340 



MESOPHYTES sect, xvi 



because man has left it almost or entirely undisturbed. Its trees remain 

 standing until they die a natural death or succumb in the struggle with 

 their neighbours, and thereafter their corpses sink to the ground, moulder 

 away, and leave a bare space where other species recommence to battle. 

 There are still primeval forests, not only on the warm humid plains 

 bounding the Amazon, but also on the storm-beaten rocky soil of Lapland 

 and Scandinavia,^ as well as in Germany and Bohemia. ip 



Tropical rain-forest is confined to regions where great heat prevails, 

 and a flood of light streams down from the sun that stands high overhead, 

 and where there are almost daily violent discharges of rain that are 

 derived from the huge volumes of air saturated with moisture which rise 

 vertically and are cooled in the upper atmospheric strata. Between the 

 crowns of the trees warm mists often arise, while, at least at certain 

 seasons of the year, water drips from the leaves during most of the day, 

 and the air may be almost saturated with moisture ; for instance, at 

 Buitenzorg in Java the relative humidity of the air is about 95 per cent, 

 from 2 to 3 p.m. until the next morning. 



The soil of rain-forest is always a rich humus, black and porous, filled 

 with mouldering remains of trunks, twigs, leaves, flowers, and fruits, and 

 presumably excavated by subterranean animals. Yet the layer of humus 

 is not so thick as is often assumed ; layers several metres in thickness 

 are not usual .^ Some writers regard the soil as being thoroughly wet 

 at all times, but others more correctly state that the rain percolates rapidly 

 downward owing to the porous nature of the soil. 



In such circumstances the plant-world develops with a luxuriant 

 diversity unrivalled elsewhere. Tropical rain-forest constitutes the climax 

 in the development of vegetation for the whole world. Its characteristics 

 are discussed in the succeeding paragraphs. 



Utilization of space. One usually finds so many storeys of plants that 

 the whole nearly forms a single complex of vegetation. As Humboldt 

 expressively remarks : ' forest is piled upon forest.' The trees forming 

 the highest storey have tall thick trunks which are unbranched up to 

 a height of 40 to 50 metres or more. Beneath them are trees of moderate 

 stature with branches not reaching those of the higher tier. Beneath 

 these, in turn, succeed slender, thin-stemmed, low palms, tree-ferns, and 

 shrubby Urticaceae, Piperaceae, Myrsinaceae, Rubiaceae, and others. 

 Scattered about are huge herbs which reach 4 or 5 metres in height and 

 belong to the scitamineous and araceous types. If there still remain space 

 available on the ground that is reached by the light, it is occupied by dark- . 

 green ferns, Selaginellae, mosses, and similar sciophytes. But often the ! 

 light is too feeble to permit of more than a very small number of plants | 

 developing on the ground, which then may be almost bare of vegetation,* i 

 with its black humus covered only by fallen, decaying, wet leaves, twigs, 

 and remnants of fruits, between which only bizarre saprophytes (Burman- 

 niaceae, Gentianaceae, and others*) or root-parasites (Rafllesiaceae and! 

 Balanophoraceae) find places. Large pileate fungi are seldom met with; 

 but there are hordes of epiphytes ^ clothing trunks and branches, and 

 belonging to the Orchidaceae, Araceae, Piperaceae, Bromeliaceae (in 



' Andersson and Hesselman, 1907. ^ Reinhardt ; Whitford, 1906. 



* Martius, 1840-7. * See p. 89. ^ See p. 87. 



