Gymnosperm

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Gymnosperm
Temporal range: CarboniferousPresent
Gymnospermae.jpg
Various gymnosperms.
Scientific classification e
Kingdom: Plantae
Clade: Tracheophytes
Clade: Spermatophytes
Clade: Gymnosperms
Divisions

Extant: Pinophyta (or Coniferophyta) – Conifers
GinkgophytaGinkgo
Cycadophyta – Cycads
GnetophytaGnetum, Ephedra, Welwitschia

Encephalartos sclavoi cone, about 30 cm long

The gymnosperms (pronunciation  lit. revealed seeds) are a group of seed-producing plants that includes conifers, cycads, Ginkgo, and gnetophytes, forming the clade Gymnospermae, the living members of which are also known as Acrogymnospermae. The term gymnosperm comes from the composite word in Greek: γυμνόσπερμος (γυμνός, gymnos, 'naked' and σπέρμα, sperma, 'seed'), literally meaning 'naked seeds'. 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 solitary as in yew, Torreya, Ginkgo.[1] Gymnosperm lifecycles involve alternation of generations. They have 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 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 comprise the spermatophytes or seed plants. The gymnosperms are subdivided into five Divisions, four of which, the Cycadophyta, Ginkgophyta, Gnetophyta, and Pinophyta (also known as Coniferophyta) are still in existence while the Pteridospermatophyta are now extinct.[2] Newer classification place the gnetophytes among the conifers.[3]

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,[4] but conifers are almost all monoecious.[5]

Some genera have mycorrhiza, fungal associations with roots (Pinus), while in some others (Cycas) small specialised roots called coralloid roots are associated with nitrogen-fixing cyanobacteria.

Classification[edit]

A formal classification of the living gymnosperms is the "Acrogymnospermae", which form a monophyletic group within the spermatophytes.[6][7] 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).[8] 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.[1][7][9]

Subclass Cycadidae

Subclass Ginkgoidae

Subclass Gnetidae

Subclass Pinidae

Extinct groupings[edit]

Diversity and origin[edit]

Over 1000 living species of gymnosperm exist.[1] 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.[10][11] The radiation of gymnosperms during the late Carboniferous appears to have resulted from a whole genome duplication event around 319 million years ago.[12] 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 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.[13][14] Evidence has also been found that mid-Mesozoic gymnosperms were pollinated by Kalligrammatid lacewings, a now-extinct family with members which (in an example of convergent evolution) resembled the modern butterflies that arose far later.[15]

Zamia integrifolia, a cycad native to Florida

All gymnosperms are perennial woody plants,[16] apart from the cycads. The soft and highly parenchymatous wood in cycads is poorly lignified,[17] and their main structural support comes from an armor of sclerenchymatous leaf bases covering the stem,[18] with the exception of species with underground stems.[19] There are no herbaceous gymnosperms and compared to angiosperms they occupy fewer ecological niches, but have evolved both parasites (parasitaxus), epiphytes (Zamia pseudoparasitica) and rheophytes (Retrophyllum minus).[20]

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).[21] Most conifers are evergreens.[22] The leaves of many conifers are long, thin and needle-like, other 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 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 Gnetales and one species of Ginkgo.[2]

Spermatophyta

Pteridospermatophyta

Acrogymnospermae

Angiospermae

Gymnospermae

Uses[edit]

Gymnosperms have major economic uses. Pine, fir, spruce, and cedar are all examples of conifers that are used for lumber, paper production, and resin. Some other common uses for gymnosperms are soap, varnish, nail polish, food, gum, and perfumes.[23]

Life cycle[edit]

Example of gymnosperm lifecycle

Gymnosperms, like all vascular plants, have a sporophyte-dominant life cycle, which means they spend most of their life cycle with diploid cells, while the gametophyte (gamete-bearing phase) is relatively short-lived. Like all seed plants, they are heterosporous, having two spore types, microspores (male) and megaspores (female) that are typically produced in pollen cones or ovulate cones, respectively. As with all heterosporous plants, the gametophytes develop within the spore wall. Pollen grains (microgametophytes) mature from microspores, and ultimately produce sperm cells. Megagametophytes develop from megaspores and are retained within the ovule. Gymnosperms produce multiple archegonia, which produce the female gamete.

