gymnosperm

—gymnospermism, n.

/jim"neuh sperrm'/, n. Bot.

a vascular plant having seeds that are not enclosed in an ovary; a conifer or cycad. Cf. angiosperm.

[1820-30; < NL gymnospermae name of type. See GYMNO-, -SPERM]

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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.

The four surviving gymnosperm divisions are Coniferophyta (conifers, the most widespread), Cycadophyta (cycads), Ginkgophyta (ginkgos), and Gnetophyta. More than half are trees; most of the rest are shrubs. Gymnosperms occur on all continents except Antarctica, and especially in the temperate latitudes. Those widely found in the Northern Hemisphere are junipers, firs, larches, spruces, and pines; in the Southern Hemisphere, podocarps (Podocarpus). The wood of gymnosperms is often called softwood to differentiate it from the hardwood of angiosperms. Many timber and pulp trees are also planted as ornamentals. Gymnosperms also are a minor source of food; of essential oils used in soaps, air fresheners, disinfectants, pharmaceuticals, cosmetics, and perfumes; of tannin, used for curing leather; and of turpentines. Gymnosperms were a major component in the vegetation that was compressed over millions of years into coal. Most are evergreen. They produce male and female reproductive cells in separate male and female strobili (see cone).

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▪ plant

Introduction

 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 (Brown, Robert) first distinguished gymnosperms from angiosperms. At one time they were considered to be a single class of seed plants, called Gymnospermae, but taxonomists now tend to recognize four distinct divisions (and orders) of extant gymnospermous plants—Pinophyta (order Pinales), Cycadophyta (Cycadales), Ginkgophyta (Ginkgoales), and Gnetophyta (Gnetales)—and to use the term gymnosperms only informally 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, 82 genera are recognized, with a total of 947 species. Gymnosperms are distributed throughout the world, with extensive latitudinal and longitudinal ranges.

General features

Diversity in size and structure

  Among the gymnosperms are plants with stems that may barely project above the ground and others that develop into the largest of trees. Cycads (cycadophyte) resemble palm trees, with fleshy stems and leathery, featherlike leaves. The tallest cycads reach 19 metres (62 feet). Zamia integrifolia, a cycad native to Florida, Georgia, and the West Indies, has a short underground stem with the leaf-bearing tip, at most, exposed. Of the gnetophytes, Ephedra (joint fir) is a shrub and some species of Gnetum are vines, while the unusual Welwitschia has a massive, squat stem that rises a short distance above the ground. The apex is about 60 centimetres in diameter. From the edge of the disk-shaped stem apex arise two leathery, straplike leaves that grow from the base and survive for the life of the plant. Most gymnosperms, however, are trees. Of the conifers (conifer), the redwoods (Sequoia) exceed 100 metres in height, and, although Sequoiadendron (giant redwood) is not as tall, its trunk is more massive.

Distribution and abundance

 Although since the Cretaceous Period (about 145.5 to 65.5 million years ago) gymnosperms have been gradually displaced by the more recently evolved angiosperms (angiosperm), they are still successful in many parts of the world and occupy large areas of the Earth's surface. Conifer forests (coniferous forest), for example, cover vast regions of northern temperate lands in North America and Eurasia. In fact, they grow in more northerly latitudes than do angiosperms. Vascular plants that occur at the highest altitudes are the gnetophyte Ephedra. Land in the Southern Hemisphere is rich in conifer forests, which tend to be more abundant at higher altitudes. Gymnosperms that occupy areas of the world with severe climatic conditions are adapted to conserving water; leaves are covered with a heavy, waxy cuticle, and pores (stomata) are sunken below the leaf surface to decrease the rate of evaporation.

 Cycads are distributed throughout the world but are concentrated in equatorial regions. As a natural population, ginkgo originally appeared to have been reduced to a small portion of the mountains of southeastern China; extensive artificial propagation has expanded this distribution. Distribution of gymnosperms in the distant past was much more extensive than at present. In fact, gymnosperms were dominant in the Mesozoic Era (about 251 to 65.5 million years ago), during which time some of the modern families originated (Pinaceae, Araucariaceae, Cupressaceae).

