Editing Seed (section)

== Functions ==
Seeds serve several functions for the plants that produce them. Key among these functions are nourishment of the [[embryo]], [[biological dispersal|dispersal]] to a new location, and [[dormancy]] during unfavorable conditions. Seeds fundamentally are means of reproduction, and most seeds are the product of [[sexual reproduction]] which produces a remixing of genetic material and [[phenotype]] variability on which [[natural selection]] acts. Plant seeds hold [[endophyte|endophytic]] microorganisms that can perform various functions, the most important of which is protection against disease.<ref>{{cite journal | vauthors = Matsumoto H, Fan X, Wang Y, Kusstatscher P, Duan J, Wu S, Chen S, Qiao K, Wang Y, Ma B, Zhu G, Hashidoko Y, Berg G, Cernava T, Wang M | display-authors = 6 | title = Bacterial seed endophyte shapes disease resistance in rice | journal = Nature Plants | volume = 7 | pages = 60–72 | date = January 2021 | issue = 1 | doi = 10.1038/s41477-020-00826-5 | pmid = 33398157 | bibcode = 2021NatPl...7...60M | s2cid = 230508404 }}</ref>

=== Embryo nourishment ===
Seeds protect and nourish the embryo or young plant. They usually give a seedling a faster start than a sporeling from a spore, because of the larger food reserves in the seed and the multicellularity of the enclosed embryo.

=== Dispersal ===
{{Main|Seed dispersal}}
Unlike animals, plants are limited in their ability to seek out favorable conditions for life and growth. As a result, plants have evolved many ways to [[Biological dispersal|disperse]] their offspring by dispersing their seeds (see also [[vegetative reproduction]]). A seed must somehow "arrive" at a location and be there at a time favorable for germination and growth. When the fruits open and release their seeds in a regular way, it is called [[dehiscence (botany)|dehiscent]], which is often distinctive for related groups of plants; these fruits include [[Capsule (fruit)|capsules]], [[Follicle (fruit)|follicles]], [[legume]]s, [[silicle]]s and [[silique]]s. When fruits do not open and release their seeds in a regular fashion, they are called indehiscent, which include the fruits [[achene]]s, [[Caryopsis|caryopses]], [[Nut (fruit)|nuts]], [[samara (fruit)|samaras]], and [[utricle (fruit)|utricles]].<ref>Jones, Samuel B., and Arlene E. Luchsinger. 1979. ''Plant systematics. McGraw-Hill series in organismic biology''. New York: McGraw-Hill. p. 195.</ref>

==== By wind (anemochory) ====
[[File:Taraxacum sect. Ruderalia MHNT.jpg|thumb|Dandelion seeds are contained within [[achene]]s, which can be carried long distances by the wind.]]
[[File:Milkweed-in-seed2.jpg|right|upright|thumb|The seed pod of [[milkweed]] (''Asclepias syriaca'')]]
* Some seeds (e.g., [[pine]]) have a wing that aids in wind dispersal.
* The dustlike seeds of [[orchid]]s are carried efficiently by the wind.
* Some seeds (e.g. [[milkweed]], [[Populus|poplar]]) have hairs that aid in wind dispersal.<ref name="california_desert_flowers_an_introduction_to_families_genera">{{cite book  | last1 = Morhardt | first1 = Sia | last2 = Morhardt | first2 = Emil | last3 = Emil Morhardt | first3 = J. | title = California desert flowers: an introduction to families, genera, and species | url = https://books.google.com/books?id=1XyN-u-Bk40C&pg=PA24| year = 2004 | publisher = University of California Press | location = Berkeley  | isbn = 978-0-520-24003-2 | page = 24 }}</ref>

Other seeds are enclosed in fruit structures that aid wind dispersal in similar ways:
* [[Dandelion]] achenes have hairs.
* [[Maple]] samaras have two wings.

