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Seed germination

Germination is the continued growth of an embryonic plant preserved in a seed; by germinating, an embryo may grow to become a seedling. The seeds of higher plants are small packages produced in a fruit, nut, or cone after the union of male and female sex cells. Most seeds go through a period of dormancy during with no active growth, during this time the seed can be safely transported to a new location and survive adverse climate conditions until they are favourable for growth. The seed contains an embryo and in most plants stored food reserves wrapped in a seed coat. If the seed finds itself in favourable conditions, it can begin to germinate, and the embryonic tissues resume growth, developing towards a seedling.

Requirements for seed germination
Water - Oxygen - Temperature - Light (or not)

The germination of seeds is dependent on both internal and external conditions. The most important external factors include: temperature, water, oxygen and sometimes light or darkness. Different plants have seeds with distinctive requirements for successful germination. Germination requirements often depend on the individual seed variety and are closely linked to the ecological conditions of the plant's natural habitat. For some seeds, future germination responses are affected by environmental conditions during seed formation; most often these responses are types of seed dormancy.

Stratification 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, African, and some Mediterranean and American native plants), or soaking in a body of water for a long period of time. Others have to be passed through an animal's digestive tract to weaken the seed coat and enable germination.

Dormancy

Many live seeds have dormancy, meaning they will not germinate even if the environment has sufficient water and warmth for the seed to germinate. Dormancy factors include conditions affecting many different parts of the seed, from the embryo to the seed coat. Dormancy is broken or ended by a number of different conditions, and is caused by internal or external and sometimes both factors. Environmental factors like light, temperature, fire, ingestion by animals, are conditions that can end seed dormancy. Internally seeds may be dormant because of plant hormones, which affect cell growth and prevent germination. One plant hormone that is a common dormancy inducing chemical is absciscic acid, while the production and application of the hormone gibberellin can break dormancy and induce seed germination.

Seedling establishment

In some definitions, the appearance of the radicle marks the end of germination and the beginning of "establishment", a period that ends when the seedling has exhausted 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. The germination index can be used as an indicator of phytotoxicity in soils. The mortality between dispersal of seeds and completion of establishment can be so high, that many species survive only by producing huge numbers of seeds.

Germination rate

In agriculture and gardening, germination rate is the number of seeds of a particular plant species, variety or particular seedlot that are likely to germinate. This is usually expressed as a percentage, e.g. an 85% germination rate indicates that about 85 out of 100 seeds will probably germinate under proper conditions. Germination rate is useful in calculating seed requirements for a given area or desired number of plants.

Dicot germination

The part of the plant that emerges from the seed first is the embryonic root, termed radicle or primary root. This allows the seedling to become anchored in the ground and start absorbing water. After the root absorbs water, the embryonic shoot emerges from the seed. The shoot comprises three main parts: the cotyledons (seed leaves), the section of shoot below the cotyledons (hypocotyl), and the section of shoot above the cotyledons (epicotyl). The way the shoot emerges differs between plant groups.

Epigeous

In epigeous (or epigeal) germination, the hypocotyl elongates and forms a hook, pulling rather than pushing the cotyledons and apical meristem through the soil. Once it reaches the surface, it straightens and pulls the cotyledons and shoot tip of the growing seedlings into the air. Beans, tamarind, and papaya are examples of plant that germinate this way.

Hypogeous

Another way of germination is hypogeous (or hypogeal) where the epicotyl elongates and forms the hook. In this type of germination, the cotyledons stay underground where they eventually decompose. Peas, and Wheat, are among many hypogeal plants. Germination also starts with one tiny seedling and then begins to sprout.

Monocot germination

In monocot seeds, the embryo's radicle and cotyledon are covered by a coleorhiza and coleoptile, respectively. The coleorhiza is the first part to grow out of the seed, followed by the radicle. The coleoptile is then pushed up through the ground until it reaches the surface. There, it stops elongating and the first leaves emerge through an opening as it is.