Importance to humans and ecology

 Some of the oldest living things on earth are gymnosperms. Redwoods live for thousands of years, and some specimens of the bristlecone pine, found in the White Mountains of California, approach 5,000 years in age.

  Gymnospermous plants are widely used as ornamentals. Conifers are often featured in formal gardens and are used for bonsai. Yews and junipers are often low-growing shrubs cultivated for ground cover and hedges. Conifers are effective windbreaks, especially those that are evergreen. Cycads are used as garden plants in warmer latitudes, and some may even thrive indoors. Their leathery green foliage and sometimes colourful cones are striking. Ginkgo is a hardy tree, and although it once approached extinction, it is now cultivated extensively and survives such challenging habitats as the streets of New York City. Some gymnosperms are weedy in that they invade disturbed areas or abandoned agricultural land. Pines and junipers are notorious invaders, making the land unusable for growing crops.

  Most of the commercial lumber (wood) in the Northern Hemisphere is derived from the trunks of conifers such as pine, Douglas fir, spruce, fir, and hemlock. Araucaria, kauri, and Podocarpus are important conifers of the Southern Hemisphere used for lumber. The wood is straight-grained, light for its strength, and easily worked. Wood of gymnosperms is often called softwood to differentiate it from the hardwood angiosperms. Wood of angiosperms typically has more kinds of elements than does softwood of gymnosperms. In addition to its use in building construction, gymnospermous wood is used for utility poles and railroad ties. Aromatic wood of cedar is frequently used in the construction of closets or clothes chests and apparently repels cloth-eating moths. Most plywood is gymnospermous. Fibres of conifers make up paper pulp and may occasionally be used for creating artificial silk or other textiles. Conifers are frequently planted in reforestation projects. Conifer bark is often the source of compounds involved in the leather tanning industry. Bark is also used extensively as garden mulch.

      From conifer resins (resin) are derived turpentine, rosin, and wood alcohol (methanol). A hardened form of resin from a kauri (Agathis australis), called copal, is used in the manufacture of paints and varnishes. Some resins, such as balsam (from hemlock) and dammar (from Agathis) are used in the preparation of mounting media for microscope slides. Many types of amber are derived from fossilized resin of conifers. Commercially useful oils (essential oil) are derived from such conifers as junipers, pines, hemlock, fir, spruces, and arborvitae. These oils serve as air fresheners, disinfectants, and scents in soaps and cosmetics.

 Seeds are often food sources. Roasted pine seeds are a delicacy eaten plain or used as a garnish on bakery products. Seeds of Ginkgo and cycads may be poisonous unless detoxified. “Berries” (in reality the fleshy cones (cone)) of juniper are used to flavour gin.

      Many species of gymnosperms have been used in traditional medicines to treat a variety of ailments. That not all these purported medicinal uses are unfounded is evidenced by recent advances in cancer treatment involving the use of the drug taxol, extracted from the bark of yew (Taxus) species. Other drugs include ephedrine and its derivatives, originally isolated from Ephedra species and used to treat respiratory ailments.

Natural history

      In all living gymnosperm groups, the visible part of the plant body, i.e., the growing stem and branches, represents the sporophyte, or asexual, generation (alternation of generations), rather than the gametophyte, or sexual, generation.

      In most gymnosperms the pollen cones, called microstrobili, contain reduced leaves called microsporophylls. Microsporangia, or pollen sacs, are borne on the lower (abaxial) surfaces of the microsporophylls. The number of microsporangia may vary from two in many conifers to hundreds in some cycads. Within the microsporangia are cells, called microsporocytes, which undergo meiotic division to produce haploid microspores.

      The gametophyte phase begins when the microspore, while still within the microsporangium, begins to germinate to form the male gametophyte. A single microspore nucleus divides by mitosis to produce a few cells. At this stage the male gametophyte (called a pollen grain) is shed and transported by wind or insects.

      Ovulate cones, called megastrobili, may be borne on the same plant that bears microstrobili (as in conifers) or on separate plants (as in cycads and Ginkgo). A megastrobilus contains many ovuliferous scales, called megasporophylls, that contain megasporangia, within which a single cell (a megasporocyte) undergoes meiotic division to produce four haploid megaspores. Typically three degenerate, leaving one functional megaspore, which is retained within the megasporangium. The female gametophyte begins development within the megaspore. Initial divisions of the megaspore nucleus are mitotic without accompanying divisions of the cytoplasm. As the number of free nuclei multiplies, the megasporangium, integument, and megaspore wall expand. Cell walls eventually develop around the nuclei. At this stage the ovule is ready to be fertilized.