==== By water (hydrochory) ====
* Some plants, such as ''[[Mucuna]]'' and ''[[Dioclea (plant)|Dioclea]]'', produce buoyant seeds termed sea-beans or drift seeds because they float in rivers to the oceans and wash up on beaches.<ref>{{cite web|url=http://www.seabean.com/|title=www.seabean.com – Sea-Beans and Drift Seeds|work=seabean.com|url-status=live|archive-url=https://web.archive.org/web/20060711180958/http://www.seabean.com/|archive-date=2006-07-11}}</ref>

==== By animals (zoochory) ====
* Seeds ([[burr (fruit)|burrs]]) with barbs or hooks (e.g. [[acaena]], [[burdock]], [[Rumex|dock]]) which attach to animal fur or feathers, and then drop off later.
* Seeds with a fleshy covering (e.g. [[apple]], [[cherry]], [[juniper]]) are eaten by animals ([[bird]]s, [[mammal]]s, [[reptile]]s, [[fish]]) which then disperse these seeds in their [[faeces|droppings]].
* Seeds ([[nut (fruit)|nuts]]) are attractive long-term storable food resources for animals (e.g. [[acorn]]s, [[hazel]]nut, [[walnut]]); the seeds are stored some distance from the parent plant, and some escape being eaten if the animal forgets them.

'''[[Myrmecochory]]''' is the dispersal of seeds by [[ant]]s. Foraging ants disperse seeds which have appendages called [[elaiosome]]s<ref>{{cite journal | author = Marinelli J | year = 1999 | title = Ants&nbsp;– The astonishing intimacy between ants & plants | url = http://www.bbg.org/gar2/topics/wildlife/1999sp_ants.html | journal = Plants & Gardens News | volume = 14 | issue = 1 | url-status=live | archive-url = https://web.archive.org/web/20060818062717/http://www.bbg.org/gar2/topics/wildlife/1999sp_ants.html | archive-date = 2006-08-18 }}</ref> (e.g. [[bloodroot]], [[trillium]]s, [[acacia]]s, and many species of [[Proteaceae]]). Elaiosomes are soft, fleshy structures that contain nutrients for animals that eat them. The ants carry such seeds back to their nest, where the elaiosomes are eaten. The remainder of the seed, which is hard and inedible to the ants, then germinates either within the nest or at a removal site where the seed has been discarded by the ants.<ref name="Ricklefs 1993">Ricklefs, Robert E. (1993) ''The Economy of Nature'', 3rd ed., p. 396. (New York: W.H. Freeman). {{ISBN|0-7167-2409-X}}.</ref> This dispersal relationship is an example of [[Mutualism (biology)|mutualism]], since the plants depend upon the ants to disperse seeds, while the ants depend upon the plants seeds for food. As a result, a drop in numbers of one partner can reduce success of the other. In [[South Africa]], the [[Argentine ant]] (''Linepithema humile'') has [[invasive species|invaded]] and displaced native species of ants. Unlike the native ant species, Argentine ants do not collect the seeds of ''[[Mimetes cucullatus]]'' or eat the elaiosomes. In areas where these ants have invaded, the numbers of ''Mimetes'' seedlings have dropped.<ref>{{cite journal | author = Bond, W.J. |author2=P. Slingsby  | year=1984 | title = Collapse of an ant-plant mutualism: The Argentine ant, ''Iridomyrmex humilis'' and myrmecochorous Proteaceae | journal = Ecology | volume=65 | issue = 4 | pages=1031–1037 | doi = 10.2307/1938311 | jstor = 1938311|bibcode=1984Ecol...65.1031B }}</ref>

=== Dormancy ===
{{Main|Seed dormancy}}

Seed dormancy has two main functions: the first is synchronizing germination with the optimal conditions for survival of the resulting seedling; the second is spreading germination of a batch of seeds over time so a catastrophe (e.g. late frosts, drought, [[herbivory]]) does not result in the death of all offspring of a plant ([[Bet-hedging (biology)|bet-hedging]]).<ref>Eira MTS, Caldas LS (2000) Seed dormancy and germination as concurrent processes. Rev Bras Fisiol Vegetal 12:85–104</ref> Seed dormancy is defined as a seed failing to germinate under environmental conditions optimal for germination, normally when the environment is at a suitable temperature with proper soil moisture. This true dormancy or innate dormancy is therefore caused by conditions within the seed that prevent germination. Thus dormancy is a state of the seed, not of the environment.<ref>{{cite journal |author=Vleeshouwers L.M. |author2=Bouwmeester H.J. |author3=Karssen C.M. | year = 1995 | title = Redefining seed dormancy: an attempt to integrate physiology and ecology | url = http://library.wur.nl/WebQuery/wurpubs/32043| journal = Journal of Ecology | volume = 83 | issue = 6| pages = 1031–1037 | doi=10.2307/2261184| jstor = 2261184 |bibcode=1995JEcol..83.1031V }}</ref> Induced dormancy, enforced dormancy or seed quiescence occurs when a seed fails to germinate because the external environmental conditions are inappropriate for germination, mostly in response to conditions being too dark or light, too cold or hot, or too dry.

