346 MESOPHYTES SECT, xvi 



Adaptation to the frequency of rain. Frequent falls of rain may have 

 an injurious effect upon plants by causing the leaf -blades to become too 

 wet and heavy. 1 But transpiration and photosynthesis are also hindered 

 by epiphyllous algae, lichens, fungi, liverworts, and, according to Haber- 

 landt, even bacteria, which settle upon the leaves. The older leaves of 

 many evergreen trees in humid tropical forest are covered with a mass of 

 epiphyllous species. 2 Therefore it would seem to be of advantage to 

 plants in rain-forest that their leaves should dry rapidly. Jungner 3 

 working in Kamerun, and Stahl 4 in Java, concluded that this aim is 

 accomplished by the following devices : 



1. A smooth cuticle which cannot be rendered wet ; this device is 

 very widespread. 



2. Drip-tips, as Stahl terms the long, often suddenly narrowed leaf- 

 apices. These are familiar in Ficus religiosa, and occur among various 

 ferns, Monocotyledones, and Dicotyledones, not only in simple but also 

 in compound leaves. The drip-tips, which occur only on leaves 

 capable of being rendered wet on the surface, enable rain to flow away 

 rapidly. They are downwardly directed. The longer and sharper the 

 tip the more rapidly does the leaf dry ; the sabre-like tip is the most 

 sufficient abductor of water, which sometimes drips away in a continuous 

 jet. Drip-tips occur neither on leaves whose surface cannot become wet 

 nor among xerophytes. 



3. Channelled nerves often occur, and serve to convey water to the 

 leaf-tip. The arcuate course of the nerves along melastomaceous leaves 

 is of use in this connexion. 



4. Velvety leaves are encountered in the shadiest and moistest parts 

 of rain-forest, in connexion with herbs forming the ground-vegetation 

 and species belonging to the lower storeys. The epidermal cells project 

 as countless papillae which give to the leaf a velvety appearance. Between 

 the papillae the water rapidly spreads by capillarity as a thin film extend- 

 ing over the whole leaf-blade, so that the water can evaporate more quickly 

 than if it were not thus spread out. The opinion has also been expressed 

 most recently that these papillae serve to increase the amount of light 

 supplied to the leaf. 5 



FLORA 



The flora of tropical rain-forest is so diversified as to be beyond detailed 

 discussion in this work. The predominant trees belong to the Leguminosae, 

 Lauraceae, Myrtaceae, Moraceae, and other families. 



DISTRIBUTION 



Rain-forest is not exacting as regards soil, and extends over vast areas 

 where the rainfall is very large (above 180 centimetres annually) and 

 the dry season does not exceed a few months. Where the rainfall is less, 

 and prolonged dry seasons prevail, rain-forest is confined to humid low- 

 lying land bounding rivers, where it forms fringing forest ; while the 

 elevations are occupied by savannah or savannah-forest. 6 



1 Concerning ombrophilous and ombrophobous species see p. 32 ; and Wiesner, 

 18930. 2 See p. 83. 3 Jungner, 1891. * Stahl, 1893; also see p. 116. 



5 Haberlandt, 1905, 1908; Guttenberg, 1905. 



6 Schimper, 1898 ; Warming, 1892 ; Passarge, 1895 ; Pechuel-Loesche, 1882. 



