Ovule
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{{ otheruses4|the plant structure|animal ovules|ovum }}
[[Image:Ovules in flower.png|thumb|Location of ovules inside a ''[[Helleborus foetidus]]'' flower]]
'''Ovule''' literally means "small [[ovum|egg]]." In [[seed plant]]s, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The '''integuments''' forming its outer layer, the '''nucellus''' (or [[sporangium|megasporangium]]), and the megaspore-derived female gametophyte (or '''megagametophyte''') in its center. The megagametophyte (also called '''embryo sac''' in flowering plants) produces the [[ovum|egg cell]] for [[fertilization]]. After fertilization, the ovule develops into a [[seed]].
== Location within the plant ==
In [[flowering plants]], the ovule is located within the actual [[flower]], the part of the [[carpel]] known as the [[Ovary (plants)|ovary]], which ultimately becomes the [[fruit]]. Depending on the plant, flowers may have one or multiple ovules per ovary. The ovule is attached to the placental wall of the ovary through a structure known as the [[funiculus]], the plant equivalent of an [[umbilical cord]]. Different patterns of ovule attachment, or [[placentation]], can be found among plants: In parietal placentation, the ovules are attached to the outer ovary wall, whereas in free central placentation, they are attached to a central column within the ovary. In axile placentation, they are attached to radial spokes within the ovary.
In [[gymnosperm]]s such as conifers and similar plants, the ovules are borne unenclosed on the surface of an ovuliferous (ovule-bearing) scale, usually within an ovulate [[conifer cone|cone]] (also called [[strobilus|megastrobilus]]).
== Ovule parts and development ==
[[Image:Ovule-Gymno-Angio-en.svg|thumb|300px|Plant ovules: Gymnosperm ovule on left, angiosperm ovule (inside ovary) on right]]
The ovule is composed of [[diploid]] maternal tissue that gives rise to the [[haploid]] tissue of the female gametophyte. The maternal tissues of the ovule include the integuments and the nucellus. The next "generation" formed within the ovule are the haploid megaspore and megagametophyte, or embryo sac. After fertilization of the egg cell and formation of a [[zygote]], the ovule contains the [[embryo]] of the next [[sporophyte]] generation and, in flowering plants, the triploid [[endosperm]].
=== Integuments, micropyle and chalaza ===
The integuments are the outer cell layers of the ovule enclosing the nucellus. Gymnosperms typically have one integument layer while angiosperms typically have two. The integuments develop into the seed coat when the ovule matures after fertilization.
The integuments do not enclose the nucellus completely but leave an opening at its apex referred to as the '''micropyle'''. The micropyle opening allows the [[pollen tube]] to enter the ovule for fertilization. In gymnosperms (e.g. conifers), the [[pollen]] itself is drawn into the ovule and the micropyle opening closes after pollination. During [[germination]], the [[seedling]]'s [[radicle]] emerges through the micropyle.
Located opposite from the micropyle is the '''[[chalaza]]''' where the nucellus is joined to the integuments. Nutrients from the plant travel through the [[phloem]] of the vascular system to the funiculus and outer integument and from there [[apoplast|apoplastically]] and [[symplast|symplastically]] through the chalaza to the nucellus inside the ovule. In chalazogamous plants, the pollen tubes enter the ovule through the chalaza instead of the micropyle opening.
=== Nucellus, megaspore and perisperm ===
The '''nucellus''' (plural: nucelli) is the central portion of the ovule inside the integuments. It consists of [[diploid]] maternal tissue and has the function of a [[sporangium|megasporangium]]. In immature ovules, it contains a megasporocyte (megaspore mother cell), which undergoes [[sporogenesis]] via [[meiosis]]. Three of the four [[haploid]] cells produced in meiosis degenerate, leaving one surviving megaspore inside the nucellus. After fertilization, the nucellus develops into the '''perisperm''' that feeds the embryo. In some plants, the diploid tissue of the nucellus can give rise to a seed through a mechanism of [[asexual reproduction]] called [[nucellar embryony]].
=== Megagametophyte and embryo sac ===
[[Image:Embryosac-en.svg|300px|thumb|Ovule with embryo sac: egg cell (yellow), synergids (orange), central cell with two polar nuclei (bright green), and antipodals (dark green)]]
The haploid megaspore inside the nucellus gives rise to the female [[gametophyte]] (megagametophyte). In gymnosperms, the female gametophyte consists of around 2000 nuclei and forms [[archegonium|archegonia]] which produce the egg cells for fertilization. In flowering plants, the megagametophyte, also referred to as '''[[embryo sac]]''', is much smaller and typically consists of only seven cells and eight nuclei. The embryo sac develops from the megaspore through three rounds of [[mitosis|mitotic]] divisions. The cell closest to the micropyle opening of the integuments differentiates into the egg cell, with two synergid cells by its side that may be involved in the production of signals that guide the pollen tube. Three antipodal cells form on the opposite (chalazal) end of ovule and later degenerate, serving no obvious function. The large central cell of the embryo sac contains two polar nuclei.
=== Zygote, embryo and endosperm ===
The pollen tube releases two [[sperm]] nuclei into the ovule. In gymnosperms, [[fertilization]] occurs within the archegonia produced by the female gametophyte. While it is possible that several egg cells are present and fertilized, typically only one [[zygote]] will develop into a mature [[embryo]] as the resources within the seed are limited.
In flowering plants, one sperm nucleus fuses with the egg cell into a zygote, the other fuses with the two polar nuclei of the central cell to give rise to the triploid [[endosperm]]. This double fertilization is unique to flowering plants. The plant stores nutrients such as [[starch]], [[protein]]s and [[oil]]s in the endosperm as a food source for the developing embryo and seedling, serving a similar function to the [[yolk]] of animal eggs. The endosperm is also called the [[albumen]] of the seed.
== References ==
* P.H. Raven, R.F. Evert, S.E. Eichhorn (2005): ''Biology of Plants'', 7th Edition, W.H. Freeman and Company Publishers, New York, ISBN 0-7167-1007-2
== See also ==
* [[Carpel]]
* [[Ovum]]
* [[Alternation of generations]]
[[Category:Plant morphology]]
[[Category:Plant anatomy]]
[[Category:Pollination]]
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