GYMNOSPERM

any woody plant that reproduces by means of a seed (or ovule) in direct contact with the environment, as opposed to an angiosperm, or flowering plant, whose seeds are enclosed by mature ovaries, or fruits. Gymnosperms is generally considered to be an informal term designating four separate extant divisions of related plantsConiferophyta, Cycadophyta, Ginkgophyta, and Gnetophyta. More than half of all gymnosperms are trees, and most of the rest are shrubs. Gymnosperms occur on all continents but Antarctica, and especially in the temperate latitudes. The most widely dispersed are the junipers (genus Juniperus), which can be found throughout the Northern Hemisphere. Others widely found in the Northern Hemisphere include the firs (Abies), larches (Larix), spruces (Picea), and pines (Pinus). The most widely dispersed gymnosperms of the Southern Hemisphere are the podocarps (Podocarpus). In contrast, many species of gymnosperms are severely limited in geographic distribution. Redwoods (Sequoia sempervirens ), for example, are limited to California and Oregon in the United States, the big trees (giant sequoias; Sequoiadendron gigantum) to California only. The spruces and larches are the most northerly of all trees. The highest known altitude to be reached by a vascular plant is that of a joint fir (Ephedra) growing at 17,500 feet (5,350 m) in Kashmir. Most of the commercial lumber in the Northern Hemisphere is produced from the trunks of such conifers as pines, Douglas firs (Pseudotsuga), spruces, firs, cedars (Cedrus), hemlocks (Tsuga), and redwoods. In the Southern Hemisphere, the kauris (dammar pines; Agathis australis), podocarps, dacrydiums (Dacrydium), and araucarias (Araucaria) are valuable timber trees. The wood of gymnosperms is often called softwood to differentiate it from the hardwood of angiosperms. Paper pulp is derived from the hemlocks, spruces, firs, and pines. Many of the timber and pulp trees are also planted as ornamentals, as are the junipers, larches, cycads, arborvitaes (Thuja), yews (Taxus), and the ginkgo (Ginkgo biloba). The Japanese use pines and junipers in the art of tree dwarfing (bonsai). The gymnosperms are only a minor source of food. There is wide commerce in pine nuts (the seeds of the pion pines), and the seeds of the araucarias, ginkgo, and gnetums are eaten locally. Although poisonous to human beings in their natural state, the seeds and stems of the cycadsespecially those of the genera Cycas, Encephalartos, Macrozamia, and Zamiawhen processed yield a starch that is an important local food in times of famine. The fleshy cones (called berries) of the junipers contain an essential oil used to flavour alcoholic beverages, particularly gin. Other essential oils are derived from spruce, pine, juniper, fir, hemlock, and arborvitae trees. The oils are used in soaps, air fresheners, disinfectants, pharmaceuticals, cosmetics, and perfumes. Some are prized for their own pleasing aromas, while others are odour-fixative or antiseptic. Tannin, used for curing leather, is obtained from the hemlock and spruce. Turpentine is an oleoresin derived from pines. It is processed to yield oil of turpentine (commonly called turpentine) and rosin. Turpentines are also obtained from the balsam fir (Abies balsamea), spruces, and European larch (Larix decidua). Some joint firs yield ephedrine, an alkaloid drug used to treat certain respiratory illnesses. Gymnosperms were a major component in the vegetation that was compressed over millions of years into coal. Gymnosperms can be considered as pests if they invade rangeland or abandoned fields that are to be reused. Nonetheless, gymnosperms are important for erosion control, and some species of conifers are widely planted for reforestation, shelter belts, and windbreaks. Most gymnosperms are evergreen, and of the evergreens, most retain their leaves more than one year. Only the cycads have compound leaves, the rest having simple leaves of various forms. Gingko leaves fork equally and repeatedly, while the leaves of cycads resemble those of palms. The veins in the leaves of gnetums form a network like those of angiosperms. Conifer leaves may be needlelike, scalelike, or broad. In the joint firs and Phyllocladus, the main photosynthetic organs are the stems, not the leaves. Gymnosperms produce male and female reproductive cells (pollen grains and megaspores, respectively) in