Gymnosperms

Gymnosperms are nonflowering seed plants that may or may not be a monophyletic group. Recent phylogenetic analyses based upon data from morphology and the chloroplast gene rbcL indicate that the gymnosperms are para-phyletic, whereas phylogenetic analyses of other gene sequences from the mitochondrial and nuclear genomes indicate that they are monophyletic. Regardless, the extant gymnosperms consist of four morphologically distinct orders: the cycads (Cycadales), the conifers (Pinales), ginkgo (Ginkgoales), and the gnetopsids (Gnetales). There are no herbaceous or truly aquatic gymnosperms, although some may grow in swamps.

Order Cycadales. Extant cycads are characterized by compound leaves; girdling leaf traces; coralloid roots containing symbiotic nitrogen-fixing cyanobacteria of the genera Anabaena and Nostoc; two unique phyto-chemicals, MAM glycosides (cycasins) and BMAA (a neurotoxic, nonprotein alpha-amino acid); simple cones; and pollination by snout weevils of the Curculionoidea. There is an excellent fossil record indicating a Pangean (worldwide) distribution in the Jurassic and Triassic, with considerable extinction during the Pleistocene, resulting in reduced distribution of extant genera as well as isolation by plate tectonics. The order contains approximately 300 species in eleven genera and three families. All cycads are considered endangered species, with four genera listed on CITES Appendix I and the other seven listed on Appendix II. They are pantropical, with levels of local endemism. Cycads are threatened by commercial overcollection and habitat destruction.

Family Cycadaceae. This family has a single genus, Cycas, with eighty-seven species in Asia, Australia, and Indian and southwest Pacific Ocean countries including Madagascar.

Family Stangeriaceae. There are two genera and three species in this family. Stangeria, with one species, is endemic to Natal, South Africa, and Bowenia, with two species, is endemic to the east coast of Queensland, Australia.

Family Zamiaceae. This is the largest family of cycads, with eight genera. Ceratozamia, with nineteen species, is endemic to southern Mexico and contiguous Belize and Guatemala. Chigua, with two species, is endemic to a small area in northern Colombia. Dioon, with eleven species, is endemic mainly to Mexico, with one species endemic to Honduras and a population in Nicaragua. Encephalartos, with sixty-three species, is endemic to central and southern Africa. Lepidozamia, with two species, and Macrozamia, with thirty-eight species, are endemic to Australia. The single species of Microcycas is found only in the Pinar del Rio area of western Cuba. Zamia, with fifty-seven known species, occurs in Florida and the Caribbean and throughout tropical Central and South America. Most species in the Zami-aceae are local endemics with small scattered populations exhibiting reproductive depression, often because of elimination of the pollinators where known.

Order Ginkgoales. This order is known from only a single species, Ginkgo biloba, which is the lone surviving species of a group with a widespread fossil record from the Permian through the early Cretaceous. Ginkgo plants are large trees with short spur shoots bearing fan-shaped leaves with dichotomous venation. In China, Ginkgo has been cultivated for centuries in temple gardens. Presumably there are still wild populations in Zhejiang province in eastern China, although that has not been confirmed. Ginkgo is widely cultivated worldwide in temperate areas as a street tree, with the original source being from temple gardens. The female trees bear fleshy, foul-smelling seeds, so most cultivated plants are pollen-bearing males. Ginkgo is wind pollinated, as evidenced by its copious seed set in cultivation in the absence of any pollinators, so its presumed extinction or at least severe range restriction in the wild is an enigma.

