Chap. IV] TROPICAL DISTRICTS CONSTANTLY MOIST 291 



tract of ground should be essentially occupied by one herbaceous species to 

 the exclusion of all others, so that there exists a far greater uniformity in 

 the herbaceous than in the woody vegetation. In the forests of Ceylon 

 and Java I saw certain species of the acanthaceous genus Strobilanthus 

 with juicy brittle stems forming delicately foliaged bushes far exceeding 

 man's height and alone constituting the herbaceous vegetation over 

 immense tracts. I have seen many other herbaceous plants growing 

 socially, though not in such great multitudes, for instance species of 

 Impatiens, Cyrtandra, Elatostema, Selaginella. Grasses are quite insigni- 

 ficant as components of the tropical forests. 



Among the more scattered plants, yet seldom absent from the rain- 

 forest, mention may be made of begonias (Fig. 136), which both in America 

 and Asia show great diversity of form, being sometimes small and erect, 

 sometimes climbers reaching high up tree-trunks and rocks ; also Aroideae 

 growing on the ground, with their variegated petioles, often reminding one 

 of snakes ; finally, as rare features, parasitic Balanophoraceae, saprophytic 

 orchids and Burmanniaceae. 



The stems of the trees are covered with a rich flora of the most diverse 

 species of lianes and epiphytes, to which detailed reference will be made 

 hereafter. The most varied forms, often characterized by large expanses 

 of foliage or by splendid flowers, form these epedaphic gardens. 



If we attempt to form a general conception of the changing aspects of 

 a virgin forest, the first point that strikes one is the struggle for light, and 

 the possibility of carrying on this struggle almost without hindrance is 

 afforded by the great and incessant humidity. 



The struggle for light indeed prevails in the forests of every zone. 

 Everywhere it drives the vegetation from the shady depths in an upward 

 direction, but this struggle is nowhere so pronounced as in the evergreen 

 forests of tropical districts, in which individuals with slender stems and erect 

 weakly branched boughs eagerly strive after the light and at the same time 

 convey to the light a crowd of guests, including lianes whose weak stems 

 cling firmly to the scaffolding of the trees, and epiphytes which germinate 

 on the branches and thus from the very first secure a suitable degree 

 of illumination. 



In the epiphytic vegetation, the struggle for light is the most thoroughly 

 successful, for it clothes the branches of the trees up to their extreme tips 

 with frequently large and even tree-like plants ; nor are the leaves spared, 

 for on them both mosses and Algae grow, and occasionally also flowering 

 orchids. 



With the epiphytic vegetation also the struggle for light is the most 

 pronounced, and results in great destruction. One may frequently hear 

 a branch of a tree crack and break under the burden of its epedaphic 

 garden which has become too luxuriant (Fig. 134); and the trellis-work of 



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