equi- 

 setum, Psilotum, and Hymenophyllaceae. In xerophilous land-plants 

 only few deviations from the normal type occur. Many xerophytes 

 possess deeply-descending roots, which aid them in securing water at 

 great depths during dry periods. This has been observed in the desert 

 of Afghanistan in species of Astragalus 2 ; in the Egyptian desert in the 

 colocynth, whose thin leaves would wither rapidly were it not for the 

 deep roots, in Calligonum comosum, and in Monsonia nivea. Volkens^ 

 observed roots in the Egyptian desert that were twenty times as long 

 as the epigeous organs. The same features are to be seen in plants of 

 the European dunes for example, in Eryngium maritimum and Carex 

 arenaria ; the latter has two kinds of roots those that are very slender, 

 branched, and lie near the surface, and others that are less branched, 



See Haberlandt, 1905. ' Aitchison, 1887. ' Volkcns, 1887. 



