Gymnosperms

Gymnosperms are seed plants that do not produce flowers. The term gymnosperm means "naked seed." However, usually when the seeds of gymnosperms are immature they are enclosed within and protected by modified leaves or a cone. In flowering plants (or angiosperms, which means "vessel seed") the ovary wall or fruit encloses the seeds, whereas in gymnosperms there is no equivalent structure; hence, the interpretation of the seeds as "naked" or not enclosed.

There are four groups of gymnosperms living today—Coniferophyta, Cycadophyta, Ginkgophyta, and Gnetophyta—but many additional groups are known from the fossil record. Seed plants evolved more than 350 million years ago and the first seed plants were gymnosperms. The relationship of the flowering plants to the gymnospermous seed plants remains a hotly contested issue within the scientific community.

Although each group of gymnosperms has its own specific characteristics, some features are shared throughout. For example, all gymnosperms produce at least some secondary growth, whereas secondary growth is lacking from most spore-bearing (nonseed) plants. Secondary growth is plant growth that does not occur directly from the tips of the plant; it is growth that occurs horizontally (or radially) rather than vertically. The most abundant product of secondary growth is the secondary xylem (xylem is the water-conducting tissue of plants), or wood.

Aspects of reproduction are also shared by all the gymnosperms. The ovule (the technical term for the seed prior to fertilization) consists of female nutritive tissue plus the female gamete, or egg, both enclosed by a

DISTINGUISHING CHARACTERS OF GYMNOSPERMS
Characters	Coniferophyta	Cycadophyta	Ginkgophyta	Gnetophyta
Extant members	50-60 genera, about 550 species	11 genera, about 160 species	1 species (Ginkgo biloba)	3 genera: Ephedra (35 species), Gnetum (30 species), Welwitschia (1 species)
Distribution	Worldwide, especially temperate regions	Tropics	Native to a small area of China	Isolated areas of temperate and tropical regions
Habit	Trees with branched woody trunk	Trees with unbranched fleshy trunk	Trees with branched woody trunk	Shrubs, vines, small trees, or weird tubers
Leaves	Simple, needlelike or broad	Large, compound (each leaf composed of many leaflets)	Simple, fan-shaped	Simple, needlelike or broad
Reproductive structure	Simple (unbranched) male, compound (branched) female cones	Simple cones (unbranched), male or female	Simple cones (unbranched), male or female	Compound (branched) male or female cones
Sex	Separate male and female cones on each plant (monoecious) or separate male and female plants (dioecious)	Individual plant male or female (dioecious)	Individual plant male or female (dioecious)	Individual plant male or female (dioecious)
Seeds	Small, without differentiated layers	Large, with outer fleshy layer and middle stony layer	Large, with outer fleshy layer and middle stony layer	Small, without differentiated layers
Sperm	No flagella; pollen tube delivers to egg	Swims using many flagella	Swims using many flagella	No flagella; pollen tube delivers to egg

layer of protective tissue (the integument). The male gamete is carried within the pollen. In most gymnosperms pollen is produced in great amounts, and the pollen grains are dispersed by the wind. Usually a small amount of sticky liquid (the pollination drop) is exuded at the tip of the ovule. Pollen grains become stuck in the pollination drop and, as the drop dries, the pollen is pulled into the ovule. Depending on the type of gymnosperm, the male ga-mete is released from the pollen and swims to the egg, or the male gamete is transported within a tube that grows to the egg. The fertilized egg then develops into the embryo of the seed. Ultimately, when the seed germinates, the embryo grows to produce the young seedling using the female nutritive tissue as a source of energy. An unusual attribute of the gymnosperms, except for the gnetophytes, is the long length of time—a year or more—that passes between the production of the egg and the sperm and the actual occurrence of fertilization.