Seed dormancy is not the same as seed persistence in the soil or on the plant, though even in scientific publications dormancy and persistence are often confused or used as synonyms.<ref>{{cite journal |vauthors=Thompson K, Ceriani RM, Bakker JP, Bekker RM | year = 2003 | title = Are seed dormancy and persistence in soil related? | journal = Seed Science Research| volume = 13 | issue = 2| pages = 97–100 | doi=10.1079/ssr2003128| bibcode = 2003SeeSR..13...97T | s2cid = 85950260 }}</ref>

Often, seed dormancy is divided into four major categories: exogenous; endogenous; combinational; and secondary. A more recent system distinguishes five classes: morphological, physiological, morphophysiological, physical, and combinational dormancy.<ref>{{cite journal | author = Baskin J.M., Baskin C.C. | year = 2004 | title = A classification system for seed dormancy | journal = Seed Science Research | volume = 14 | issue = 1 | pages = 1–16 | doi=10.1079/ssr2003150| doi-access = free | bibcode = 2004SeeSR..14....1B }}</ref>

Exogenous dormancy is caused by conditions outside the embryo, including:
* '''Physical dormancy''' or '''hard seed coats''' occurs when seeds are [[Permeability (fluid)|impermeable]] to water. At dormancy break, a specialized structure, the 'water gap', is disrupted in response to environmental cues, especially temperature, so water can enter the seed and germination can occur. Plant families where physical dormancy occurs include [[Anacardiaceae]], [[Cannaceae]], [[Convulvulaceae]], [[Fabaceae]] and [[Malvaceae]].<ref>Baskin JM, Baskin CC, Li X (2000) "Taxonomy, anatomy and evolution of physical dormancy in seeds". ''Plant Species Biology'' 15:139–152</ref>
* '''Chemical dormancy''' considers species that lack physiological dormancy, but where a chemical prevents germination. This chemical can be leached out of the seed by rainwater or snow melt or be deactivated somehow.<ref>Baskin, C.C. and Baskin, J.M. (1998) Seeds: Ecology, biogeography, and evolution of dormancy and germination.San Diego, Academic Press</ref> Leaching of chemical inhibitors from the seed by rain water is often cited as an important cause of dormancy release in seeds of desert plants, but little evidence exists to support this claim.<ref>Gutterman, Y. (1993) Seed germination in desert plants. Springer Verlag, Berlin/Heidelberg.</ref>

