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Concept
Ovule
Summary
In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the integument, forming its outer layer, the nucellus (or remnant of the megasporangium), and the female gametophyte (formed from a haploid megaspore) in its center. The female gametophyte — specifically termed a megagametophyte— is also called the embryo sac in angiosperms. The megagametophyte produces an egg cell for the purpose of fertilization. The ovule is a small structure present in the ovary. It is attached to the placenta by a stalk called a funicle. The funicle provides nourishment to the ovule.
In flowering plants, the ovule is located inside the portion of the flower called the gynoecium. The ovary of the gynoecium produces one or more ovules and ultimately becomes the fruit wall. Ovules are attached to the placenta in the ovary through a stalk-like structure known as a funiculus (plural, funiculi). Different patterns of ovule attachment, or placentation, can be found among plant species, these include:
Apical placentation: The placenta is at the apex (top) of the ovary. Simple or compound ovary.
Axile placentation: The ovary is divided into radial segments, with placentas in separate locules. Ventral sutures of carpels meet at the centre of the ovary. Placentae are along fused margins of carpels. Two or more carpels. (e.g. Hibiscus, Citrus, Solanum)
Basal placentation: The placenta is at the base (bottom) of the ovary on a protrusion of the thalamus (receptacle). Simple or compound carpel, unilocular ovary. (e.g. Sonchus, Helianthus, Asteraceae)
Free-central placentation: Derived from axile as partitions are absorbed, leaving ovules at the central axis. Compound unilocular ovary. (e.g. Stellaria, Dianthus)
Marginal placentation: Simplest type. There is only one elongated placenta on one side of the ovary, as ovules are attached at the fusion line of the carpel's margins . This is conspicuous in legumes. Simple carpel, unilocular ovary. (e.g.
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