      Before fertilization can take place, however, the mature male gametophyte (the pollen grain) must be transported to the female gametophyte—the process of pollination. In many gymnosperms, a sticky “pollination droplet” oozes from a tiny hole in the megasporangium (the micropyle) and pollen grains are caught in the droplet. The droplet is then resorbed through the micropyle into the megasporangium. The pollen grain settles on the surface of the megasporangium, where the male gametophyte develops further. A pollen tube emerges from the grain and grows through the megasporangium toward the multicellular egg-containing structure called the archegonium. The egg and sperm continue to mature, the nucleus of the latter undergoing additional divisions resulting in two male gametes, or sperm. (Sperm cells have flagella in cycads and Ginkgo but not in any other seed plants.) By the time the pollen tube reaches the archegonium, both the egg and sperm are fully mature, and the egg is ready to be fertilized.

      The nuclei of the two sperm are injected into the egg cell; one nucleus dies, and the other unites with the egg nucleus to form a diploid zygote. The nucleus of the fertilized (fertilization) egg begins the development of a new sporophyte generation by dividing a number of times; the resulting multicellular structure becomes the embryo of the seed. Food for the developing embryo is provided by the massive, starch-filled female gametophyte that surrounds it. The time interval between pollination and maturation of the embryo into a new sporophyte generation varies among different groups, ranging from a few months to over one year (in pine, for example). The integument develops into the seed coat, while the female gametophyte is a source of food for the developing embryo during germination.

      In some gymnosperms (e.g., cycads, Ginkgo) the seed coat (sarcotesta) consists of two layers. In some cycads the sarcotesta is brightly coloured. The sarcotesta of Ginkgo seeds is foul-smelling when ripe. Attached to the seed coat in pine and related conifers is a thin membranous winglike structure, which remains with the seed at its release and serves as a wing that may assist in the distribution of the seed. Members of the order Taxales have a fleshy structure, an aril, surrounding the actual seed. Cones of juniper are fleshy, and the entire fleshy unit drops off or is picked off by birds. Juniper seeds pass through the digestive tracts of birds and are thus distributed effectively.

      At maturity, a gymnosperm embryo has two or more seed leaves (the cotyledons (cotyledon)). Cycads, Ginkgo, and gnetophytes have two cotyledons in the embryo; pine and other conifers may have several (eight is common; some have as many as 18). Below the attachment point of the cotyledons is the hypocotyl, which emerges through the seed coat during germination, bends downward, and eventually establishes the root system. Above the attachment point of the cotyledons is the epicotyl, the tip of which contains the shoot tip and leaves. In cycads and Ginkgo the cotyledons remain within the seed and serve to digest the food in the female gametophyte and absorb it into the developing embryo. Conifer cotyledons typically emerge from the seed and become photosynthetic after digesting and absorbing the food in the female gametophyte.

Form and function

General features

 The visible part of the gymnospermous plant body represents the sporophyte generation. Typically, a sporophyte has a stem with roots and leaves and bears the reproductive structures. The vascular system contains two conducting tissues, the xylem and phloem. The xylem is a tissue containing nonliving cells whose walls form a conducting system of “pipes” through which water and minerals are conducted from the roots to the shoots. The sturdy nature of the xylem makes it useful in support as well. The phloem, like the xylem, is a conducting tissue; its cells, however, are living and distribute the sugars, amino acids, and organic nutrients manufactured in the leaves to the nonphotosynthetic tissues of the plant. When the plant is actively growing, the phloem may also conduct stored nutrients from the roots to the developing shoots.

