Write the Characteristics of Ovule.
Answers
The ovule is part of the makeup of the female reproductive organ in seed plants. It’s the place where female reproductive cells are made and contained, and it is what eventually develops into a seed after fertilization, only for the seed to then ripen and produce a complete adult plant. Ovules are contained in ovaries at the bottom of a vase-like structure, the carpel, which has a neck called a style and an opening at the top, called a stigma.
After fertilization the ovule starts to swell and its wall starts to toughen up in preparation to become a seed, while the ovary starts to grow around it and becomes the fruit. Keep in mind that some plants, like the avocado, have a single ovule in their ovary, while others, like the kiwifruit, have many, which develop into many seeds in the fruit. Another way that plants differ with regards to their ovules is the place where the ovules are found. Specifically, in gymnosperms, such as conifers, the ovules are found on the scales of female cones, while in angiosperms, which are flowering plants, the ovules are found inside of the ovary within the carpel.
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.
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, Compositae)
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. Pisum)
Parietal placentation: Placentae on inner ovary wall within a non-sectioned ovary, corresponding to fused carpel margins. Two or more carpels, unilocular ovary. (e.g. Brassica)
In gymnosperms such as conifers, ovules are borne on the surface of an ovuliferous (ovule-bearing) scale, usually within an ovulate cone (also called megastrobilus). In the early extinct seed ferns, ovules were borne on the surface of leaves. In the most recent of these taxa, a cupule (a modified branch or group of branches) surrounded the ovule (e.g. Caytonia or Glossopteris).