A genus of about ten closely related species, natives of the northern hemisphere, including Central America as far south as Guatemala and Costa Rica. They are medium-sized trees, with deciduous, alternate, parallel-nerved leaves, quite closely related to the hornbeams (Carpinus), and in the foliage especially similar. The chief botanical differences are in the female flowers and fruits. In both genera the female flowers are borne on slender catkins, and in pairs at the base of deciduous scales. In Ostrya, however, each flower is set in a bag-like husk (involucre), which at first is open at the top, but closes up after fertilisation takes place. The husk afterwards grows very considerably, and is the pale, membranous, ovate, flattish, bladder-like organ which, congregated and overlapping in hoplike clusters, and completely enclosing the nutlet, gives the trees of this genus their popular name. In Carpinus this involucre remains open and does not enclose the nutlet.
The four ostryas described here should be raised from seed; they thrive in any soil of good or moderate quality, all being perfectly hardy.