Stems (stem)

  The stems, roots, and branches of vascular plants undergo secondary growth, which takes place from stem and branch growth tissue, called the vascular cambium. Stems of conifers are characteristically woody with a dense mass of secondary xylem. They are usually branched, with basal branches dropping off as the stem elongates, resulting in a main stem that is often tall and straight. The wood is simpler than that of angiosperms; it consists primarily of elongated water- and mineral-conducting cells (tracheids) in the xylem and living cells that store materials and provide for lateral conduction (vascular rays) in the phloem. The growth tissue of the stem and branches (the vascular cambium) contributes more xylem each growing season, forming concentric growth rings (growth ring) in the wood. Tracheids produced by the vascular cambium early in the growing season are larger, and the walls thinner, than those formed later in the growing season. This results in the characteristic light and dark bands of wood. Some conifers have additional cell types, such as fibres and axially elongated xylem parenchyma cells that store food. Phloem is also simpler than that of angiosperms, consisting of food-conducting cells (sieve cells) and storage cells. Phloem rays traverse the phloem tissue.

  Stems of Ginkgo are anatomically similar to those of conifers. Ginkgo and cedar have two kinds of branches: elongated major branches and dwarf lateral branches. The dwarf shoot bears a cluster of leaves; at the end of the growing season the shoot develops a terminal bud that elongates the following year to produce a new set of leaves. After several years these dwarf shoots develop into short, stubby outgrowths from the stem. Stems of cycads are typically short and squat, although the Australian cycad Macrozamia hopei may reach 19 metres. In the centre is a large, fleshy pith surrounded by a cylinder of xylem and phloem. There never is as much secondary vascular tissue as is found in conifers, however. Interspersed among the thin-walled tracheids are abundant vascular rays. The wood, consequently, is not as dense as in conifers.

Leaves (leaf)

   Leaves of gymnospermous plants are extremely variable. Most gymnosperms are evergreen, with leaves lasting more than one growing season. Others are deciduous and drop their leaves at the end of every growing season. Bald cypress (Taxodium), larch (Larix), and dawn redwood (Metasequoia) are examples of deciduous conifers. Ginkgo also sheds its leaves in the autumn.

  Among the conifers, leaves are always simple; that is, the blade is a single unit. Leaves may be small and scalelike (e.g., Thuja) or needlelike (Abies, Picea, Pinus) or have a broad blade (Araucaria, Agathis). In some conifers (Taxodium) small branch fragments with numerous needlelike leaves are dropped at the end of the growing season.

 Cycad leaves are compound, with thick, leathery leaflets borne in a featherlike (pinnate) arrangement on a main axis (rachis). Produced among the normal photosynthetic leaves of cycads are reduced, pointed, stiff, scalelike leaves called cataphylls. These contribute to the persistent “armour” on the trunk surfaces.

 Ginkgo resembles an angiospermous tree in that the woody stem is frequently and irregularly branched and bears broad leaves, which are fan-shaped with dichotomously branched veins. The leaves of Gnetum look much like those of dicotyledonous angiosperms. Ephedra has small, scalelike leaves.

      In certain conifers, such as pine and cedar, leaves are borne on dwarf lateral branches that do not elongate. In cedar, the dwarf lateral shoots grow forward each year producing a new cluster of needles each season. In pine, however, the number of needles per cluster is small (one to eight) and no more needles are produced on the dwarf shoot after the first year.

 Ephedra and Gnetum do not produce extensive vascular cylinders; Ephedra is shrubby, while some species of Gnetum are vines. The stem of Welwitschia is somewhat turnip-shaped and does not project very high above the ground. The apex is broad and concave, with leaves and reproductive structures borne along the edges. Gnetum, unlike most gymnosperms, has vessels in the xylem. Perforations (pores) at the ends of the conducting elements connect them to adjacent elements.

Roots (root)

      Filaments of the fungi called endomycorrhizae live within the cells of the roots of certain gymnosperms, especially conifers. Endomycorrhizal fungi are apparently parasitic, but not destructively so. In cycads, blue-green algae grow in nodules in the roots. These roots may grow opposite to the force of gravity and may form corallike masses on the ground surface, hence the term “coralloid roots.” It is thought that these fungi and blue-green algae fix atmospheric nitrogen (nitrogen cycle) into a form usable by the plant.