separate male and female strobili (conelike structures), which may be borne on different plants (as in the cycads) or on the same plants (as in most conifers). Male cones, or microstrobili, vary in size from 1/8 inch (2 mm) long in the junipers to 30 inches (80 cm) long in some cycads. Each cone consists of a number of scales arranged in a spiral on a central axis. Each scale bears two or more pollen sacs on its undersurface. Female cones, or megastrobili, can be of great size. Those of the cycad Macrozamia denisonii of Australia may be almost 100 cm (40 inches) long and weigh up to 38 kg (84 pounds). Certain junipers produce the smallest seed cones, only 3/16 inch in diameter. Scales of the seed cones may be fleshy and fused together, as in the berries of junipers, or papery or woody and distinct, as in spruces and pines. Some gymnosperms bear their seeds in structures of a fleshy outer covering and a hard, stonelike interior, resembling plums. This occurs in gingkos, yews, podocarps, Torreya, and Cephalotaxus. Gymnosperm megaspores are borne exposed to the air in ovules, on the surfaces of the scales of the megastrobili, or on the stalks of leaves. Pollen grains are carried to the ovules and germinate there, sending extensions called pollen tubes into the ovular tissues. After fertilization, an embryo develops; the surrounding integument hardens into a seed coat. Most gymnosperms are pollinated by the wind, a few by insects. Gymnospermous seeds are dispersed by the wind and animals, and a few (the bald cypresses, Taxodium) by water. The earliest gymnosperms were members of the now-extinct division Pteridospermophyta (or seed ferns), which originated in the Late Devonian Epoch (374 million to 360 million years ago). They were abundant until the Triassic Period (245 million to 208 million years ago), when their dominance was displaced by the angiosperms. Four divisions of gymnosperms are now extinct. The division Coniferophyta (conifers) is the most widespread today. any vascular plant that reproduces by means of an exposed seed, or ovule, as opposed to an angiosperm, or flowering plant, whose seeds are enclosed by mature ovaries, or fruits.The seeds of many gymnosperms (literally, naked seed) are borne in cones and are not visible. These cones, however, are not the same as fruits. During pollination, the immature male gametes, or pollen grains, sift among the cone scales and land directly on the ovules, which contain the immature female gametes, rather than on elements of a flower (the stigma and carpel) as in angiosperms. Furthermore, at maturity, the cone expands to reveal the naked seeds. It was in 1825 that the Scottish botanist Robert Brown first distinguished gymnosperms from angiosperms. At one time they were considered to be a class of seed plants, called Gymnospermae, but taxonomists now tend to recognize four distinct divisions of extant gymnospermous plants (Coniferophyta, Cycadophyta, Ginkgophyta, Gnetophyta) and to use the term gymnosperms only when referring to the naked-seed habit. Not all divisions of gymnosperms are closely related, having been distinct groups for hundreds of millions of years. Currently, about 6070 genera are recognized, with a total of 700800 species. Gymnosperms are distributed throughout the world, with extensive latitudinal and longitudinal ranges. Additional reading General works providing comprehensive coverage of the gymnosperms include K.R. Sporne, The Morphology of Gymnosperms: The Structure and Evolution of Primitive Seed-Plants, 2nd ed. (1974), a compact summary discussing both living and extinct groups; Thomas N. Taylor, Paleobotany: An Introduction to Fossil Plant Biology (1981), an excellent survey of fossil plants, including the history of the various gymnosperm groups and especially strong on the evolution of seeds; Charles Joseph Chamberlain, Gymnosperms: Structure and Evolution (1935, reprinted 1966), a classic description of the life history and morphology of all extant groups; W. Dallimore and A. Bruce Jackson, A Handbook of Coniferae and Ginkgoaceae, 4th ed., rev. by S.G. Harrison (1966), a well-illustrated discussion of many representative types, including cultivated forms; and Ernest M. Gifford and Adriance S. Foster, Morphology and Evolution of Vascular Plants, 3rd ed. (1989), focusing on the structure and reproduction of vascular plants, including the gymnosperms.