Gymnosperm

Gymnosperms are a group of woody, perennial seed-producing plants that lack the protective outer covering surrounding seeds in flowering plants. They include conifers, cycads, ginkgos, and gnetophytes, forming the clade Gymnospermae. The name derives from Ancient Greek for 'naked seed,' referring to the unenclosed condition of their seeds, which develop on the surface of scales or leaves, often modified into cones.

Gymnosperms and angiosperms together constitute the spermatophytes, or seed plants. Over 1,000 living species exist, with conifers being the largest group, followed by cycads, gnetophytes, and a single species of Ginkgo biloba. Most conifers are monoecious and wind-pollinated, while about 65% of all gymnosperms are dioecious. Unlike flowering plants, gymnosperm seeds are not enclosed within an ovary; they form on the surface of scales or leaves, as seen in cones, or individually in yew, Torreya, and Ginkgo.

Their life cycle features alternation of generations, with a dominant diploid sporophyte and a reduced, dependent haploid gametophyte. All gymnosperms are perennial and woody, though cycads have soft, poorly lignified wood and rely on a protective armor of leaf bases for structural support. Some species form specialized root associations, such as ectomycorrhiza in pines or coralloid roots in cycads that host nitrogen-fixing cyanobacteria.

Diversity and origin

Gymnosperms occupy fewer ecological niches than angiosperms but include parasites like Parasitaxus, epiphytes like Zamia pseudoparasitica, and rheophytes like Retrophyllum minus. The group is the most threatened of all plant lineages. Phylogenetic evidence suggests gymnosperms diverged from angiosperm ancestors during the Early Carboniferous, with a major radiation in the Late Carboniferous following a whole genome duplication. Fossil progymnosperms from the late Devonian show early seed plant characteristics.

Taxonomy

During the mid-Mesozoic, some extinct gymnosperms were pollinated by scorpionflies with specialized proboscides, and by kalligrammatid lacewings, representing pollination mutualisms that predate similar insect-flower coevolution. The term "gymnosperm" is used in paleobotany for all non-angiosperm seed plants, a paraphyletic group; the modern monophyletic group is sometimes called Acrogymnospermae. Extinct gymnosperm groups include seed ferns, Bennettitales, glossopterids, and Caytonia.