Strobili

 In most conifers the pollen-bearing and ovule-bearing components (the microsporangia and megasporangia, respectively) are borne on the same plant, though separately (monoecious). A pollen-bearing cone, the microstrobilus, consists of a central axis on which are borne, in a close helical arrangement, reduced, fertile leaves (the microsporophylls). On the lower surfaces of the microsporophylls are borne elongated microsporangia; two microsporangia per microsporophyll are common, but some genera have more. The ovulate cone, the megastrobilus, is more complex than the microstrobilus. The megastrobilus bears seeds on flattened dwarf branches, all of the parts of which are fused (ovuliferous scales). Subtending the ovuliferous scale on the cone axis is a reduced scale leaf, or bract. In some conifers the bract is not recognizable because it has been fused to the ovuliferous scale.

      In Ginkgo, microsporangia and megasporangia are borne on separate trees (i.e., it is dioecious). A Ginkgo microstrobilus is borne on a dwarf shoot among the fan-shaped leaves. The microstrobilar axis bears stalked appendages at the ends of each of which are two microsporangia directed downward. A megastrobilus is not recognized as such. Among the leaves of a dwarf shoot on a plant other than one bearing microstrobili are borne elongated, slender stalks, each with a pair of terminal ovules. Usually only one ovule matures into a seed.

      Cycads also are dioecious, and all genera bear microstrobili consisting of an axis with microsporophylls inserted in a close, helical arrangement. The microsporophylls are reduced leaves with abaxial sporangia. In the genus Cycas, ovules are borne among the edges of the stalk of a reduced leaf with a bladelike region still present. These modified leaves, or megasporophylls, are clustered at the apex of the plant but not arranged in a cone. All other genera of cycads, however, have megastrobili, with the megasporophylls reduced and not leaflike in appearance. Each megasporophyll has a stalk with an expanded distal portion, on the inner face of each of which develop two seeds.

      Ephedra, of the Gnetophyta, may rarely have both microstrobili and megastrobili on the same plant, or more commonly they occur on separate plants. The two remaining genera of Gnetophyta, Gnetum and Welwitschia, are dioecious.

evolution and paleobotany

      The first seed plants to have evolved were gymnospermous in the sense that the seeds were naked. The earliest seedlike bodies are found in rocks of the Upper Devonian Series (about 385 to 359 million years ago). During the course of the evolution of the seed habit, a number of morphological modifications were necessary. First, all seed plants are heterosporous: two kinds of spores (microspores and megaspores) are produced by the sporophyte. Hence, it is assumed that the ancestors of seed plants must have been heterosporous. Sporangia of plants that do not bear seeds typically lack an integument. The origin of the integument in seed plants was made clear by a study of ovules discovered in Scotland from the Mississippian subdivision of the Carboniferous Period (about 359 to 318 million years ago). One example, Genomosperma kidstonii, consists of an elongated megasporangium with one functional megaspore. Arising from the base of the megasporangium were eight elongated, fingerlike processes that loosely surrounded the megasporangium. In a related species, G. latens, these eight fingerlike processes were fused at the base into a cup, with eight free tips. These tips tended to cover the megasporangium rather closely, as opposed to the flared appendages in G. kidstonii. Ultimately these fingerlike appendages were almost completely fused into a continuous integument surrounding the megasporangium. A small hole, the micropyle, is left at the apex of the megasporangium where the integument does not quite cover its tip.

      In searching for seed-plant ancestors it is necessary to look for a heterosporous type of plant bearing leaves and also having an internal structure similar to that of seed plants. The extinct division Progymnospermophyta provides such an ancestral condition. The best-known progymnosperm is the Devonian Archaeopteris, originally assumed to be a fern, with wedge-shaped, subdivided leaflets (pinnules) and sporangia borne on appendages taking the place of pinnules. What was first interpreted as the frond axis was shown to have internal structure like that of Callixylon, known as Devonian stems and wood fragments assumed to be gymnospermous. Callixylon wood is like that of many conifers, consisting of tracheids and vascular rays, with closely spaced circular bordered pits on the radial walls of the tracheids. Pits are clustered, separated from other clusters by an area of the wall lacking pits. What were assumed to be pinnae of the frond of Archaeopteris are actually branches, and the so-called pinnules are helically arranged leaves. At least some species are known to be heterosporous, hence Archaeopteris has many of the features to be anticipated in a seed-plant ancestor.

