On the lower slopes of the Andes are found oak, beech, cedar, Winter's bark, pine (Araucaria imbricata), laurel and calden (Prosopis algarobilla).
The provinces of Santa Fe, Cordoba and Santiago del Estero are only partially wooded; large areas of plains are intermingled with scrubby forests of algarrobo (Prosopis), quebracho-blanco (Aspido-sperma quebracho), tala (Celtis tala, Sellowiana, acuminata), acacias and other genera.
The western dry areas have the old-world leguminous Astragalus and Prosopis (Mesquit), but are especially characterized by the northward extension of the new-world tropical Cactaceae, Mgmmillaria, Cereus and Opuntia, by succulent Amar llideae such as A gave (of which the so-called American aloe is a type), and by arborescent Liliaceae (Yucca).
In a few hollows which are reached by moisture the trees of the desert find support, the algarrobo (Prosopis horrida), a low tree of very scraggy growth, the vichaya (Capparis crotonoides), and the zapote del perro (Colicodendrum scabridum), mere shrubs.
The " tongue," for example, is short and obtuse or emarginate in Colletes and Prosopis, while in all other bees it is pointed at the tip. But in Andrena and its allies it is comparatively short, while in the higher genera, such as A pis and Bombus, it is elongate and flexible, forming a most elaborate and perfect organ for taking liquid food.