ERICALES

order of dicotyledonous flowering plants, a division of the subclass Dilleniidae. Mostly shrubs and small trees that are found throughout the world, the order includes such ornamentals as the azaleas and rhododendrons as well as such food plants as blueberries and cranberries. order of flowering plants that belongs to the class Magnoliopsida (dicotyledons; characterized by two seed leaves). It comprises 8 families, some 160 genera, and about 4,500 species of mostly shrubs and small trees distributed in all areas of the world. They are widely distributed throughout the tropical and temperate regions of the Northern and Southern hemispheres, and a few genera extend into the Arctic and Antarctic regions. The Ericales include azaleas, rhododendrons, and heath plants, all prized for their flowers and abundant growth. The most common characteristic of the Ericales is the single integument of the ovule, the outer layer that becomes the seed coat. Other features, less constant in this order, include the shrub habit, simple and leathery leaves, perfect flowers (male and female components in the same flower), and pollen that is shed in the four-grouped form. Bogs and peaty soils with a high sand content are favourite habitats for many members of the Ericales; when planted in rock gardens, well-drained, moist soil with a high organic content is required. Rhododendrons and many members of the heath family (Ericaceae) do not thrive on clay soils or on limestone. The Ericaceae family is widely distributed, extending into the subarctic and along mountain chains through the tropics. It is by far the most important family of the Ericales; a large percentage of its approximately 110 genera and 4,000 species are under cultivation. There are more than 800 species of Rhododendron (which includes the azaleas). Many Himalayan species, in a variety of beautiful colours, are now grown in warmer temperate regions. Although azaleas, with funnel-form flowers, can be distinguished visually from taller rhododendrons, with bell-shaped flowers and evergreen leaves, no clear distinction between these two groups is possible on the basis of floral and vegetative analyses; azaleas are included in the genus Rhododendron. Other ornamental genera include Calluna (heather of northern climates), Erica (heaths of southern climates), Kalmia (mountain laurel), and Pieris (andromeda). Cultivars (horticultural varieties) of the genus Vaccinium, derived largely from the highbush blueberries V. corymbosum and V. ashei, support a major industry in the eastern United States. In New England and maritime Canada native populations of the low-growing blueberries cover marginally productive lands. The other families of the Ericales have distinctive distributions and contain a wide variety of species. After Ericaceae, the next largest family is Epacridaceae, with 30 genera and some 400 species of small trees and shrubs, found primarily in Australia and New Zealand, with a few species in Southeast Asia and South America. The other families in the order are relatively small and include Cyrillaceae, Clethraceae, Grubbiaceae, Empetraceae (the crowberry family), Pyrolaceae (the shinleaf family), and Monotropaceae (the Indian pipe family), which is almost entirely saprophytic, living off decaying vegetable matter with the aid of fungi. Threadlike cells (hyphae) of soil fungi belonging to the mushroom and related groups of molds envelop the roots of the Monotropaceae and many other members of the Ericales. Outer layers of roots, changed in structure by this association with fungi, are known as mycorrhizae. This arrangement aids in the absorption of mineral nutrients and organic substances by the host plants. The production of seeds is the primary method of reproduction in the Ericales. Inflorescences are usually borne at the ends of branches in elongate, often branched clusters. Species of many families bear a single flower just above the juncture of leaf and stem. A generalized flower of the Ericales possesses sepals, white- or pink-coloured petals, stamens, and a pistil within the same flower. The ovary is superior (flower parts arise below the ovary). Five sepals are joined basally to form a green calyx, and the five petals of the corolla are separate from each other. Stamens occur in two whorls when they number 10. The ovary of the pistil contains many ovules in each of the five chambers; a single style and stigma arise above the ovary. Additional reading Fred C. Galle, Azaleas, rev. and enlarged ed. (1987), a well-illustrated book for a general audience, on native and cultivated azaleas; Peter A. Cox, The Larger Rhododendron Species, rev. ed. (1990); H.H. Davidian, The Rhododendron Species, 3 vol. (198292); Richard A. Jaynes, Kalmia (1988), also well illustrated and for a general audience, on the mountain laurels and their cultivated forms; A.H. Baker and E.G.H. Oliver, Ericas in Southern Africa (1967), a technical book with detailed scientific descriptions; J.L. Luteyn, Speciation and Diversity of Ericaceae in Neotropical Montane Vegetation, in L.B. Holm-Nielsen, I.C. Nielsen, and H. Balslev (eds.), Tropical Forests: Botanical Dynamics, Speciation, and Diversity (1989), pp. 297310, a technical account of the South American members of this family and how they have adapted to montane habits; P.F. Stevens, Phytogeography and Evolution of the Ericaceae of New Guinea, in J.L. Gressitt (ed.), Biogeography and Ecology of New Guinea, vol. 1 (1982), pp. 331354, a technical account.