      From progymnosperms such as Archaeopteris could have arisen more than one group of gymnosperms. Those with compound leaves (e.g., pteridosperms and cycads) have leaves that would correspond to a flattened branch system of Archaeopteris. Those with simple leaves (e.g., conifers) have leaves that are probably the equivalent of the wedge-shaped Archaeopteris leaves.

Earliest gymnosperms

      The earliest recognized group of gymnospermous seed plants are members of the division Pteridospermophyta (pteridosperms, or seed ferns). These plants originated in the Devonian Period and were widespread by the Carboniferous. In habit, Paleozoic seed ferns resembled some progymnosperms in that they were small trees with fernlike leaves (the equivalent of a progymnospermous flattened branch) bearing seeds. While Paleozoic seed ferns resembled ferns externally, the internal structure was like that of gymnosperms. Secondary vascular tissues were common in stems of seed ferns. The wood, however, was composed of thin-walled tracheids and abundant vascular rays, suggesting that stems were fleshy like those of cycads. Pteridosperm seeds were very similar to those of cycads. Many were large, with an outer, softer seed coat and a harder, inner seed coat. Within an ovule ready for fertilization was a massive female gametophyte with several archegonia. There has been one report of a pollination droplet in a Carboniferous pteridosperm ovule and a report of a pollen tube emerging from a pollen grain in the micropyle of a seed-fern ovule, suggesting that transport of the sperm through a pollen tube (siphonogamy) was in existence as far back as the Paleozoic. In some pteridosperms the seed was contained within a cupule; some botanists interpret the cupule as a precursor of an angiosperm carpel. Pollen grains, however, landed directly on the micropyle of the ovule. Pollen-bearing organs were variable among the pteridosperms; in many cases the microsporangia were elongated and fingerlike and were produced in clusters or were fused into compound organs. Mesozoic seed ferns are less well defined, and the concept of pteridospermy is used loosely to refer to plants with fernlike foliage bearing seeds; many botanists assign these fossils to other taxonomic groups.

Appearance of gymnosperm divisions

      It is generally conceded that from the pteridosperms arose members of the division Cycadophyta. The first cycads appeared in the Permian Period (299 to 251 million years ago), although fragmentary fossils of older age suggest that cycads were present during the preceding Carboniferous Period. Some of these presumed cycads differ from extant members in that megasporophylls were undivided, unlike those of Cycas, considered to be primitive among cycads, in which the distal portion of the megasporophyll may be pinnately divided. Other Permian megasporophylls, from China, are more like those of Cycas. Cycad remains, especially leaves, are abundant in Mesozoic rocks. For this reason paleobotanists often refer to the Mesozoic Era as the “age of cycads.” The earliest well-known cycads appear to have had slender stems, sometimes branched, with leaves not borne close together, unlike the situation in extant cycads in which leaves are densely crowded at the apex of the plant. There is evidence that these earliest cycads were deciduous. Megasporophylls of Mesozoic cycads are essentially like those of extant cycads. The megasporophyll of the Triassic Palaeocycas is like that of Cycas. Jurassic megasporophylls are like those of most other cycads (cycadophyte). Extant cycads are now limited in geographic distribution to the warmer parts of the Earth.

      Coexisting with the cycads during the Mesozoic was another group of gymnosperms, the cycadeoids (division Cycadeoidophyta—sometimes called the Bennettitophyta). Although they are superficially similar in habit to the cycads, with a squat trunk and often pinnately divided leaves, their reproductive structures were different, and their actual relationship is not close. Typically seeds were borne on the surface of a fleshy receptacle. Among the seeds were sterile structures, called interseminal scales, that held the seeds tightly together. Pollen organs were quite similar among the forms in the sense that all had a whorl of modified leaves (microsporophylls) on which were borne compound microsporangia.

      Conifers (division Pinophyta) appeared first toward the end of the Carboniferous Period (about 359 to 299 million years ago). Some of the earliest conifers (class Cordaitopsida) were trees with long, strap-shaped leaves. Trunks were similar to those of extant conifers, with dense, compact wood; small, thick-walled tracheids; and narrow vascular rays. Reproductive axes were slender, bearing narrow bracts in the axils of which were small, budlike shoots with helically arranged scales. On some of the topmost scales were borne elongated microsporangia. Buds on other axes bore ovules instead of microsporangia.

