This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
A branching oil-bearing shrub (the Asiatic species trees), with thin alternate pinnately-veined entire short-petioled deciduous leaves, and dioecious or polygamous small greenish racemose flowers. Staminate flowers with a campanulate 3-5-cleft calyx, the lobes valvate, recurved or spreading, pubescent at the base within; disk of 3-5 distinct glands or scales; stamens 4 or 5, inserted between the glands and opposite the calyx-lobes; filaments short; anthers ovate. Pistillate and perfect flowers with a top-shaped calyx adnate to the obovoid ovary; style short, stout; stigma capitate, depressed. Fruit a pear-shaped or oval drupe, the endocarp thin and endosperm of the seed very oily. [Name from Pyrus, the pear, from the similar shape of the fruit.]
Three species, the following typical one, and two Asiatic.
A straggling or erect much branched shrub, 3°-15° tall, with terete twigs, the young foliage pubescent. Leaves oblong, oblong-lanceolate or somewhat obo-vate, nearly glabrous when mature, acute or acuminate at both ends, 3'-5' long, 8"-1 1/2' wide; petioles 2"-4" long; racemes terminating short branches, the staminate many-flowered, 1-2' long, the pistillate few-flowered and shorter; pedicels slender, 1 1/2"-2" long; staminate flowers about 2" broad; calyx 3-5-cleft; drupe about 1' long, crowned by the ovate acute calyx-lobes.