gymnosperm

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Gymnosperms Fossil range: 370–0 Ma PreЄ Є O S D C P T J K Pg N Devonian - Recent
White Spruce needles
Scientific classification
Kingdom:	Plantae
Divisions
Pinophyta (or Coniferophyta) - Conifers Ginkgophyta - Ginkgo Cycadophyta - Cycads Gnetophyta - Gnetum, Ephedra, Welwitschia

Gymnosperm (Gymnospermae) is a group of spermatophyte seed-bearing plants with ovules on scales,^[1] which are usually arranged in cone-like structures.

The term "gymnosperm" comes from the Greek word gymnospermos (γυμνόσπερμος), meaning "naked seeds" and referring to the unenclosed condition of the seeds, since, when they are produced, they are found naked on the scales of a cone or similar structure.

By far the largest group of living gymnosperms are the conifers (pines, cypresses, and relatives), followed by cycads, the Gnetophyta (Gnetum, Ephedra and the rather unusual desert plant Welwitschia), and finally the tree Ginkgo (with just one living species).

Classification

Coast Douglas-fir cone

In early classification schemes, the gymnosperms (Gymnospermae) were regarded as a "natural" group. There is conflicting evidence on the question of whether the living gymnosperms form a clade or whether angiosperms evolved from gymnosperms,^[2] although according to most analyses of recent molecular data the gymnnosperms appear to be monophyletic.^[3] Numerous fossil gymnosperm clades once existed that are morphologically at least as distinctive as the four living gymnosperm groups, such as Bennettitales, Caytonia and the glossopterids. When these groups are considered, the question of gymnosperm and angiosperm relationships becomes even more complicated.

Diversity and origin

There are between 700 and 900 extant or currently living species of Gymnosperms.

It is widely accepted ^[4] that the gymnosperms originated in the late Carboniferous Period. Early characteristics of seed plants were evident in fossil progymnosperms of the late Devonian period around 380 million years ago. It has been suggested that during the mid-Mesozoic period, pollination of some extinct groups of gymnosperms were by extinct species of scorponflies that had specialized proboscis for feeding on pollination drops. The scorponflies likely engaged in pollination mutualisms with gymnosperms, long before the similar and independent coevolution of nectar-feeding insects on angiosperms.^[5][6]

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).^[7] Conifers are woody plants and most are evergreens.^[8] The leaves of many conifers are long, thin and needle-like, others species, including most of the Cupressaceae and some of the Podocarpaceae, have flat, triangular scale-like leaves. The Agathis in Araucariaceae and Nageia in Podocarpaceae, have broad, flat strap-shaped leaves.

Cycads are the next most abundant group of Gymnosperms, with about 130 species. The other extant groups are the 75 - 80 species of Gnetales and one species of Ginkgo.

Pteridosperms are sometimes used as a root.^[9]

Uses

Often gymnosperms are used for economical uses and as folk medicines. Some common uses for them are as soap, varnish, lumber, paint, food, and perfumes.

Life cycle

Gymnosperms are sporophytes, producing microspores from microsporangia that develop into pollen grains and megaspores from megasporangia that are retained in an ovule. Gymnosperm gametophytes are retained in cones of two types. Microscopic male gametophytes are stored in pollen cones and microscopic female gametophytes are kept in ovulate cones. A male gametophyte develops into a pollen grain which is stored within a pollen wall. A female gametophyte develops from a megaspore and produces one or more eggs. After pollination and fertilization (joining of the micro- and megaspore), the resulting embryo, along with other cells comprising the ovule, develops into a seed. The seed is a sporophyte resting stage. Gymnosperms have a sporophyte dominant life cycle as with all other vascular plants. The seed consists of an embryo, the remains of the female gametophyte, which serve as a food supply, and the seed coat or integument.^[10] Reproduction in gymnosperms varies greatly. Cycads and Ginkgo have motile sperm that swim directly to the egg inside the ovule, while conifers and gnetophytes have sperm with no flagella that are conveyed to the egg along a pollen tube which grows through the micropyle which is a microscopic gap in the integument of the ovule tissue.

References

External links

Wikimedia Commons has media related to: Gymnosperm

• Gymnosperm Database
• Gymnosperms on the Tree of Life

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