Endogenous dormancy is caused by conditions within the embryo itself, including:
* In '''morphological dormancy''', germination is prevented due to morphological characteristics of the embryo. In some species, the embryo is just a mass of cells when seeds are dispersed; it is not differentiated. Before germination can take place, both differentiation and growth of the embryo have to occur. In other species, the embryo is differentiated but not fully grown (underdeveloped) at dispersal, and embryo growth up to a species specific length is required before germination can occur. Examples of plant families where morphological dormancy occurs are [[Apiaceae]], [[Cycadaceae]], [[Liliaceae]], [[Magnoliaceae]] and [[Ranunculaceae]].<ref name="Baskin, C.C 1998">Baskin, C.C. and Baskin, J.M. (1998) Seeds: Ecology, biogeography, and evolution of dormancy and germination.San Diego, Academic Press.</ref><ref name="Baskin, J.M 2004">Baskin, J.M. and Baskin, C.C. (2004) A classification system for seed dormancy. Seed Science Research 14:1–16.</ref>
* '''Morphophysiological dormancy''' includes seeds with underdeveloped embryos, and also have physiological components to dormancy. These seeds, therefore, require a dormancy-breaking treatments, as well as a period of time to develop fully grown embryos. Plant families where morphophysiological dormancy occurs include [[Apiaceae]], [[Aquifoliaceae]], [[Liliaceae]], [[Magnoliaceae]], [[Papaveraceae]] and [[Ranunculaceae]].<ref name="Baskin, C.C 1998" /> Some plants with morphophysiological dormancy, such as ''[[Asarum]]'' or ''[[Trillium]]'' species, have multiple types of dormancy, one affects radicle (root) growth, while the other affects plumule (shoot) growth. The terms "double dormancy" and "two-year seeds" are used for species whose seeds need two years to complete germination or at least two winters and one summer. Dormancy of the radicle (seedling root) is broken during the first winter after dispersal while dormancy of the shoot bud is broken during the second winter.<ref name="Baskin, C.C 1998" />
* '''Physiological dormancy''' means the embryo, due to physiological causes, cannot generate enough power to break through the seed coat, endosperm or other covering structures. Dormancy is typically broken at cool wet, warm wet, or warm dry conditions. [[Abscisic acid]] is usually the growth inhibitor in seeds, and its production can be affected by light.
**'''Drying''', in some plants, including a number of grasses and those from seasonally arid regions, is needed before they will germinate. The seeds are released, but need to have a lower moisture content before germination can begin. If the seeds remain moist after dispersal, germination can be delayed for many months or even years. Many herbaceous plants from temperate climate zones have physiological dormancy that disappears with drying of the seeds. Other species will germinate after dispersal only under very narrow temperature ranges, but as the seeds dry, they are able to germinate over a wider temperature range.<ref>International Workshop on Seeds, and G. Nicolas. 2003. ''The biology of seeds recent research advances : proceedings of the Seventh International Workshop on Seeds, Salamanca, Spain 2002''. Wallingford, Oxon, UK: CABI Pub. p. 113.</ref>
* In seeds with '''combinational dormancy''', the seed or fruit coat is impermeable to water and the embryo has physiological dormancy. Depending on the species, physical dormancy can be broken before or after physiological dormancy is broken.<ref name="Baskin, J.M 2004" />
* '''Secondary dormancy'''* is caused by conditions after the seed has been dispersed and occurs in some seeds when nondormant seed is exposed to conditions that are not favorable to germination, very often high temperatures. The mechanisms of secondary dormancy are not yet fully understood, but might involve the loss of sensitivity in receptors in the plasma membrane.<ref>Bewley, J. Derek, and Michael Black. 1994. ''Seeds physiology of development and germination. The language of science''. New York: Plenum Press. p. 230.</ref>

The following types of seed dormancy do not involve seed dormancy, strictly speaking, as lack of germination is prevented by the environment, not by characteristics of the seed itself (see [[Germination]]):
* '''Photodormancy''' or light sensitivity affects germination of some seeds. These photoblastic seeds need a period of darkness or light to germinate. In species with thin seed coats, [[light]] may be able to penetrate into the dormant embryo. The presence of light or the absence of light may trigger the germination process, inhibiting germination in some seeds buried too deeply or in others not buried in the soil.
* '''Thermodormancy''' is seed sensitivity to heat or cold. Some seeds, including cocklebur and amaranth, germinate only at high temperatures (30&nbsp;°C or 86&nbsp;°F); many plants that have seeds that germinate in early to midsummer have thermodormancy, so germinate only when the soil temperature is warm. Other seeds need cool soils to germinate, while others, such as celery, are inhibited when soil temperatures are too warm. Often, thermodormancy requirements disappear as the seed ages or dries.

Not all seeds undergo a period of dormancy. Seeds of some [[mangrove]]s are viviparous; they begin to germinate while still attached to the parent. The large, heavy root allows the seed to penetrate into the ground when it falls. Many garden plant seeds will germinate readily as soon as they have water and are warm enough; though their wild ancestors may have had dormancy, these cultivated plants lack it. After many generations of selective pressure by plant breeders and gardeners, dormancy has been selected out.

For [[annual plant|annuals]], seeds are a way for the species to survive dry or cold seasons. Ephemeral plants are usually annuals that can go from seed to seed in as few as six weeks.<ref>{{cite journal | author = Patten D.T. | year = 1978 | title = Productivity and production efficiency of an Upper Sonoran Desert ephemeral community | journal = American Journal of Botany | volume = 65 | issue = 8| pages = 891–895 | doi = 10.2307/2442185 | jstor = 2442185 }}</ref>

=== Persistence and seed banks ===
{{further|Seed dormancy}}