Order Pinales. Conifers are resin-producing, evergreen, woody shrubs or trees with scalelike or needle leaves and pollen and seed cones. All conifers are wind pollinated, and most have wind-dispersed seeds. A few exceptions occur. Larches (Larix and Pseudolarix), bald cypress (Taxodium), dawn redwood (Metasequoia), and Glyptostrobus are deciduous, and the yews and their relatives produce only small amounts of resin. Yews (Taxaceae and Cephalotaxaceae), podocarps (Podocarpaceae), and Araucariaceae are animal dispersed. The

A forest of white pines (USDA Forest Service)

conifers are basically a temperate group that form vast forest stands as a principal component of temperate rain forests, boreal forest, and taiga in the Northern Hemisphere, and araucaria forests in the Southern Hemisphere. When conifers occur in subtropical to tropical regions they are usually at higher, cooler elevations (for example, South America) or in very dry areas such Callitris in Australia. Generally they are absent from true oceanic islands. Conifers have a good fossil record. Coniferous wood is known from the Upper Carboniferous, and unequivocal conifer fossils begin in the Lower Permian, with all extant families reaching their zenith in diversity in the Lower Cretaceous and then declining in the Upper Cretaceous, accompanied by the diversification and rise of the angiosperms. As a major source of timber and wood for paper products, conifers are extremely important economically, and conifer forests are subjected to extreme habitat destruction and degradation. Conifers are cultivated in pure stand plantations, and that results in habitat degradation of mesophytic broadleaved forests worldwide.

There are eight families forming two distinctive groups (sometimes treated as orders) within the Pinales. Cephalotaxaceae, Podocarpaceae, and Taxaceae form one of these groups. Cephalotaxaceae, found in temperate habitats in Asia, has two genera: Amen-totaxus, with four species in widely disjunct small populations in China and Taiwan; and Cephalotaxus, with six species ranging from the Himalayas to Japan, which is widely cultivated in temperate areas. Podocarpaceae is the largest family, with as many as 125 species in seventeen genera currently recognized. The family is pantropical, with extensions into the subtropics mainly in the Southern Hemisphere. A majority of the species occur in montane regions, and some even reach alpine areas. Podocarpus, with 100 species, is widespread in the tropics worldwide. The other sixteen genera are more restricted. Afrocarpus is centered in equatorial Africa and extends south to southeastern Africa. Saxegothaea is endemic to temperate South America. The two species of Acmopyle are endemic to New Caledonia and Fiji. The single species of Par-asitaxus, the only known parasitic gym-nosperm, is endemic to New Caledonia, where it grows on the roots of another member of the family, Falcatifolium taxoides. Microcachys and one species each of Microstrobos and Lagen-strobos are endemic to Tasmania. The second species of Microstrobos occurs in a limited area of New South Wales, Australia, and the same is true for Lagenstrobos in New Zealand, where Halocarpus is also endemic. The other genera, Dacrydium, Dacrycarpus, Falcatifolium, and Sundacarpus, are centered in the region extending from Indo-China to Australia, with three others—Retrophyllum, Prumnopitys, and Lep-idothamnus—also occurring in South America. Phyllocladus, with five species in moist, cool habitats from Luzon to New Zealand and Tasmania, is unusual in lacking well-developed leaves and instead having flattened photo-synthetic stems (cladodes or phylloclades). Taxaceae contains four genera with sixteen species that are associated primarily with old-growth forests and are becoming quite rare worldwide as a result of habitat destruction. Pseudotaxus, with one species, is known only from a few disjunct populations in China; Austrotaxus, with one species, is endemic to New Caledonia. Torreya (stinking yew), with six species, is highly disjunct, occurring in limited areas of California, Florida, China, and Japan. More widespread is Taxus (the yews), with eight species, found in temperate areas of the Northern Hemisphere and extending at higher elevations to southern Mexico and Indonesia. Taxus is cultivated widely as an ornamental. The bark of Taxus brevifolia, endemic to the Pacific Northwest of North America, is the primary source of taxol, used in cancer treatment. It has become endangered as a result of the destructive collection of the bark from mature trees.

