Ovule
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An ovule is a structure found in seed plants that develops into a seed after fertilization. In Angiosperms, (the flowering plants, meaning enclosed seed), the ovule is found within part of the carpel known as the ovary, which ultimately becomes the fruit. In conifers and similar plants known as Gymnosperms (meaning naked seed), the ovules are borne unenclosed on the surface of an ovuliferous (ovule-bearing) scale, usually within a cone. 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. Gymnosperms typically have one integument layer while angiosperms typically have two. The following description applies to the typical Angiosperm ovule— Gymnosperm ovule structures are not all 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 organ 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 the integument, which will become the seed coat. Both megasporangium and integument are derived from sporophytic (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, each undergoing a separate fertilization event. The first sperm nucleus fuses with the egg cell nucleus to form the zygote, while the second sperm nucleus fuses with the two polar nuclei within the central cell resulting in a triploid cell that proliferates through mitosis, giving rise to the endosperm. The endosperm is a source of nutrition for the developing embryo after fertilization, and is often a starchy tissue.
Plants are described as having alternation of generations whereby both the sporophytic (diploid) and gametophytic (haploid) phases of the life cycle are multicellular in nature. In land plant evolution, there has been an increased reduction in the female gametophyte such that it is entirely dependent upon, and enclosed within, the parent sporophyte in angiosperms. In vascular plants such as ferns, gymnossperms, and angiosperms the sporophyte is the dominant phase in the life cycle of the plant. This is in contrast with non-vascular plants such as mosses where the gametophyte is the dominant phase in the life cycle.
