This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
. Trees very similar to the Elms in habit and foliage, though usually with a naked trunk and branched head. Flowers polygamous. Perianth 4- or 5-lobed; stamens 4 or 5. Fruit 1-or 2-celled, 1- or 2-seeded, nut-like, and covered with protuberances, not winged. The species are natives of North America, the Caucasus, Siberia, and Japan. This genus was named after a German botanist.
1. P. Richardii, syn. P. crenata. - This is a handsome deciduous tree from 60 to 80 feet high with oblong deeply crenate leaves and smooth deciduous bark. A native of the Caucasus, by no means so common in our parks as it deserves to be, for it is perfectly hardy, though it rarely flowers in this country.
P. aquatica, syn. P. ulmifolia, is a North American species with serrated foliage, of much smaller stature and less hardy. P. Japonica is a similar Japanese species recently introduced.