The second group of conifer families consists of Araucariaceae (for example, Norfolk Island pine, monkey puzzle tree), Pinaceae (for example, pines, spruces, firs, larches, cedars), Sciadopityaceae (umbrella pine), Cupressaceae (for example, arbor vitae, junipers, Leyland cypress), and Taxodiaceae (for example, bald cypress, dawn redwood, redwood, sequoias). The latter three appear to be related, and current evidence strongly suggests that the Cupressaceae is in fact a component of the Taxodiaceae, with Sciadopity-aceae being problematic. There are thirty-two species in three genera in the Araucariaceae. All are found in the Southern Hemisphere except in Southern Africa. Araucaria (for example, Norfolk Island Pine), with eighteen species, has thirteen species endemic to New Caledonia, one species on Norfolk Island, and the rest in New Guinea, Australia, and southern South America, where they form large stands. Agathis, with thirteen species, ranges from Malaya to Fiji, with five species endemic to New Caledonia. In 1995 a new, very locally endemic genus, Wollemei, was found in a national park in New South Wales, Australia. The Cupressaceae, with 125 species in twenty genera, occur primarily in cool to warm temperate areas of both hemispheres of both the Old and New Worlds. They are widely cultivated ornamentals with small leaves arranged in either whorls of three to four or in decussate pairs. The most widely distributed genera,

Juniperus, Cupressus, Chaemaecyparis, and Thuja (for example, arbor vitae), are limited to the Northern Hemisphere. Other genera show high degrees of endemism. Widdringtonia is South African; Actinostrobus is Australian; Tetraclinis is Mediterranean and North African; Neocallitropsis is New Caledonian; Platycladus and Microbiota are temperate East Asian; Pil-gerdendron, Fitzroya, and Austrocedrus are southern South American; Diselma is Tas-manian; Thujiopsis is Japanese; Fokenia is IndoChinese; and Papuacedrus is New Guinean, with an outlier in the Moluccas.

Callitris is primarily Australian, with endemic species in Tasmania and New Caledonia. Calocedrus is found in temperate rain forests of Pacific North America and far eastern Asia. Libocedrus is from New Zealand and New Caledonia. The Sciadopityaceae, endemic to Japan, has a single genus, Sciado-pitys, with one species and an unusual morphology in that the leaves are actually two laterally fused leaves with inconspicuous free tips. Taxodiaceae, with sixteen species in nine genera, commonly have vegetative shoots growing from the cone tips and are mostly confined to warm-temperate areas of the Northern Hemisphere, with one genus, Athro-taxis, endemic to Tasmania. Other locally endemic genera are: Sequoia (coast redwood) and Sequoiadendron (giant redwood) in western North America, and Glyptostrobus and Metasequoia (dawn redwood) in central and southern China. In contrast, the more widespread genera include Taiwania in China, Taiwan, and northern Burma; Cunninghamia in northern China and Taiwan; Cryptomeria in China and Japan; and Taxodium, ranging in local disjunct populations from the northeastern United States to Florida and the Gulf States to central Mexico. Pinaceae, with 200 species in twelve genera, are confined to the Northern Hemisphere. The larger genera

(Pinus, Larix, Picea, and Abies) are distributed widely across the Old and New Worlds, where they reach high northerly latitudes as the major components of boreal forests. There are concentrations of species of these genera in North America and eastern Asia. Tsuga (hemlock) and Pseudotsuga (Douglas fir) have a similar pattern but are absent from Eurasia. Nothotsuga and Cathaya, each with a single species, are endemic to southern China, where they are very rare. Hesperopeuce is found at high elevations in western North America from Alaska to California. Keetelaria is common in China but rare in the rest of the Sino-Himalayan region. Cedrus (cedars) ranges from the Atlas Mountains of North Africa to the western Himalayas. Pinaceae are absent from South America (except for naturalized introductions), Australasia, and Africa, with the exception of one species of Cedrus in Algeria and Morocco. The Central American and southeast Asian species are found almost exclusively at higher elevations in montane forests and above.

