ACACIA, ȧ-kā′sha, (Gr. akē, spine, from their spiny stalks), a genus of plants, family Mimosaceæ. They are trees or shrubs with compound pinnate leaves and small leaflets,– in some species wholly or partially undeveloped, with the petiole or leaf-stalk expands into a blade resembling a leaf, hence called phyllodium. Certain species yield gum arabic, gum senegal and other gums; some have astririgent barks and pods, used in tanning. Some of the Australian species contain considerable tannin, and hence are exposed to a large extent. An Indian species yields an astringent called catechu. Some of the species of tropical America, known as hull-horn Acacias, are interesting because of their large hollow spines which are penetrated and inhabited by ants.