The density of the forest is greatly augmented by the cipos, or lianas, which overgrow the largest trees to their tops, and by a profusion of epiphytes which cover the highest branches.
Its forests are not composed of one or a few dominating species, as in the cold temperate zone, but of countless genera and species closely interwoven together - a confused mass of giant trees, lianas and epiphytes struggling to reach the sunlight.
The greater number of them are epiphytes or plants that grow on others without absorbing nourishment from them, and heat and moisture afford all or nearly all the nourishment they require.
An epiphytic fungus is not necessarily a parasite, however, as many saprophytes (moulds, &c.) germinate and develop a loose mycelium on living leaves, but only enter and destroy the tissues after the leaf has fallen; in some cases, however, these saprophytic epiphytes can do harm by intercepting light and air from the leaf (Fumago, &c.), and such cases make it difficult to draw the line between saprophytism and parasitism.
In epiphytic forms the rhizoids of the epiphyte often penetrate into the tissue of the host, and certain epiphytes are not known to occur excepting in connexion with a certain host; but to what extent, if any, there is a partial parasitism in these cases has not been ascertained.