Editing Seed (section)

== Development ==
[[File:Seed Development Cycle.svg|thumb|right|'''Stages of seed development''':

{{Col-begin}}
{{Col-break}}
 '''I''' Zygote<br />
 '''II''' Proembryo<br />
'''III''' Globular
 {{Col-break}}
'''IV'''  Heart<br />
'''V'''  Torpedo<br />
'''VI'''  Mature Embryo
{{col-end}}
'''Key''': ''1. Endosperm 2. Zygote 3. Embryo 4. Suspensor 5. Cotyledons 6. Shoot Apical Meristem 7. Root Apical Meristem 8. Radicle 9. Hypocotyl 10. Epicotyl 11. Seed Coat'']]

Angiosperm seeds are "enclosed seeds", produced in a hard or fleshy structure called a fruit that encloses them for protection. Some fruits have layers of both hard and fleshy material. In gymnosperms, no special structure develops to enclose the seeds, which begin their development "naked" on the bracts of cones. However, the seeds do become covered by the [[Conifer cone|cone]] scales as they develop in some species of [[conifer]].

[[Angiosperm]] (flowering plants) seeds consist of three genetically distinct constituents: (1) the embryo formed from the zygote, (2) the endosperm, which is normally triploid, (3) the seed coat from tissue derived from the maternal tissue of the ovule. In angiosperms, the process of seed development begins with [[double fertilization]], which involves the fusion of two male gametes with the egg cell and the central cell to form the primary [[endosperm]] and the zygote. Right after fertilization, the zygote is mostly inactive, but the primary endosperm divides rapidly to form the endosperm tissue. This tissue becomes the food the young plant will consume until the roots have developed after [[germination]].

=== Ovule ===
{{Main|Ovule}}
[[File:Ovule-Gymno-Angio-en.svg|thumb|300px|Plant ovules: Gymnosperm ovule on left, angiosperm ovule (inside ovary) on right]]

After fertilization, the [[Ovule|ovules]] develop into the seeds. The ovule consists of a number of components:
* The '''funicle''' (''funiculus, funiculi'') or seed stalk which attaches the ovule to the [[placentation|placenta]] and hence [[Ovary (botany)|ovary]] or fruit wall, at the '''[[Fruit anatomy#Pericarp layers|pericarp]]'''.
* The '''nucellus''', the remnant of the [[megasporangium]] and main region of the ovule where the megagametophyte develops.
* The '''micropyle''', a small pore or opening in the apex of the integument of the ovule where the [[pollen tube]] usually enters during the process of fertilization.
* The '''[[chalaza]]''', the base of the ovule opposite the micropyle, where integument and nucellus are joined.<ref>{{cite book |chapter=Chapter One |author1=Galili G |author2=Kigel J |title=Seed development and germination |publisher=M. Dekker |location=New York |year=1995 |isbn=978-0-8247-9229-9 }}</ref>

The shape of the ovules as they develop often affects the final shape of the seeds. Plants generally produce ovules of four shapes: the most common shape is called '''anatropous''', with a curved shape. '''Orthotropous''' ovules are straight with all the parts of the ovule lined up in a long row producing an uncurved seed. '''Campylotropous''' ovules have a curved megagametophyte often giving the seed a tight "C" shape. The last ovule shape is called '''amphitropous''', where the ovule is partly inverted and turned back 90 degrees on its stalk (the funicle or '''funiculus''').

In the majority of flowering plants, the zygote's first division is transversely oriented in regards to the long axis, and this establishes the polarity of the embryo. The upper or chalazal pole becomes the main area of growth of the embryo, while the lower or [[micropylar]] pole produces the stalk-like suspensor that attaches to the micropyle. The suspensor absorbs and manufactures nutrients from the endosperm that are used during the embryo's growth.<ref>Raven, Peter H., Ray Franklin Evert, and Helena Curtis. 1981. ''Biology of plants''. New York: Worth Publishers. p. 410.</ref>

=== Embryo ===
[[File:Ginkgo embryo and gametophyte.jpg|thumb|The inside of a ''[[Ginkgo]]'' seed, showing a well-developed embryo, nutritive tissue ([[megagametophyte]]), and a bit of the surrounding seed coat]]

The main components of the embryo are:
* The '''[[cotyledon]]s''', the seed leaves, attached to the embryonic axis. There may be one ([[Monocotyledons]]), or two ([[Dicotyledons]]). The cotyledons are also the source of nutrients in the non-endospermic dicotyledons, in which case they replace the endosperm, and are thick and leathery. In endospermic seeds, the cotyledons are thin and papery. Dicotyledons have the point of attachment opposite one another on the axis.
* The '''[[epicotyl]]''', the embryonic axis above the point of attachment of the cotyledon(s).
* The '''plumule''', the tip of the epicotyl, and has a feathery appearance due to the presence of young leaf primordia at the apex, and will become the shoot upon germination.
* The '''[[hypocotyl]]''', the embryonic axis below the point of attachment of the cotyledon(s), connecting the epicotyl and the radicle, being the stem-root transition zone.
* The '''[[radicle]]''', the basal tip of the hypocotyl, grows into the primary root.