During pollination, pollen grains are physically transferred between plants from the pollen cone to the ovule. Pollen is usually moved 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 flagellated motile sperm[24] that swim directly to the egg inside the ovule, whereas conifers and gnetophytes have sperm with no flagella that are moved along a pollen tube to the egg. After syngamy (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. The mature seed comprises the embryo and the remains of the female gametophyte, which serves as a food supply, and the seed coat.[25]

Genetics[edit]

The first published sequenced genome for any gymnosperm was the genome of Picea abies in 2013.[26]

References[edit]

  1. ^ a b c "Gymnosperms on The Plant List". Theplantlist.org. Archived from the original on 2013-08-24. Retrieved 2013-07-24.
  2. ^ a b Raven, P.H. (2013). Biology of Plants. New York: W.H. Freeman and Co.
  3. ^ Recent advances on phylogenomics of gymnosperms and a new classification
  4. ^ Walas, Łukasz; Mandryk, Wojciech; Thomas, Peter A.; Tyrała-Wierucka, Żanna; Iszkuło, Grzegorz (2018-09-01). "Sexual systems in gymnosperms: A review". Basic and Applied Ecology. 31: 1–9. doi:10.1016/j.baae.2018.05.009. ISSN 1439-1791. S2CID 90740232.
  5. ^ Walas Ł, Mandryk W, Thomas PA, Tyrała-Wierucka Ż, Iszkuło G (2018). "Sexual systems in gymnosperms: A review" (PDF). Basic and Applied Ecology. 31: 1–9. doi:10.1016/j.baae.2018.05.009. S2CID 90740232.
  6. ^ Cantino 2007.
  7. ^ a b Christenhusz, M.J.M.; Reveal, J.L.; Farjon, A.; Gardner, M.F.; Mill, R.R.; Chase, M.W. (2011). "A new classification and linear sequence of extant gymnosperms" (PDF). Phytotaxa. 19: 55–70. doi:10.11646/phytotaxa.19.1.3.
  8. ^ Hilton, Jason; Bateman, Richard M. (January 2006). "Pteridosperms are the backbone of seed-plant phylogeny 1". The Journal of the Torrey Botanical Society. 133 (1): 119–168. doi:10.3159/1095-5674(2006)133[119:PATBOS]2.0.CO;2. S2CID 86395036.
  9. ^ Christenhusz, M. J. M.; Byng, J. W. (2016). "The number of known plants species in the world and its annual increase". Phytotaxa. 261 (3): 201–217. doi:10.11646/phytotaxa.261.3.1.
  10. ^ Li, Hong-Tao; Yi, Ting-Shuang; Gao, Lian-Ming; Ma, Peng-Fei; Zhang, Ting; Yang, Jun-Bo; Gitzendanner, Matthew A.; Fritsch, Peter W.; Cai, Jie; Luo, Yang; Wang, Hong (May 2019). "Origin of angiosperms and the puzzle of the Jurassic gap". Nature Plants. 5 (5): 461–470. doi:10.1038/s41477-019-0421-0. PMID 31061536. S2CID 146118264.
  11. ^ Morris, Jennifer L.; Puttick, Mark N.; Clark, James W.; Edwards, Dianne; Kenrick, Paul; Pressel, Silvia; Wellman, Charles H.; Yang, Ziheng; Schneider, Harald; Donoghue, Philip C. J. (2018-03-06). "The timescale of early land plant evolution". Proceedings of the National Academy of Sciences of the United States of America. 115 (10): E2274–E2283. Bibcode:2018PNAS..115E2274M. doi:10.1073/pnas.1719588115. PMC 5877938. PMID 29463716.