      By the late Paleozoic there came into existence another group of extinct conifers, the Voltziales (division Pinophyta). In general habit they must have resembled some of the extant araucarias (e.g., Norfolk Island pine), with whorled, flattened branches bearing helically arranged, needlelike leaves. Reproductive axes were generally homologous with those of the Cordaitales, but they were more compact, with the bracts on the ovule-bearing axes obscuring the axillary fertile buds. During the end of the Paleozoic and in the early Mesozoic, these axillary buds underwent further transformation. The sterile, non-seed-bearing part became flattened, with the scales fused together. The ovule-bearing portion was situated toward the upper surface (away from the bract). The ovuliferous scale of a conifer seed cone, then, may be interpreted as an axis bearing bracts in the axils of which are modified woody ovuliferous scales derived from lateral buds.

      Modern families of conifers began to appear in the Mesozoic Era. Members of the Taxodiaceae, the family to which redwoods and bald cypress are assigned, appeared first in the Jurassic Period. Metasequoia, the dawn redwood, is also a member of this family. Discovered first as fossils in Miocene (23.7 to 5.3 million years ago) deposits, it was assumed to have become extinct until it was discovered growing in Szechwan province in China. Its distribution in the late Mesozoic and Tertiary (65.5 to 2.6 million years ago) was throughout the Northern Hemisphere. The plant has since been introduced to a variety of places in the world.

      During late Triassic times there existed a type of conifer (Compsostrobus) that had many features of the Pinaceae. Seed cones (cone) had woody ovuliferous scales subtended by bracts with two ovules on the upper surface of each ovuliferous scale. More typical pinaceous remains occurred later in the Mesozoic. Conifers were the dominant vegetation just before the appearance of the angiosperms.

      The division Ginkgophyta, represented today by only one living species, Ginkgo biloba, was much more widespread in past ages. Gymnosperms that were presumed to be ginkgophytes existed as far back as the Permian period. In Mesozoic rocks, Ginkgo leaves are commonly found throughout the world. The oldest fossil ginkgophytes had leaves much more dissected than the typical Ginkgo leaf, resembling more closely the leaves found on new growth in extant ginkgoes.

      The fossil record of the division Gnetophyta (gnetophyte) is obscure, and its origin is not clear. Pollen grains similar to those of Ephedra and Welwitschia are found as far back as the Permian Period. Megafossil remains of possible gnetophytan plants occur in Upper Cretaceous (99.6 to 65.5 million years ago) deposits. The plant is unlike any existing one, but venation of the foliage is similar to that of leaves of Welwitschia. Pollen grains are typically gnetophytan.

Classification

Distinguishing taxonomic features

      Gymnosperms differ from angiosperms most obviously on the basis of the naked-seed habit in the former and the enclosure of seeds within a fruit in the latter. The pollen grain of gymnosperms, when shed from the microsporangium, has more than two cells (three in cycads and four in ginkgo and conifers). Furthermore, gymnosperm pollen lands on the ovule directly, whereas in angiosperms pollen lands on the stigma of a carpel and germinates there, with the pollen tube growing through stigmatic and stylar tissues to reach the ovule. In angiosperm pollen tubes, a total of three cells make up the male gametophyte; gymnosperms have more. The female gametophyte in gymnosperms is much larger than that of angiosperms and serves as the source of food for the developing embryo sporophyte. The female gametophyte of angiosperms consists normally of just a few cells. Both sperm cells in an angiosperm pollen tube are functional, one fertilizing the egg, the other joining with two other nuclei of the female gametophyte. Division of this latter cell forms a multicellular tissue (endosperm) in which food is stored for the embryo. Gymnospermous ovules typically have only one integument; most angiosperms have two.

Annotated classification

      Older classifications considered all seed plants to be assignable to a single division, Spermatophyta. The Angiospermae and Gymnospermae were two classes that made up the division. More recent classifications recognize that the characteristic of naked seeds is not important enough to be used to tie all plants with that feature into one group. Classification of gymnosperms now recognizes four separate divisions. Groups marked with a dagger (†) are known only from fossils and have no living members.