Monocotyledonous plants have two additional structures in the form of sheaths. The plumule is covered with a '''[[coleoptile]]''' that forms the first leaf while the radicle is covered with a '''[[coleorhiza]]''' that connects to the primary root and [[adventitious]] roots form the sides. Here the hypocotyl is a rudimentary axis between radicle and plumule. The seeds of corn are constructed with these structures; pericarp, scutellum (single large cotyledon) that absorbs nutrients from the endosperm, plumule, radicle, coleoptile, and coleorhiza&nbsp;– these last two structures are sheath-like and enclose the plumule and radicle, acting as a protective covering.

=== {{anchor|Seed coat}}Seed coat ===
{{Redirect|Seed coat|artificial coat|Seed coating}}
The maturing ovule undergoes marked changes in the integuments, generally a reduction and disorganization but occasionally a thickening. The seed coat forms from the two integuments or outer layers of cells of the ovule, which derive from tissue from the mother plant, the inner integument forms the '''tegmen''' and the outer forms the '''testa'''. (The seed coats of some monocotyledon plants, such as the grasses, are not distinct structures, but are fused with the fruit wall to form a [[Fruit anatomy#Anatomy of simple fruits|pericarp]].) The testae of both monocots and dicots are often marked with patterns and textured markings, or have wings or tufts of hair. When the seed coat forms from only one layer, it is also called the testa, though not all such testae are [[Homology (biology)|homologous]] from one species to the next. The funiculus abscisses (detaches at fixed point&nbsp;– abscission zone), the scar forming an oval depression, the '''[[Hilum (biology)|hilum]]'''. Anatropous ovules have a portion of the funiculus that is adnate (fused to the seed coat), and which forms a longitudinal ridge, or '''raphe''', just above the hilum. In bitegmic ovules (e.g. ''Gossypium'' described here) both inner and outer integuments contribute to the seed coat formation. With continuing maturation the cells enlarge in the outer integument. While the inner epidermis may remain a single layer, it may also divide to produce two to three layers and accumulates starch, and is referred to as the colourless layer. By contrast, the outer epidermis becomes [[tannin|tanniferous]]. The inner integument may consist of eight to fifteen layers.<ref name="kozlowski">{{cite book|title=Seed Biology Volume III|year=1972|publisher=Elsevier|isbn=978-0-323-15067-5|url=https://books.google.com/books?id=Rl04Uvkwq-sC|editor=T.T. Kozlowski|access-date=17 February 2014}}</ref>

As the cells enlarge, and starch is deposited in the outer layers of the pigmented zone below the outer epidermis, this zone begins to lignify, while the cells of the outer epidermis enlarge radially and their walls thicken, with nucleus and cytoplasm compressed into the outer layer. these cells which are broader on their inner surface are called '''palisade''' cells. In the inner epidermis, the cells also enlarge radially with plate like thickening of the walls. The mature inner integument has a palisade layer, a pigmented zone with 15–20 layers, while the innermost layer is known as the fringe layer.<ref name="kozlowski" />

==== Gymnosperms ====

In gymnosperms, which do not form ovaries, the ovules and hence the seeds are exposed. This is the basis for their nomenclature&nbsp;– naked seeded plants. Two sperm cells transferred from the pollen do not develop the seed by double fertilization, but one sperm nucleus unites with the egg nucleus and the other sperm is not used.<ref>{{cite book | author1=Rost, Thomas L. | author2=Weier, T. Elliot | author3=Weier, Thomas Elliot | title=Botany: a brief introduction to plant biology | year=1979 | publisher=Wiley | location=New York | isbn=978-0-471-02114-8 | pages=[https://archive.org/details/botanybriefintro00rost/page/319 319] | url=https://archive.org/details/botanybriefintro00rost/page/319 }}</ref> Sometimes each sperm fertilizes an egg cell and one zygote is then aborted or absorbed during early development.<ref name=Bot2000>{{cite journal |author1=Filonova LH |author2=Bozhkov PV |author3=von Arnold S |title=Developmental pathway of somatic embryogenesis in Picea abies as revealed by time-lapse tracking |journal=J Exp Bot |volume=51 |issue=343 |pages=249–264 |date=February 2000 |pmid=10938831 |doi=10.1093/jexbot/51.343.249 |doi-access= }}</ref> The seed is composed of the embryo (the result of fertilization) and tissue from the mother plant, which also form a cone around the seed in coniferous plants such as [[pine]] and [[spruce]].