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Gymnosperm

Gymnosperms
Fossil range: 370â0 Ma Devonian - Recent
Picea glauca (White Spruce) needles
Scientific classification
Kingdom: Plantae
Divisions

Pinophyta (or Coniferophyta) - Conifers
Ginkgophyta - Ginkgo
Cycadophyta - Cycads
Gnetophyta - Gnetum, Ephedra, Welwitschia

Gymnosperms are seed-bearing plants, whose seeds do not form inside fruits but outside the ovum.

Once an authoritative and major classification level in the plant kingdom, the gymnosperms form today a rather heterogeneous group of seed-bearing plants that includes conifers, cycads, Ginkgo and Gnetales. The term "gymnosperm" comes from the Greek word gymnospermos (îυîîόσπîρîî¿ς), meaning "naked seeds", after the unenclosed condition of their seeds (called ovules in their unfertilized state). Their naked condition stands in contrast to the seeds or ovules of flowering plants (angiosperms) which are enclosed during pollination. Gymnosperm seeds develop either on the surface of scale- or leaf-like appendages of cones, or at the end of short stalks (Ginkgo).

The gymnosperms and angiosperms together comprise the spermatophytes or seed plants. By far the largest group of living gymnosperms are the conifers (pines, cypresses, and relatives), followed by cycads, Gnetales (Gnetum, Ephedra and Welwitschia), and Ginkgo (a single living species).

Contents

[edit] Classification

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.[1][2] 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.)[3] When fossil gymnosperms such as Bennettitales, Caytonia and the glossopterids are considered, it is clear that angiosperms are nested within a larger gymnosperm clade, although which group of gymnosperms are their closest relatives remains unclear.

[edit] 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 scorpionflies that had specialized proboscis 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.[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 Cupressaceae and some Podocarpaceae, have flat, triangular scale-like leaves. 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.

[edit] Uses

Gymnosperms have major economic uses. Pine, fir, spruce and cedar are all examples of conifers which we use for lumber. Some other common uses for them are as soap, varnish, paint, food and perfumes.

[edit] Life cycle

Gymnosperms are spore-bearing plants (sporophytes), with a sporophyte-dominant life cycle; as in all other vascular plants the gametophyte (gamete-bearing phase) is relatively short-lived. Two spore types, microspores and megaspores, are generally produced in pollen cones or ovulate cones, respectively. A short-lived multicellular haploid, gamete-bearing phase (gametophyte) develops inside the spore wall. Pollen grains (microgametophytes) mature from microspores, and ultimately produce sperm cells; megagametophyte tissue develops in the megaspore of each ovule, and produces multiple egg cells. Thus, megaspores are enclosed in ovules (unfertilized seeds) and give rise to megagametophytes and ultimately to egg cells. During pollination, pollen grains are physically transferred between plants, from pollen cone to the ovule, being transferred by wind or insects. Whole grains enter each ovule through a microscopic gap in the ovule coat (integument) called the micropyle. The pollen grains mature further inside the ovule and produce sperm cells. Two main modes of fertilization are found in gymnosperms. 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. After fertilization (joining of the sperm and egg cell), the zygote develops into an embryo (young sporophyte). More than one embryo is usually initiated in each gymnosperm seed. Competition between the embryos for nutritional resources within polyembryonic seeds produces programmed cell death to all but one embryo. The mature seed comprises the embryo and the remains of the female gametophyte, which serves as a food supply, and the seed coat (integument).[9]

[edit] References

  1. ^ Jeffrey D. Palmer, Douglas E. Soltis and Mark W. Chase (2004). "The plant tree of life: an overview and some points of view". American Journal of Botany 91: 1437â1445. doi:10.3732/ajb.91.10.1437. http://www.amjbot.org/cgi/content/full/91/10/1437. 
  2. ^ Stevens, P. F. (2001 onwards). "Angiosperm Phylogeny Website - Seed Plant Evolution". http://www.mobot.org/mobot/research/APweb/orders/Cycadales.html#Seedplants. 
  3. ^ Hilton, Jason, and Richard M. Bateman. 2006. Pteridosperms are the backbone of seed-plant phylogeny. Journal of the Torrey Botanical Society 133: 119-168 (abstract)
  4. ^ Campbell and Reece; Biology, Eighth edition
  5. ^ Ollerton J. Coulthard E. (2009). Evolution of Animal Pollination. Science, 326: 808-809. doi:10.1126/science.1181154
  6. ^ Ren D, Labandeira CC, Santiago-Blay JA, Rasnitsyn A, Shih CK, Bashkuev A, Logan MA, Hotton CL, Dilcher D. (2009). Probable Pollination Mode Before Angiosperms: Eurasian, Long-Proboscid Scorpionflies. Science, 326 (5954), 840-847. doi:10.1126/science.1178338
  7. ^ Catalogue of Life: 2007 Annual checklist - Conifer database
  8. ^ Campbell, Reece, "Phylum Coniferophyta."Biology. 7th. 2005. Print. P.595
  9. ^ Walters, Dirk R Walters Bonnie By (1996). Vascular plant taxonomy. Dubuque, Iowa: Kendall/Hunt Pub. Co.. pp. 124. ISBN 9780787221089. http://books.google.com/?id=ZbaNxSnNoecC&pg=PA124&dq=Gymnosperm+seeds. 

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