genus of about five species of trees and shrubs in the elm family (Ulmaceae) native to Asia. The Japanese zelkova, or keaki (Z. serrata), up to 30 m (100 feet) tall and with sharply toothed deep green leaves, is an important timber tree and bonsai subject in Japan. It is widely planted elsewhere as a shade tree substitute for the disease-ravaged American elm, and, while not as cold-hardy as the latter, it is more resistant to Dutch elm disease. Z. carpinifolia, from the Caucasus, and Z. sinica, from China, are somewhat shorter trees with wavy-toothed leaves.