"Seeds" and "Fruit"

achene: a dry, indehiscent, seed like,fruit containing one seed which is free from the pericarp.
eg. Strawberries.
Formed from a superior ovary of one carpel cf. cypsela, follicle.
aggregate fruit: a cluster of fruits formed from the free carpel of one flower. cf.syncarp.
angiosperm: a seed-bearing plant whose ovules, and hence seeds, develop within an enclosed ovary. cf. gymnosperm.
baccate: berry-like; of seeds, having a succulent or pulpy testa; of fruits, having the seeds embedded in pulp.
berry: a fleshy or pulpy indehiscent fruit with the seed(s) embedded in the fleshy tissue of the pericarp. cf. drupe, pyrene.
carpel: an organ, evolved from a leaf, at the centre of a flower, bearing one or more ovules.
having its margins fused together or with other carpels to enclose the ovule(s) in an ovary, and consisting also of a stigma and usually a style.
cone: in gymnosperms and club-mosses, a group of sporophylls arranged compactly on a central axis.
In Casuarina, a woody multiple fruit incorporating the bracts and bracteoles associated with the flowers.
cypsela: a dry, indehiscent, one-seeded fruit formed from an inferior ovary. cf. achene.
drupe: a succulent fruit formed from one carpel, having the seed(s) enclosed in an inner stony layer of the fruit wall.
adj. drupaceous (which is often used to mean drupe-like but not strictly a drupe). cf. berry, pyrene.
follicle: a dry, dehiscent fruit formed from one carpel and dehiscing along the line of fusion of its margin. Contains few to many seeds attached along it's margins.
fruit: the seed-bearing structure in an angiosperm formed from the ovary after flowering.
grain: a fruit characteristic of grasses (= caryopsis); pollen grain, a microspore of a seed plant, or the partially developed gametophyte formed from it.
gymnosperm: a seed plant with the ovules borne on the surface of a sporophyll. cf. angiosperm.
legume: a fruit characteristic of the families Mimosaceae, Caesalpiniaceae and Fabaceae, formed from one carpel and either dehiscent along both sides, or indehiscent.
An edible crop species in the family Fabaceae.
lomentum: a legume having distinct constrictions or lines of abscission between the seeds and breaking into one-seeded segments when mature.
nut: a hard, dry, indehiscent fruit formed from two or more carpels but containing only one seed.
pepo: literally, a pumpkin (latin); a fruit with firm skin, pulpy interior, many seeds and a single locule.
pod: a leguminous fruit.
pome: a fleshy (false) fruit, formed from an inferior ovary, in which the receptacle or hypanthium has enlarged to enclose the true fruit.
propagule: a structure with the capacity to give rise to a new plant, e.g. a seed, a spore, part of the vegetative body capable of independent growth if detached from the parent.
pyrene: the 'stone' (endocarp plus seed) of a succulent fruit. cf. berry, drupe.
samara: a dry, indehiscent fruit with its wall expanded into a wing.
schizocarp: a dry fruit formed from more than one carpel but breaking apart into 1 -carpel units when ripe. cf. mericarp.
seed: a propagating organ formed in the sexual reproductive cycle of gymnosperms and angiosperm, consisting of a protective coat (testa) enclosing an embryo and food reserves.
siliqua: a dry, dehiscent fruit formed from a superior ovary of two carpels, with two parietal placentas and divided into two loculi by a false septum between the placentas.
spore: a simple propagule, produced either sexually or asexually, and consisting of one or a few cells.
sporocarp: a fruiting body containing sporangia.
strobilus: a 'cone' consisting of sporophylls borne close together on an axis.
syconium: a multiple fruit with a hollow centre, e.g. in ficus (fig).
synangium: of fruit, several fruits united in a single structure.
syncarp: a structure consisting of several united fruits, usually fleshy. cf. aggregate fruit.

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