  12. ^ Jiao, Yuannian; Wickett, Norman J.; Ayyampalayam, Saravanaraj; Chanderbali, André S.; Landherr, Lena; Ralph, Paula E.; Tomsho, Lynn P.; Hu, Yi; Liang, Haiying; Soltis, Pamela S.; Soltis, Douglas E. (2011-04-10). "Ancestral polyploidy in seed plants and angiosperms". Nature. 473 (7345): 97–100. Bibcode:2011Natur.473...97J. doi:10.1038/nature09916. PMID 21478875. S2CID 4313258.
  13. ^ Ollerton, J.; Coulthard, E. (2009). "Evolution of Animal Pollination". Science. 326 (5954): 808–809. Bibcode:2009Sci...326..808O. doi:10.1126/science.1181154. PMID 19892970. S2CID 856038.
  14. ^ Ren, D; Labandeira, CC; Santiago-Blay, JA; Rasnitsyn, A; et al. (2009). "A Probable Pollination Mode Before Angiosperms: Eurasian, Long-Proboscid Scorpionflies". Science. 326 (5954): 840–847. Bibcode:2009Sci...326..840R. doi:10.1126/science.1178338. PMC 2944650. PMID 19892981.
  15. ^ Labandeira, Conrad C.; Yang, Qiang; Santiago-Blay, Jorge A.; Hotton, Carol L.; Monteiro, Antónia; Wang, Yong-Jie; Goreva, Yulia; Shih, ChungKun; Siljeström, Sandra; Rose, Tim R.; Dilcher, David L.; Ren, Dong (2016). "The evolutionary convergence of mid-Mesozoic lacewings and Cenozoic butterflies". Proceedings of the Royal Society B: Biological Sciences. 283 (1824): 20152893. doi:10.1098/rspb.2015.2893. PMC 4760178. PMID 26842570.
  16. ^ The tortoise and the hare: ecology of angiosperm dominance and gymnosperm persistence
  17. ^ Tissue Responses and Solution Movement After Stem Wounding in Six Cycas Species
  18. ^ Green Plants: Their Origin and Diversity
  19. ^ Introduction to Plant Fossils
  20. ^ A Monographic Revision of Retrophyllum (Podocarpaceae)
  21. ^ Catalogue of Life: 2007 Annual checklist – Conifer database Archived January 15, 2009, at the Wayback Machine
  22. ^ Campbell, Reece, "Phylum Coniferophyta."Biology. 7th. 2005. Print. P.595
  23. ^ Biswas, C.; Johri, B.M. (1997). "Economic Importance". The Gymnosperms (PDF). Springer, Berlin, Heidelberg. pp. 440–456. doi:10.1007/978-3-662-13164-0_23. ISBN 978-3-662-13166-4.
  24. ^ Southworth, Darlene; Cresti, Mauro (September 1997). "Comparison of flagellated and nonflagellated sperm in plants". American Journal of Botany. 84 (9): 1301–1311. doi:10.2307/2446056. JSTOR 2446056. PMID 21708687. Retrieved 26 March 2022.
  25. ^ Walters, Dirk R Walters Bonnie By (1996). Vascular plant taxonomy. Dubuque, Iowa: Kendall/Hunt Pub. Co. p. 124. ISBN 978-0-7872-2108-9. Gymnosperm seeds.
  26. ^ Nystedt, B; Street, NR; Wetterbom, A; et al. (May 2013). "The Norway spruce genome sequence and conifer genome evolution". Nature. 497 (7451): 579–584. Bibcode:2013Natur.497..579N. doi:10.1038/nature12211. PMID 23698360.

General bibliography[edit]

  • Cantino, Philip D.; Doyle, James A.; Graham, Sean W.; Judd, Walter S.; Olmstead, Richard G.; Soltis, Douglas E.; Soltis, Pamela S.; Donoghue, Michael J. (August 2007). "Towards a phylogenetic nomenclature of Tracheophyta". Taxon. 56 (3): 822–846. doi:10.2307/25065864. JSTOR 25065864.

External links[edit]