Inseed plants, theovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: theintegument, forming its outer layer, thenucellus (or remnant of themegasporangium), and the femalegametophyte (formed from a haploidmegaspore) in its center. The female gametophyte — specifically termed amegagametophyte — is also called theembryo sac inangiosperms. The megagametophyte produces anegg cell for the purpose offertilization. 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. On the basis of the relative position of micropyle, body of the ovule, chalaza and funicle, there are six types of ovules.
Inflowering plants, the ovule is located inside the portion of theflower called thegynoecium. Theovary of the gynoecium produces one or more ovules and ultimately becomes thefruit wall. Ovules are attached to the placenta in the ovary through a stalk-like structure known as afuniculus (plural, funiculi). Different patterns of ovule attachment, orplacentation, can be found among plant species, these include:[1]
Ingymnosperms such as conifers, ovules are borne on the surface of an ovuliferous (ovule-bearing) scale, usually within an ovulatecone (also calledmegastrobilus). In the early extinctseed 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 orGlossopteris).
Ovule orientation may beanatropous, such that when inverted the micropyle faces the placenta (this is the most common ovule orientation in flowering plants),amphitropous,campylotropous, ororthotropous (anatropous are common and micropyle is in downward position and chalazal end in on the upper position hence, inamphitropous the anatropous arrangement is tilted 90 degrees and inorthotropous it is completely inverted) .The ovule appears to be a megasporangium with integuments surrounding it. Ovules are initially composed ofdiploid maternal tissue, which includes a megasporocyte (a cell that will undergo meiosis to produce megaspores). Megaspores remain inside the ovule and divide by mitosis to produce thehaploid female gametophyte or megagametophyte, which also remains inside the ovule. The remnants of the megasporangium tissue (the nucellus) surround the megagametophyte. Megagametophytes produce archegonia (lost in some groups such as flowering plants), which produce egg cells. After fertilization, the ovule contains a diploidzygote and then, after cell division begins, anembryo of the nextsporophyte generation. In flowering plants, a second sperm nucleus fuses with other nuclei in the megagametophyte forming a typically polyploid (often triploid)endosperm tissue, which serves as nourishment for the young sporophyte.
Anintegument is a protective layer of cells surrounding the ovule. Gymnosperms typically have one integument (unitegmic) while angiosperms typically have two integuments (bitegmic). The evolutionary origin of the inner integument (which is integral to the formation of ovules from megasporangia) has been proposed to be by enclosure of a megasporangium by sterile branches (telomes).[2]Elkinsia, a preovulate taxon, has a lobed structure fused to the lower third of the megasporangium, with the lobes extending upwards in a ring around the megasporangium. This might, through fusion between lobes and between the structure and the megasporangium, have produced an integument.[3]
The origin of the second or outer integument has been an area of active contention for some time. The cupules of some extinct taxa have been suggested as the origin of the outer integument. A few angiosperms produce vascular tissue in the outer integument, the orientation of which suggests that the outer surface is morphologically abaxial. This suggests that cupules of the kind produced by theCaytoniales orGlossopteridales may have evolved into the outer integument of angiosperms.[4]
The integuments develop into the seed coat when the ovule matures after fertilization.
The integuments do not enclose the nucellus completely but retain an opening at the apex referred to as themicropyle. The micropyle opening allows the pollen (a malegametophyte) to enter the ovule for fertilization. In gymnosperms (e.g., conifers), the pollen is drawn into the ovule on a drop of fluid that exudes out of the micropyle, the so-called pollination drop mechanism.[3] Subsequently, the micropyle closes. In angiosperms, only a pollen tube enters the micropyle. Duringgermination, theseedling'sradicle emerges through the micropyle.
Located opposite from the micropyle is thechalaza where the nucellus is joined to the integuments. Nutrients from the plant travel through thephloem of the vascular system to the funiculus and outer integument and from thereapoplastically andsymplastically 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.
