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An ovule is a structure found in seed plants that develops into a seed after fertilization. In Angiosperms, or flowering plants, the ovule is found within an ovary which becomes fruit. In conifers and other similar plants known as Gymnosperms, the ovules are borne on the surface of an ovuliferous (ovule-bearing) scale, usually within a cone, and are not enclosed. Prior to fertilization, the ovule consists of the female gametophyte, the nucellus, and integuments, which in Angiosperms are attached to the placental wall of the fruit through a structure known as the funiculus. The following description applies to the typical Angiosperm ovule— Gymnosperm ovule structure is not identical. It is also important to note that there are many variations and modifications of these structures within the Angiosperms. The mature female gametophyte (or megagametophyte) is a seven-celled, eight nucleate organism in the haploid gametophyte phase of the plant life cycle that develops within the ovule. The ovule consists of megasporangium (or nucellus) tissue encased within two separate tissue layers known as integument which will become the seed coat. Both megasporangium and integument are derived from sporophytic (or parental) tissue. Within the megasporangium of the ovule, a special cell type called the megasporocyte (or megaspore mother cell) undergoes meiosis to produce 4 haploid cells called megaspores. One of the 4 megaspores survives to undergo 3 rounds of mitosis and produce an 8-celled, immature female gametophyte. Only one of these 8 cells will differentiate into the female gamete, or egg cell. In addition to the egg cell, the mature female gametophyte consists of two synergid cells, three antipodal cells and a central cell containing two polar nuclei (a result of two polar cells fusing). The role of the synergid cells appears to be involved in the production of signals that help guide the pollen tube, which contains two sperm nuclei prior to fertilization, to the micropyle. The role of the antipodals has yet to be determined. The role of the central cell with 2 polar nuclei is discussed below (see double fertilization). Unique to Angiosperms, flowering plants exhibit double fertilization whereby two sperm nuclei are deposited in the female gametophyte, with eac
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