Editing Germination (section)

== Seed plants ==
[[Image: Horticulture Tray3.jpg|thumb|150px|A seed pot used in horticulture for sowing and taking plant cuttings and growing [[Plug (horticulture)|plugs]]]]
[[File:Sprossenglas.JPG|thumb|Germination glass (glass sprouter jar) with a plastic [[sieve]]-lid]]
[[Image:Raapstelen gekiemde zaden (Brassica campestris germinating seeds).jpg|thumb|250px|''[[Brassica|Brassica campestris]]'' germinating seeds]]
[[File:Mung bean germination.ogv|right|250px|Time-lapse video of mung bean seeds germinating]]

Germination is usually the growth of a plant contained within a seed resulting in the formation of the seedling. It is also the process of reactivation of metabolic machinery of the seed resulting in the emergence of [[radicle]] and [[plumule]]. The seed of a [[vascular plant]] is a small package produced in a [[fruit]] or [[conifer cone|cone]] after the union of male and female [[gamete|reproductive cells]]. All fully developed seeds contain an [[embryo]] and, in most plant species some store of food reserves, wrapped in a seed coat. Dormant seeds are viable seeds that do not germinate because they require specific internal or environmental stimuli to resume growth. Under proper conditions, the seed begins to germinate and the embryo resumes growth, developing into a seedling.{{clarify|date=April 2019}}

[[File: Seed Germination.png|thumb|Step 1: Water imbibition, the uptake of water, results in rupture of seed coat.

Step 2: The imbibition of the seed coat results in emergence of the [[radicle]] (1) and the [[plumule]] (2); the [[cotyledons]] are unfolded (3).<br />
Step 3: This marks the final step in the germination of the seed, where the cotyledons are expanded, which are the true leaves. Note: Temperature must be kept at an optimum level.]]

Disturbance of soil can result in vigorous plant growth by exposing seeds already in the soil to changes in environmental factors where germination may have previously been inhibited by depth of the seeds or soil that was too compact. This is often observed at gravesites after a burial.<ref>{{cite book |title=Forensic Botany |publisher=Wiley-Blackwell |year=2012 |page=10}}</ref>

