The hazel (Corylus) is a genus of deciduous trees and large shrubs native to the temperate Northern Hemisphere. The genus is usually placed in the birch family Betulaceae, though some botanists split the hazels (with the hornbeams and allied genera) into a separate family Corylaceae. The fruit of the hazel is the hazelnut. The shape and structure of the involucre, and also the growth habit (whether a tree or a suckering shrub), are important in the identification of the different species of hazel.