Order Ephedrales (=Gnetales). This is an extremely diverse order of seed plants. There are three genera—Ephedra, Gnetum, and Wel-witschia—and each is treated as belonging to its own family. All species are functionally dioecious and have vessel elements in the wood that appear to be independently derived when compared with those found in angiosperms, as does the phenomenon of double fertilization. The three genera are easily distinguished based on growth habit, leaf features, and habitat preferences. Ephedra (known as Mormon tea in the western United States), with fifty species, is xerophytic, heliophilious, and somewhat cold resistant. Eurasian species are distributed from the Canary Islands through the Mediterranean and semiarid regions of inner Asia, with disjuncts in the Arabian Gulf. New World species are found in western

North America and the Andes from Peru to Patagonia. Plants of Ephedra are profusely branched erect to prostrate shrubs, vines, or small trees with leaves reduced to small scales and with photosynthetic stems. Seeds are enclosed winged bracts for wind dispersal or in fleshy white, orange, or bright bracts indicating bird dispersal. Pollination by Diptera has been demonstrated in some Eurasian species. They have been widely used as medicinals since antiquity, and some species are known to produce ephedrine.

Gnetum, with thirty-seven species, is found in the lowland tropics of Asia, western Africa, and the New World. Generally the plants are twining vines, but two Asian species are trees. The leaves are well developed, opposite, and exhibit a reticulate venation pattern similar to that of angiosperms. The seeds are large and enclosed in fleshy orange to red bracts and appear, at least in South America species, to be dispersed by primates. Otherwise, herbivory including insect predation are unknown in the genus, even though toxic compounds have never been identified. The mode of pollination has not been established.

Welwitschia, with a single species, is found only in a narrow, extremely xeric coastal belt of the Namibia Desert from Kuiseb, Southwest Africa, to Cabo Negro, Angola. Welwitschia is one of the most bizarre and unique vascular plants. Each plant is single stemmed and produces only one set of two continuously growing foliage leaves and a massive, deep-growing taproot. The leaves can reach lengths of several meters. After twelve to fifty years the axillary buds of the foliage leaves begin annually to produce highly branched cone-bearing axes. Large plants of Welwitschia have been radiocarbon dated at 1,000 to 2,000 years old. Pollination is assumed to be anemophilous, but there is some evidence of entomophily by Hymenoptera and mosquitoes. The seeds are lightweight and winged for wind dispersal.

—Dennis Wm. Stevenson

See also: Angiosperms; Biogeography; Botany; Extinction, Direct Causes of; Phylogeny; Plate Tectonics; Pleistocene Epoch

Bibliography

Johnson, Laurie A. S., and Karen L. Wilson. 1990. "Cycadatae." In The Families and Genera of Vascular Plants. Vol. I: Pteridophytes and Gymnosperms, edited by K. Kramer and P. Green, pp. 363-377. Berlin: Springer-Verlag; Kubitzki, Klaus. 1990. "Gnetatae." In The Families and Genera of Vascular Plants. Vol. I: Pteridophytes and Gymnosperms, edited by K. Kramer and P. Green, pp. 378-391. Berlin: Springer-Verlag; Norstog, Knut J., and Trevor J. Nicholls. 1997. The Biology of the Cycads. Ithaca, NY: Cornell University Press; Page, Christopher N. 1990. "Coniferophytina." In The Families and Genera of Vascular Plants. Vol. I: Pteridophytes and Gymnosperms, edited by K. Kramer and P. Green, pp. 282-362. Berlin: Springer-Verlag; Price, Robert A. 1996. "Systematics of the Gnetales: A Review of Morphological and Molecular Evidence." International Journal of Plant Sciences 157: S40-S49; Stevenson, Dennis W., Knut J. Norstog, and Priscilla K. Fawcett. 1998. "Pollination Biology of Cycads." In Reproductive Biology: In Systematics, Conservation, and Economic Botany, edited by S. Owens and P. Rudall, pp. 277-294. Kew: Royal Botanic Gardens; Taylor, Thomas N., and Edith L. Taylor. 1993. The Biology and Evolution of Fossil Plants. Englewood Cliffs, NJ: Prentice-Hall; Walters, Kerry S., and Harriet J. Gillett, eds. 1998. 1997 IUCN Red List of Threatened Plants. Gland, Switzerland: IUCN World Conservation Union.

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