Coniferophyta

The Coniferophyta, or conifers, are the most abundant group of living gymnosperms and the first of the living gymnosperm groups to appear in the fossil record. They have been important components of Earth's vegetation for almost three hundred million years. The oldest (bristlecone pine), the tallest (coast redwood), and the biggest (giant sequoia) organisms on the planet today are conifers. Most conifers are large trees that make abundant wood, have small evergreen leaves, and produce their seeds within woody cones.

Although most conifers fit that standard description, there are numerous exceptions. First, not all conifers are evergreen. A few types, such as the larch, are deciduous, losing all their leaves each fall and growing new leaves in the spring. Even the leaves of the evergreen type are not immortal; it is just that the senescence (aging and death) of leaves occurs individually rather than all at once. Second, not all conifers have small or needlelike leaves. Some conifers, native to the Southern Hemisphere, have broad leaves. Third, although all conifers are woody plants, not all conifers are large trees. A number are shrubs or small trees, and one is even a parasite on the roots of other coniferous trees. Finally, not all conifers have cones as people usually think of them. Some conifers, such as the yew, have solitary seeds covered with a fleshy, colorful tissue.

The female cones produced by most conifers are complex structures that are made of repeating units, each consisting of ovules on a woody platform (ovuliferous scale) beneath which is a bract (e.g., the "mouse tail" that peeks out of Douglas-fir cones). Cones of the other gymnosperms lack the equivalent of the ovuliferous scale. Conifers make rather small ovules and lack swimming sperm.

Cycadophyta

The Cycadophyta, or cycads, are restricted to tropical latitudes and were more abundant in the geologic past. These plants have unbranched, fleshy stems. The trunks of some species can grow fairly tall (15 to 18 meters), but all cycads lack extensive wood development. Because so much water is present in the stems, the plants are very vulnerable to damage from freezing (think of a soda can that explodes after you forget you have placed it in the freezer). Thus the cycads are restricted to parts of the world where freezing temperatures are rare or absent. A few types are found naturally in sub-tropical areas where mild freezing temperatures occasionally occur; these types of cycads have short, squat, subterranean stems. Some additional cycad species are hardy as ornamentals in similar subtropical to warm temperate areas. Excellent outdoor collections of cycads can be seen in the United States at Fairchild Botanical Garden in Miami, Florida, or at Huntington Botanical Garden east of Los Angeles, California.

The leaves of cycads occur as a crown at the top of the stem. Most often the leaves are very large (up to 1.5 meters in length), leathery, and compound; that is, the blade of the leaf is made of many separate leaflets. When the leaves fall off, usually the base of the leaf remains attached to the stem. Thus, the trunk is protected from herbivory and to some extent from freezing by the remnant bases.

Individual cycad plants are either male or female. At the tip of the trunk within the crown of leaves a cone develops that will either contain pollen or ovules, depending on the sex of the plant. Cycads make very large ovules and swimming sperm.

Ginkgophyta

Only one species of the Ginkgophyta group remains living today: the maidenhair tree, Ginkgo biloba. Twenty to thirty million years ago the ancestor of the modern species was found throughout the Northern Hemisphere. However, climatic changes have led to the plants gradually becoming reduced to a smaller and smaller territory. Since the last ice age, wild populations have been restricted to a small area in China. It is debated whether or not any populations are truly wild. The tree still exists today because it was a sacred plant maintained in monasteries. Now the ginkgo tree is commonly used as a street tree and so has regained, in some sense, much of its earlier territory. Individual plants can live very long lives (more than one thousand years), and the species is unusually resistant to pollution and disease. The Ginkgo plant is a fairly tall (25 meters or more), much-branched tree with abundant wood, bearing fan-shaped leaves.

Individual ginkgo trees are either male or female. The sex of the plant does not become apparent until the tree is fifteen to twenty years old. Male plants are preferred as ornamentals. The ovule produced by the female is large with a stony interior and a fleshy outer covering that produces a smell usually equated with rotting butter. Ginkgo also produces swimming sperm.

