The dense forests also contain many varieties of lianas or rubber vines, huge bombax and bamboos.
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.
These widely divergent conditions give to Mexico a flora that includes the genera and species characteristic of nearly all the zones of plant life on the western continents - the tropical jungle of the humid coastal plains with its rare cabinet-woods, dye-woods, lianas and palms; the semi-tropical and temperate mountain slopes where oak forests are to be found and wheat supplants cotton and sugar-cane; and above these the region of pine forests and pasture lands.
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.
To these lianas (species of which yield one kind of the rubber of commerce) is due largely the weird aspect of the forest.