CHAP, xcin EVERGREEN DICOTYLOUS FOREST 345 



Haberlandt x found that some hydathodes possess the power of absorb- 

 ing dyes, and he therefore concluded that they also serve to absorb water 

 and hand it on to the plant. This is presumably possible only at a definite 

 time of day, namely, when the first showers of rain fall, some hours after 

 midday. When the plant has transpired too vigorously the hydathodes 

 would thus aid the plant in rapidly recovering its turgidity. Hydathodes 

 may accordingly act as regulators of the plant's water-supply, by expelling 

 any excess of water, and by absorbing water when pressing necessity 

 so demands. 



What has been said so far refers to plants forming the upper storeys, 

 so that their leaves are at the roof of the forest and are directly insolated. 

 But different relations may be expected to subsist in regard to humbler 

 plants standing beneath others inside the forest. And here we do find 

 plants that are eminently adapted to live in shade and in moist air ; 

 among such are the Hymenophyllaceae, whose filmy fronds show only 

 one or few layers of cells, no proper epidermis, and no intercellular spaces, 

 and whose stems bear root-hairs. 2 



Adaptation to rain. Other structural features seem to relate to the 

 discharges of rain, and concern 



a. The violence of these, which in this respect are unparalleled in 

 the European climate. 



b. The frequency of the rainfall. 



Adaptation to the mechanical action of rain? The great distance at 

 which one can hear the noise of rain falling upon a tropical forest gives 

 us some idea as to the violence of the impact ; but the trees are adapted 

 to withstand this. Many simple leaves are firm and coriaceous, and the 

 epidermis may be so impregnated with silica that the lamina is rigid and 

 brittle, as in Medinilla magnifica, and the blade may resemble a * green 

 lacquered sheet of metal '. The leaves of mimosas, acacias, and other 

 Leguminosae, also of palms, are divided into many leaflets, or segments, 

 so that they oppose less resistance to rain. In addition very often they 

 can execute movements causing their leaflets to close together, so that 

 they expose a smaller surface or even merely an edge to the falling rain- 

 drops. In other plants the leaf shows folds or is trough-like, and thus 

 acquires additional mechanical power of resistance ; this is most distinct 

 in palm-leaves whose pinnately or palmately arranged segments are folded, 

 inasmuch as their lateral parts are inclined upwards or downwards. The 

 leaf-stalk is often directed upwards, presumably for a reason different 

 from that in connexion with xerophytic communities, namely, in order 

 to oppose greater resistance to the force of rain. In many other cases 

 the same object is accomplished by the pendent arrangement of the young 

 leaf-blades and young twigs ; in Araceae many large leaves retain this 

 lie, but others subsequently erect themselves. The huge leaves of palms 

 and Scitamineae have large sheaths which embrace the stem, and thus add 

 strength to stem and leaves and enable these to resist the violence of wind 

 and rain. 



1 Haberlandt, 1894, 1895, 1904. 



2 For further information concerning transpiration in moist tropical climates 

 I'eaders should consult the works of Haberlandt, 1892, 1897 ; Stahl, 1894 ; Burger- 

 stein, 1904; Giltay, 1897, 1898; and Holtermann, 1902, 1907. 



3 Wiesner, 1895 a, 1897. 