Gnetophyta

The Gnetophyta are a small, odd group of living gymnosperms. Three types are known, each being somewhat different from the other gnetophytes and unusual among the seed plants in its own way. Species of the genus Ephedra are shrubby plants of arid temperate regions. They have tiny leaves and the stems are often green and photosynthetic. Species of the genus Gnetum are tropical plants that are small trees or, more usually, vines. The gnetums are unusual among the gymnosperms in that they produce broad leaves with netted venation similar to that found in flowering plants. The third group is represented by a single species, Welwitschia mirabilis. Welwitschia has been described as the "weirdest plant on Earth," a plant that has "lost its head" or a giant seedling that can reproduce. Welwitschia is native to the Namib Desert of southwest Africa and consists of a large tuber or taproot and two long, strap-shaped leaves that grow from their bases and are retained for the life of the plant.

Most gnetophytes produce separate female and male cones born on separate (male or female) plants. Each type of gnetophyte has a slightly different reproductive structure but all are variations on one theme: pairs of papery bracts (modified leaves) surround the ovule or pollen organs. Some botanists compare these bracts to the petals and sepals of angiosperm flowers, whereas others equate them to components of the cone found in conifers. Other aspects of the gnetophyte morphology are also ambiguous and, depending on their interpretation, suggests an evolutionary link to either the conifers or the angiosperms. For example, vessels, specialized water-conducting cells, occur in the wood of gnetophytes. These cells are dead at maturity, empty of any contents. Vessels lack any end walls; they are open-ended tubes that line up end to end and act as a pipe to transport water without any obstructions. Most flowering plants have vessels but most other types of plants lack this cell type. Some plant scientists see the gnetophyte vessels as evidence that gnetophytes and angiosperms share a common ancestor. However, because of differences in vessel cell anatomy and development between gnetophytes and flowering plants, other botanists consider the gnetophyte vessel to be an independent evolution of a water transport cell that lost an end wall. Molecular comparisons of the seed plants using different types of deoxyribonucleic acid (DNA) data have not settled this debate: gnetophytes remain a problematic group in terms of evolutionary placement and morphological interpretation.

Ecological Significance

Conifers are the gymnospermous group with the most profound ecological role in Earth's vegetation. Conifers dominate some vegetation types, such as the taiga of high northern latitudes or boreal forests of lower latitudes. Some temperate forests, for example in Argentina, Australia, or northwestern North America, are also composed almost exclusively of coniferous trees.

Conifers can also be important as successional species or as the climax vegetation of odd environments in other temperate or tropical areas. For example, in environments prone to fire, with a long enough growing season and enough precipitation to support the growth of trees, conifers are often present. Conifer bark is thicker than most flowering plant tree bark and so coniferous trees are better able to survive ground fires. Some conifer cones open to release their seeds only after a fire.

Economic Significance

Conifers are also considered the most important gymnospermous group from an economic perspective. Coniferous trees are a very important source of timber for lumber and paper. They are harvested in North America, parts of Europe and Asia, and in Australia. In addition to timber, conifers provide Christmas trees, ornamental trees and shrubs, turpentine, and resin. Pine nuts (or pignoli), the seeds of some pine trees, are used as food. An important cancer-fighting drug, taxol, has been derived from the bark and leaves of the Pacific Coast yew (Taxus). Other gymnosperms also are the source of drugs and herbal medications. The powerful stimulant ephedrine derived from the gnetophyte Ephedra is often used in cold and allergy medications, and compounds shown to improve the mental capacities of the elderly have been discovered in Ginkgo. Ginkgo seeds are also quite nutritious and are used as food in Asia. Ginkgo and cycads are also important as ornamentals.

Linda A. Raubeson

Bibliography

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Hori, T., ed. Ginkgo Biloba—A Global Treasure. New York: Springer-Verlag, 1997.

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Nimsch, Hubertus. A Reference Guide to the Gymnosperms of the World. Champaign, IL: Balogh Scientific Books, 1995.

Norstog, Kurt J., and Trevor J. Nicholls. The Biology of the Cycads. Ithaca, NY: Cornell University Press, 1998.

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