URTICALES

nettle order of dicotyledonous flowering plants, the largest order in the subclass Hamamelidae. The order includes a diversity of plant types that range from small herbaceous species to large trees. Among the members are stinging nettles; mulberry, fig, and elm trees; hop vines; and the hemp, or marijuana, plant. David E. Boufford the nettle order of flowering plants, containing about 110 genera and approximately 2,200 species in six families and belonging to the class called dicotyledon (characterized by two seed leaves). The order includes a diversity of plant types ranging from small herbaceous (nonwoody) species to large trees. Among the group are stinging nettles; mulberry, fig, and elm trees; hop vines; and the hemp, or marijuana, plant. Members of the order grow abundantly throughout the world. The large mulberry family (Moraceae) contains mostly trees and shrubs of the tropics and subtropics, with a few temperate species. The nettle family (Urticaceae) is another large group, consisting predominantly of herbs, some of them with stinging hairs. They occur mainly in the tropics, where they are shrubby and treelike, but many herbaceous nettles are native to temperate areas of the Northern Hemisphere. Ulmaceae, the elm family, includes trees and shrubs distributed over the north temperate, tropical, and subtropical regions. Some species grow to impressive heights. The family Cecropiaceae's most prominent members are fast-growing trees in the New World tropics. The hemp family (Cannabaceae) is the second smallest of the order, containing aromatic herbs of the hemp and hop genera, which are widespread in northern temperate areas. The taxonomic status of the family Barbeyaceae, which has only one species, remains uncertain. Many species in the elm and mulberry families, including red mulberry (Morus rubra), letterwood (Brosimum guianense), and many members of Ficus, are used as timber trees and ornamentals. The rock elm (Ulmus thomassi), the hackberry (Celtis occidentalis), and other members of the Ulmaceae yield a variety of useful woods. The mulberry family is the source of the chief edible fruits in the order. The breadfruit tree (Artocarpus altilis) is one of the highest yielding of food plants, and the carbohydrate-rich breadfruits constitute a staple food in many tropical lands. The edible fig (F. carica) has been cultivated since antiquity. Silkworms feed almost exclusively on mulberry leaves, particularly those of Morus alba. Some members of the Moraceae have a milky juice, latex, which furnished the earliest source of rubber, and species of Ficus and Castilla are still grown commercially to make some types of rubber and other products. Paper and bark cloth (tapa cloth) are products of the paper mulberry (Broussonetia papyrifera). The two genera of the hemp family, Humulus and Cannabis, have been economically important for many centuries. The flowers of H. lupulus are the hops used in brewing to flavour beer. Various parts of the hemp plant (C. sativa) are dried to make the drug marijuana. Hemp is also the source of coarse fibres used for rope and sacks. A member of the nettle family, the Chinese grass plant (Boehmeria nivea), yields a fine fibre called ramie that is made into silklike cloth. Urticales is distinguished from other plant orders chiefly by floral characteristics. In most members of the order, flowers are unisexual; that is, male and female structures occur in separate flowers. The ovary (female) has two carpels (ovule-bearing structures) but contains just one ovule (potential seed), which produces a single seed. Male flowers have only a few pollen-bearing stamens. Flowers are inconspicuous, lack petals, and are composed of four or five small greenish sepals. Leaves are simple, grow alternately on the stems, and have appendages called stipules. The stinging nettle (Urtica ferox), showing secretory (glandular), or stinging, hairs Some species of nettles have stinging hairs tipped by a sharp terminal cell that penetrates skin, breaks off, and releases an irritating fluid. In the wood nettle of eastern North America (Laportea canadensis), the stinging fluid is formic acid. The more common species of Urtica (U. dioica and U. gracilis) produce an irritation that may last for hours, and other species can cause more severe reactions. The nettle family typifies Urticales in being wind-pollinated. Some species are specially adapted for this process by having explosive stamens. In the artillery plant (Pilea microphylla) and many other genera of Urticaceae, the stamens spring out suddenly when the male flower bud opens, releasing puffs of pollen that the wind carries to female flowers of other plants. Among the Moraceae, some species are insect-pollinated, notably the common fig. Small gall wasps use the clustered flower structures for laying their eggs, and they pollinate the fig trees in the process. Some of the flowers form an abnormal tissue, the gall, on which the wasp larvae feed. This close relationship between figs and gall wasps is a type of symbiosis, or mutual interdependence, that is also exemplified in the Urticaceae family. There the tropical American trumpet tree (Cecropia peltata) provides living space and special food bodies for ants of the Azteca genus, which in turn attack leaf-cutter ants that attempt to chew up the foliage. Many fruits of the Moraceae are multiple fruits, formed by the fusion of several flowers. Each seed in a fig is actually an individual fruit. The jackfruit (Artocarpus heterophyllus) produces immense fruits that grow along the main tree trunk. Two kinds of fruits appear in the Ulmaceae family. Elms (Ulmus) have a seed enclosed in a dry, winged structure adapted for wind dispersal. In other family members, such as the hackberries (Celtis), the fruit is fleshy, with a stony pit, and the seed is dispersed by birds and animals that eat the fruit. In some species of Ficus, including the strangler figs, birds and bats drop fig seeds onto the branches of other trees. There the seeds develop aerial roots that grow down along the trunk of the supporting tree, enlarge, and gradually crush the host to death. Banyans (F. benghalensis), which grow thick prop roots from their wide-spreading branches, may reach an immense size and resemble a dense grove of trees. Additional reading A useful study is C.C. Berg, Urticales, Their Differentiation and Systematic Position, in Plant Systematics and Evolution, Supplementum, vol. 1 (1977), pp. 349374.