Among the characteristics of tropical forests is the presence of many kinds of plants on a limited area, hundreds of species struggling upward to the light where there seems room but for one; the variety of mosses and lichens; the profusion of flowering parasites; the luxuriance of the vines, many of which are armed with spines; and the abundance of the remarkable aerial roots termed _lianas_. Of the last there is a great variety, some of them of large size and surprising length; they frequently descend from plants entwined among the topmost branches of great trees, looking not unlike the cordage of a forest of masts in some crowded port, and on reaching the ground send out rootlets in the humid soil. In the depths of a tropical forest it is always twilight. Even at noontide no shafts of yellow sunlight reach the ground to glorify mossy banks and flower-gemmed dells, as in the open woods of temperate climes, but a diffused greenish light, producing weird effects, alone penetrates the dense leafy canopy far overhead. The roots of even the larger trees in these hot, humid forests do not have to descend deeply in order to find the necessary moisture or to receive protection from frost and sudden changes of temperature, but are usually widely expanded and thickly interwoven over the surface. The earth from which the dense vegetation derives nourishment is surprisingly deficient in vegetable mould, which is a characteristic feature in the moist forests of temperate and even subarctic regions, where the complete decay of dead vegetation is long delayed. In the tropical forests the annual supply of dead vegetable matter suitable to be transformed into humus is far greater than on a corresponding area in the woods of more northern regions, but decay is so rapid, owing to the uniformly high temperature and the conditions favouring the multiplication of bacteria, that even great trees on falling quickly disappear; in many instances, the forms of prostrate tree trunks are preserved and overgrown with luxuriant mosses or gorgeously festooned with ferns and orchids, but soon become fragile shells from which nearly all the woody tissues have been removed by decay or by swarming colonies of insects. Where life is so exuberant and the wants of growing plants so great it seems as if the food supply was insufficient, and that none could be spared to accumulate on the ground and form a soil. The two most characteristic and distinctive classes of plants in the tropical forests are the palms and the ferns, each of which is represented by many genera, a large number of species, and multitudes of individuals, and in each class there is a gradation in size from low herbaceous growths to arboreal forms.