The zone of tropical forests shows, especially on the Atlantic side behind the coast region, a strip of from twenty to twenty-two miles in width, with lofty trees of _Rubiaceæ_, _Myrtaceæ_, _Melastomaceæ_, _Sterculiaceæ_, _Euphorbiaceæ_, _Meliaceæ_, _Urticaceæ_, _Moraceæ_, _Anacardiacæ_, _Sapindaceæ_, _Leguminosæ_ and _Palmæ_. It is relatively free from ligneous undergrowth, having more monocotyledonous plants, such as _Cycadeæ_, _Scitamineæ_, _Cannaceæ_, _Marantaceæ_, _Cyperaceæ_, _Filices_ and _Bromeliaceæ_, underneath. The latter orders figure, also with _Orchideæ_ and _Loranthaceæ_ among the epiphytes and parasites which cover the trees. Among the most characteristic plants of this region we name the coyol palm (_Acrocomia_), corozo (_Attalea cohune_), biscoyol (_Bactris horrida_), palmiche (_Elæis melanococca_) and _Raphia nicaraguensis_ which forms almost forests along the River San Juan; further, _Tecoma pentaphylla_, _Bombax ceiba_, _Eriodendron_, _Spondias_, _Croton gossypifolius_, _Hymenæa courbaril_, rubber trees (_Castilloa costaricencis_ and _C. elastica_), _Geoffræa superba_, _Simaba cedron_, species of _Enterolobium_, _Cæsalpinia_, _Liquidambar_, _Copaifera_, _Cedrela_, _Swietenia_, _Sapota_, _Pithecolobium_, _Palicourea_, _Cinchona_, _Piper_, _Ficus_, _Cecropia_; still further, smilax, vanilla, etc. Many of these characteristic plants are largely social, such as the piper, ferns, palms and others. Moritz Wagner states that all along the southern limits of Costa Rica a likeness of climatic and geological conditions gives to the vegetation a nearly uniform character, while further northward a notable contrast is observed between the Atlantic and Pacific slopes of the mountain groups and on the interior terrace lands. The Atlantic slope, with more constant humidity of air, is characterized by vast, dense, evergreen, virgin forests, while the Pacific lands, with a relatively dry climate and rainless summer, present more open forests and savannas, with many deciduous trees and shrubs. However, deep river valleys and some slopes near the water-shed have dense, evergreen forests, and their vegetation does not differ much from that of the Atlantic slope. The flora of the high terrace lands has been so altered by thorough cultivation as to have almost lost its original character. The Atlantic virgin forests, as well as those in the region of the San Juan River and of Lake Nicaragua, which comprise two-thirds of Costa Rican territory, show such a dense vegetation that its interior can be penetrated almost only by way of the rivers, and its general character and its enormous extension be studied only from high mountains. Owing to the very mountainous character of the country, over half of its area lies between 900 and 2100 meters above the sea, and is almost wholly covered with virgin forest. This forest here and there ascends still higher, reaching the upper limit of the oak region about 2700 meters above the sea.