They are found in the Compoeitae (Cickoriaceae), Campanulaceae, Papaveraceae, Loheliaceae, Papayaceae, in some Aroideae and Musaceae, and in Euphorbiaceae (Manihot, Ilevea).
Amongst arboreous families Leguminosae and Euphorbiaceae are prominent; Hevea belonging to the latter is widely distributed in various species in the Amazon basin, and yields Para and other kinds of rubber.
Euphorbiaceae and Scrophulariaceae and Orchidaceae are universally present, the last in specially large proportions.
The more common plants in the most characteristic part of this region in southern Arabia are Capparidaceae, Euphorbiaceae, and a few Leguminosae, a Reseda and Dipterygium; palms, Polygonaceae, ferns, and other cryptogams, are rare.
The trees and plants whose latices furnish caoutchouc in considerable quantity chiefly belong to the natural orders Euphorbiaceae, Urticaceae, Apocynaceae, Asclepiadaceae.