Seed germination depends on both internal and external conditions. The most important external factors include right [[temperature]], [[water]], [[oxygen]] or [[air]] and sometimes [[light]] or [[darkness]].<ref name="Raven" /> Various plants require different variables for successful seed germination. Often this depends on the individual seed variety and is closely linked to the [[Ecology|ecological conditions]] of a plant's [[natural habitat]]. For some seeds, their future germination response is affected by environmental conditions during seed formation; most often these responses are types of [[seed dormancy]].
* '''Water''' is required for germination. Mature seeds are often extremely dry and need to take in significant amounts of water, relative to the dry weight of the seed, before cellular [[metabolism]] and growth can resume. Most seeds need enough water to moisten the seeds but not enough to soak them. The uptake of water by seeds is called [[imbibition]], which leads to the swelling and the breaking of the seed coat. When seeds are formed, most plants store a food reserve with the seed, such as [[starch]], [[protein]]s, or [[oil]]s. This food reserve provides nourishment to the growing embryo. When the seed imbibes water, [[hydrolytic enzyme]]s are activated which break down these stored food resources into metabolically useful [[chemical]]s.<ref name="Raven"/> After the seedling emerges from the seed coat and starts growing roots and leaves, the seedling's food reserves are typically exhausted; at this point [[photosynthesis]] provides the energy needed for continued growth and the seedling now requires a continuous supply of water, nutrients, and light.
* '''Oxygen''' is required by the germinating seed for [[metabolism]].<ref>{{cite journal | vauthors = Siegel SM, Rosen LA | year = 1962 | title = Effects of Reduced Oxygen Tension on Germination and Seedling Growth | journal = Physiologia Plantarum | volume = 15 | issue = 3| pages = 437–444 | doi = 10.1111/j.1399-3054.1962.tb08047.x | bibcode = 1962PPlan..15..437S }}</ref> Oxygen is used in [[aerobic respiration]], the main source of the seedling's energy until it grows leaves.<ref name="Raven"/> Oxygen is an [[atmospheric gas]] that is found in [[soil]] pore spaces; if a seed is buried too deeply within the soil or the soil is waterlogged, the seed can be oxygen starved. Some seeds have impermeable seed coats that prevent oxygen from entering the seed, causing a type of physical dormancy which is broken when the seed coat is worn away enough to allow gas exchange and water uptake from the environment.
** In a small number of plants, such as [[Oryza sativa|rice]], anaerobic germination can occur in waterlogged conditions. The seed produces a hollow [[coleoptile]] that acts like a 'snorkel', providing the seed with access to oxygen.<ref>{{cite journal |last1=Magneschi |first1=Leonardo |last2=Perata |first2=Pierdomenico |title=Rice germination and seedling growth in the absence of oxygen |journal=Annals of Botany |date=25 July 2008 |volume=103 |issue=2 |pages=181–196 |doi=10.1093/aob/mcn121 |pmid=18660495 |pmc=2707302 |url=https://academic.oup.com/aob/article/103/2/181/187259 |access-date=27 March 2022}}</ref>
* '''Temperature''' affects cellular metabolism and growth rates. Seeds from different species and even seeds from the same plant germinate over a wide range of temperatures. Seeds often have a temperature range within which they will germinate, and they will not do so above or below this range. Many seeds germinate at temperatures slightly above 60–75 F (16–24 C) [room-temperature in centrally heated houses], while others germinate just above freezing and others germinate only in response to alternations in temperature between warm and cool. Some seeds germinate when the soil is cool 28–40 F (-2 – 4 C), and some when the soil is warm 76–90 F (24–32 C). Some seeds require exposure to cold temperatures ([[vernalization]]) to break dormancy. Some seeds in a dormant state will not germinate even if conditions are favorable. Seeds that are dependent on temperature to end dormancy have a type of physiological dormancy. For example, seeds requiring the cold of winter are inhibited from germinating until they take in water in the fall and experience cooler temperatures. Cold [[stratification (seeds)|stratification]] is a process that induces the dormancy breaking prior to light emission that promotes germination .<ref>{{Cite book|title=Variation in Seed Dormancy and Germination within and between Individuals and Populations of a Species. Seeds: Ecology, Biogeography, and, Evolution of Dormancy and Germination.| first1 = Carol C | last1 = Baskin | first2 = Jerry M | last2 = Baskin | name-list-style = vanc |publisher=Elsevier Science|year=2014|isbn=9780124166837|location=Burlington|pages=5–35}}</ref> Four degrees Celsius is cool enough to end dormancy for most cool dormant seeds, but some groups, especially within the family [[Ranunculaceae]] and others, need conditions cooler than -5 C. Some seeds will only germinate after hot temperatures during a [[forest fire]] which cracks their seed coats; this is a type of physical dormancy.
Most common annual [[vegetable]]s have optimal germination temperatures between 75–90 F (24–32 C), though many species (e.g. [[radish]]es or [[spinach]]) can germinate at significantly lower temperatures, as low as 40 F (4 C), thus allowing them to be grown from seeds in cooler climates. Suboptimal temperatures lead to lower success rates and longer germination periods.
* '''Light or darkness''' can be an environmental trigger for germination and is a type of physiological dormancy. Most seeds are not affected by light or darkness, but many [[photoblasticism|photoblastic]] seeds, including species found in forest settings, will not germinate until an opening in the canopy allows sufficient light for the growth of the seedling.<ref name="Raven"/>
* '''[[Scarification (botany)|Scarification]]''' mimics natural processes that weaken the seed coat before germination. In nature, some seeds require particular conditions to germinate, such as the heat of a fire (e.g., many Australian native plants), or soaking in a body of water for a long period of time. Others need to be passed through an animal's [[digestive tract]] to weaken the seed coat enough to allow the seedling to emerge.<ref name="Raven"/>
[[Image:Sjb whiskey malt.jpg|thumb|250px|Malted (germinated) [[barley]] grains]]

=== Dormancy ===
Some live seeds are [[dormancy|dormant]] and need more time, and/or need to be subjected to specific environmental conditions before they will germinate. [[Seed dormancy]] can originate in different parts of the seed, for example, within the embryo; in other cases the seed coat is involved. Dormancy breaking often involves changes in membranes, initiated by dormancy-breaking signals. This generally occurs only within hydrated seeds.<ref>{{cite book | first1 = J Derek | last1 = Bewley | first2 = Michael | last2 = Black | first3 = Peter | last3 = Halmer | name-list-style = vanc | title = The encyclopedia of seeds: science, technology and uses Cabi Series| url = https://books.google.com/books?id=aE414KuXu4gC&pg=PA203| year = 2006| pages = 203| isbn = 978-0-85199-723-0 }}</ref> Factors affecting seed dormancy include the presence of certain plant hormones, notably [[abscisic acid]], which inhibits germination, and [[gibberellin]], which ends seed dormancy. In [[brewing]], barley seeds are treated with gibberellin to ensure uniform seed germination for the production of barley [[malt]].<ref name="Raven"/>

=== Seedling establishment ===
In some definitions, the appearance of the [[radicle]] marks the end of germination and the beginning of "establishment", a period that utilizes the food reserves stored in the seed. Germination and establishment as an independent organism are critical phases in the life of a plant when they are the most vulnerable to injury, disease, and water stress.<ref name="Raven"/> The germination index can be used as an indicator of [[phytotoxicity]] in soils. The mortality between dispersal of seeds and completion of the establishment can be so high that many species have adapted to produce large numbers of seeds.{{cn|date=June 2024}}