†Division Pteridospermophyta (seed fern)

 Late Devonian to Jurassic; seed plants resembling tree ferns with compound, frondlike leaves; seeds and microsporangia borne on the leaves; most stems with secondary vascular tissues.Division Cycadophyta (cycadophyte)

 Permian to the present; palmlike plants; leaves usually pinnately compound; dioecious; seeds borne in megastrobili with reduced megasporophylls, each bearing inwardly directed seeds (except for the living genus Cycas); microstrobili with microsporophylls bearing abaxial microsporangia; 11 extant genera usually classified into 4 families and about 150 species.†Division Cycadeoidophyta (Bennettitophyta)

 Triassic (Permian?) to the Cretaceous; cycadlike plants; leaves usually pinnately compound (some entire); ovules borne on the surface of a fleshy receptacle and separated by interseminal scales; microsporangia compound and borne on fingerlike structures fused at the base.†Division Glossopteridophyta

 Permian; trees with tongue-shaped leaves with net venation; trunks with compact conifer-like wood; seeds borne on, or associated with, leaves; microsporangia borne on tongue-shaped leaves.Division Pinophyta

 Late Carboniferous to the present; woody plants, usually trees, with simple leaves; wood compact; microstrobilus bearing microsporophylls with elongated abaxial microsporangia; seeds borne on megastrobili; ovule with a single integument.

      †Class Cordaitopsida

 Mississippian to the Permian (or perhaps into the Mesozoic); trees; leaves elongated, strap-shaped; wood compact, conifer-like; fertile shoots slender and elongated with fertile buds borne in the axils of reduced leaves or bracts.

      Class Pinopsida

 Late Carboniferous to the present; mostly trees; leaves scalelike, needlelike, or flat and bladelike; wood compact; microstrobili with reduced microsporophylls with abaxial sporangia; megastrobili usually bearing woody ovuliferous scales derived from flattened dwarf branches; seeds borne on the upper surface; 6 living families, with 62 genera and 515 species.

      Class Taxopsida

 Triassic to the present; trees or shrubs; leaves needlelike; microstrobili with microsporophylls bearing abaxial microsporangia; seeds not in megastrobili but terminating dwarf shoots and surrounded by a fleshy aril; 1 family (Taxaceae), with 6 genera and about 30 species.

Division Ginkgophyta (ginkgophyte)

 Permian to the present; dioecious trees with fan-shaped leaves; bilobed or with more lobes, especially in fossil forms; microstrobili borne among leaves on dwarf shoots; ovules on stalks borne among leaves; 1 extant genus with 1 species.Division Gnetophyta (gnetophyte)

 An artificial group containing 3 orders, each with a single family and genus: Ephedrales (Ephedra, 65 species), shrubs with reduced leaves and jointed stems; Welwitschiales (Welwitschia, 1 species), plants with a massive, fleshy stem bearing 2 large, leathery, strap-shaped leaves; and Gnetales (Gnetum about 30 species), vines, shrubs, or trees with flattened angiospermoid (angiosperm) leaves.

Critical appraisal

      The classification (taxonomy) presented above emphasizes that all gymnospermous plants are not closely related to each other. Alternatively, some botanists prefer to treat most or all the gymnosperms as classes of a single division. However, most botanists now believe that the characteristic of naked seeds was apparently derived among seed plants more than once. The fossil record indicates that the seed-fern–cycadophyte complex was separate from the conifer line from the very beginning. Plants called pteridosperms may not all belong together; Mesozoic forms were quite different from the Paleozoic ones. In fact, it cannot even be determined in some instances that seeds were actually borne on leaves in the Mesozoic forms. Glossopterids, in a sense, have seed-fern characters in that seeds were borne on fernlike leaves (although they were entire). The wood of glossopterids, however, is more like that of conifers (conifer).

      Several fossil groups were not included in this classification because their relationships are still obscure. Some of these groups are in the orders Pentoxylales, Vojnovskyales, and Czekanowskiales.

T. Delevoryas

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 and Edith L. Taylor, The Biology and Evolution of Fossil Plants (1993), 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; 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; K.U. Kramer and P.S. Green (eds.), Pteridophytes and Gymnosperms (1990), an overview of diversity and taxonomy of living gymnosperms.

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