JUGLANDALES

the walnut order of flowering plants, consisting of two families, Rhoipteleaceae and Juglandaceae, belonging to the class called dicotyledon (q.v.; characterized by two seed leaves). Members of this order are trees bearing inconspicuous flowers, usually aggregated into conelike, pliable catkins. Walnut, pecan, and hickory (qq.v.) are among the many trees of Juglandales that are prized for both their edible nuts and their strong, attractive woods. The Rhoipteleaceae family consists of only a single species growing in northern Vietnam and in China. The large and economically important Juglandaceae family comprises 8 genera and about 60 species distributed mainly in the North Temperate Zone, but extending through Central America along the Andes Mountains to Argentina and, in scattered stands, from temperate Asia to Java and New Guinea. Whether growing in forests of temperate regions, in mountainous areas, or along seashores of the tropics, members of Juglandaceae require a reliable level of soil moisture. Those thriving on dry hillsides depend on well-spaced rains. A few species grow along water courses in moist canyons and in fog belts of mountains. Walnuts and hickories seldom grow in solid stands; instead, they are intermingled with understory shrubs and oaks in deciduous forests as in the eastern United States and eastern Mexico. Stems of black walnut trees attain heights of 53 m (175 feet). Massive trunks are also developed by pecans and some hickories. Tree stems consist almost entirely of secondary woodi.e., wood produced by a cambial cylinder composed of cells that differentiate inwardly, to add to the wood already present. Cambium, also present in roots, is active mainly in the spring. Woods of this family are especially noted for their grain patterns and for the lustre of local bands of cells (wood rays) that radiate toward the perimeter of the trunk. Black walnut (Juglans nigra) and pecan (Carya illinoinensis) are among the Juglandaceae that are in great demand for their strength and beauty; many natural stands have been depleted. Leaves of the walnut family are pinnately compound. The leaf-stalk (petiole), which is a lateral outgrowth of a twig (stem), continues as an elongate unit (the rachis) from which the oppositely positioned leaflets arise. The rachis is the equivalent of the midrib of a simple leaf. The number of leaflets in Juglans ranges from 7 to 23. The undersurface of the leaflets of many members bears yellow dots or tiny, resinous scales that give the species a pungent, characteristic aroma. Buds, flowers, and fruits often are more aromatic than the leaves. Platycarya, a tree of China, and several species of Pterocarya, native to a wide area in Asia and attaining heights of about 30 m, produce roots on buried twigs that eventually are separated from the parent plant. Pterocarya and several hickories bear suckers and root sprouts to reproduce the species vegetatively. Otherwise, members of the walnut family multiply by seeds. The inflorescence (flower cluster) of the Juglandaceae consists of an elongate stalk bearing many flowers. The staminate (male) cluster is a catkin in which stalkless masses of the pollen-producing stamens are associated with small, leaflike bracts. As many as 100 stamens may be produced by a single male flower of a catkin. The pistillate (female) flowers are spaced farther apart to form a spike that, in some genera, consists of only one or two flowers. Both kinds of inflorescences are borne on the same tree, except in Engelhardtia. Pollination is achieved by wind. The fruit of nut-bearing trees is composed of three layers: an outer skin, a middle pulpy zone, and an inner, stony layer (endocarp). These features typically define a drupe, but it is generally named a drupaceous nut, or simply a nut. In areas where squirrels are present, many nuts are stored underground and are not reclaimed, with the result that, by spring, many seedlings appear.