in botany, a genus of the monococcia triandria clas. The amentum of the male is imbricated; it has no corolla; and the calyx consists of one leaf. The amentum of the female is likewise imbricated; the corolla is wanting; and the calyx consists of one leaf; the nectarium is inflated and three-teethed; there are three stigmata; and the seeds are triangular and contained within the nectarium. There are 37 species, 26 of which are natives of Britain.