Thenucellus (plural: nucelli) is part of the inner structure of the ovule, forming a layer of diploid (sporophytic) cells immediately inside the integuments. It is structurally and functionally equivalent to themegasporangium. In immature ovules, the nucellus contains a megasporocyte (megaspore mother cell), which undergoessporogenesis viameiosis. In themegasporocyte ofArabidopsis thaliana,meiosis depends on the expression ofgenes that facilitateDNA repair andhomologous recombination.[5]
In gymnosperms, three of the fourhaploid spores produced in meiosis typically degenerate, leaving one surviving megaspore inside the nucellus. Among angiosperms, however, a wide range of variation exists in what happens next. The number (and position) of surviving megaspores, the total number of cell divisions, whether nuclear fusions occur, and the final number, position and ploidy of the cells or nuclei all vary. A common pattern of embryo sac development (thePolygonum type maturation pattern) includes a single functional megaspore followed by three rounds of mitosis. In some cases, however, two megaspores survive (for example, inAllium andEndymion). In some cases all four megaspores survive, for example in theFritillaria type of development (illustrated byLilium in the figure) there is no separation of the megaspores following meiosis, then the nuclei fuse to form a triploid nucleus and a haploid nucleus. The subsequent arrangement of cells is similar to thePolygonum pattern, but the ploidy of the nuclei is different.[6]
After fertilization, the nucellus may develop into theperisperm that feeds the embryo. In some plants, the diploid tissue of the nucellus can give rise to the embryo within the seed through a mechanism ofasexual reproduction callednucellar embryony.
The haploid megaspore inside the nucellus gives rise to the femalegametophyte, called themegagametophyte.
In gymnosperms, the megagametophyte consists of around 2000 nuclei and formsarchegonia, which produce egg cells for fertilization.
In flowering plants, the megagametophyte (also referred to as theembryo sac) is much smaller and typically consists of only seven cells and eight nuclei. This type of megagametophyte develops from the megaspore through three rounds ofmitotic divisions. The cell closest to the micropyle opening of the integuments differentiates into the egg cell, with twosynergid cells by its side that are involved in the production of signals that guide the pollen tube. Threeantipodal cells form on the opposite (chalazal) end of the ovule and later degenerate. The largecentral cell of the embryo sac contains twopolar nuclei.
The pollen tube releases twosperm 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 onezygote will develop into a matureembryo as the resources within the seed are limited.[citation needed]
In flowering plants, one sperm nucleus fuses with the egg cell to produce a zygote, the other fuses with the two polar nuclei of the central cell to give rise to the polyploid (typically triploid)endosperm. This double fertilization is unique to flowering plants, although in some other groups the second sperm cell does fuse with another cell in the megagametophyte to produce a second embryo. The plant stores nutrients such asstarch,proteins, andoils in the endosperm as a food source for the developing embryo and seedling, serving a similar function to theyolk of animal eggs. The endosperm is also called thealbumen of the seed.[citation needed] the zygote then develops into a megasporophyte, which in turn produces one or more megasporangia. The ovule, with the developing megasporophyte, may be described as either tenuinucellate or crassinucellate. The former has either no cells or a single cell layer between the megasporophyte and the epidermal cells, while the latter has multiple cell layers between.[7]
Embryos may be described by a number of terms includingLinear (embryos have axile placentation and are longer than broad), orrudimentary (embryos are basal in which the embryo is tiny in relation to the endosperm).[8]
Megagametophytes of flowering plants may be described according to the number of megaspores developing, as eithermonosporic,bisporic, ortetrasporic.[citation needed](RF)
Media related toOvule diagrams at Wikimedia Commons Media related toOvules at Wikimedia Commons| Subdisciplines | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Plant groups | |||||||||||
| Plant anatomy |
| ||||||||||
| Plant physiology Materials | |||||||||||
| Plant growth and habit | |||||||||||
| Reproduction | |||||||||||
| Plant taxonomy | |||||||||||
| Practice | |||||||||||
| |||||||||||
