The gymnosperms ( nə-spurmz, -noh-; from Ancient Greek γυμνός, gymnós 'naked' and σπέρμα, spérma 'seed', thus 'naked seed') are a group of woody, perennial seed-producing plants, typically lacking the protective outer covering which surrounds the seeds in flowering plants, that include conifers, cycads, ginkgos, and gnetophytes, forming the clade Gymnospermae. The name is based on the unenclosed condition of their seeds (called ovules in their unfertilized state). The non-encased condition of their seeds contrasts with the seeds and ovules of flowering plants (angiosperms), which are enclosed within an ovary. Gymnosperm seeds develop either on the surface of scales or leaves, which are often modified to form cones, or on their own as in yew, Torreya, and Ginkgo. The life cycle of a gymnosperm involves alternation of generations, with a dominant diploid sporophyte phase, and a reduced haploid gametophyte phase, which is dependent on the sporophytic phase. The term "gymnosperm" is often used in paleobotany to refer to (the paraphyletic group of) all non-angiosperm seed plants. In that case, to specify the modern monophyletic group of gymnosperms, the term Acrogymnospermae is sometimes used. The gymnosperms and angiosperms together constitute the spermatophytes or seed plants. The spermatophytes are subdivided into five divisions, the angiosperms and four divisions of gymnosperms: the Cycadophyta, Ginkgophyta, Gnetophyta, and Pinophyta (also known as Coniferophyta). Newer classification place the gnetophytes among the conifers. Numerous extinct seed plant groups are recognised including those considered pteridosperms/seed ferns, as well other groups like the Bennettitales. By far the largest group of living gymnosperms are the conifers (pines, cypresses, and relatives), followed by cycads, gnetophytes (Gnetum, Ephedra and Welwitschia), and Ginkgo biloba (a single living species). About 65% of gymnosperms are dioecious, but conifers are almost all monoecious. Some genera have ectomycorrhiza fungal associations with roots (Pinus), while in some others (Cycas) small specialised roots called coralloid roots are associated with nitrogen-fixing cyanobacteria.
Diversity and origin
Over 1,000 living species of gymnosperm exist. It was previously widely accepted that the gymnosperms originated in the Late Carboniferous period, replacing the lycopsid rainforests of the tropical region, but more recent phylogenetic evidence indicates that they diverged from the ancestors of angiosperms during the Early Carboniferous. The radiation of gymnosperms during the late Carboniferous appears to have resulted from a whole genome duplication event around 319 million years ago. Early characteristics of seed plants are evident in fossil progymnosperms of the late Devonian period around 383 million years ago. It has been suggested that during the mid-Mesozoic era, pollination of some extinct groups of gymnosperms was by extinct species of scorpionflies that had specialized proboscides for feeding on pollination drops. The scorpionflies likely engaged in pollination mutualisms with gymnosperms, long before the similar and independent coevolution of nectar-feeding insects on angiosperms. Some mid-Mesozoic gymnosperms were pollinated by kalligrammatid lacewings. All gymnosperms are perennial woody plants. Unlike in other extant gymnosperms the soft and highly parenchymatous wood in cycads is poorly lignified, and their main structural support comes from an armor of sclerenchymatous leaf bases covering the stem, with the exception of species with underground stems. There are no herbaceous gymnosperms and compared to angiosperms they occupy fewer ecological niches, but the gymnosperms include parasites (Parasitaxus), epiphytes (Zamia pseudoparasitica) and rheophytes (Retrophyllum minus). Conifers are by far the most abundant extant group of gymnosperms with six to eight families, with a total of 65–70 genera and 600–630 species (696 accepted names). Most conifers are evergreens. Conifers produce their seeds inside a protective cone called a strobilus. Most species are monoecious, with male and female cones on the same tree. All conifers are wind-pollinated. Cycads, small palm-like trees, are the next most abundant group of gymnosperms, with two or three families, 11 genera, and approximately 338 species. A majority of cycads are native to tropical climates and are most abundantly found in regions near the equator. The other extant groups are the 95–100 species of Gnetophytes and one species of Ginkgo. The ginkgo or maidenhair trees are tall and have bilobed leaves, while gnetophytes are a diverse groups of plants and shrubs including the horizontally growing Welwitschia. Gymnosperms are the most threatened of all plant groups.
Evolution
Phylogeny
The phylogeny of the gymnosperms has been revised by multiple authors.
Taxonomy A formal classification of the living gymnosperms is the "Acrogymnospermae", which form a monophyletic group within the spermatophytes. The wider "Gymnospermae" group includes extinct gymnosperms and is thought to be paraphyletic. The fossil record of gymnosperms includes many distinctive taxa that do not belong to the four modern groups, including seed-bearing trees that have a somewhat fern-like vegetative morphology (the so-called "seed ferns" or pteridosperms). When fossil gymnosperms such as these and the Bennettitales, glossopterids, and Caytonia are considered, it is clear that angiosperms are nested within a larger gymnospermae clade, although which group of gymnosperms is their closest relative remains unclear. The extant gymnosperms include 12 main families and 83 genera which contain more than 1000 known species. Subclass Cycadidae
Order Cycadales Family Cycadaceae: Cycas Family Zamiaceae: Dioon, Bowenia, Macrozamia, Lepidozamia, Encephalartos, Stangeria, Ceratozamia, Microcycas, Zamia Subclass Ginkgoidae
Order Ginkgoales Family Ginkgoaceae: Ginkgo Subclass Gnetidae
Order Welwitschiales Family Welwitschiaceae: Welwitschia Order Gnetales Family Gnetaceae: Gnetum Order Ephedrales Family Ephedraceae: Ephedra Subclass Pinidae
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