Ovule
From Free net encyclopedia
An ovule is a structure found in seed plants that develops into a seed after fertilization. In angiosperms, which are the flowering plants the ovule is within an ovary or gynoecium, but in gymnosperms which are the conifers and their allies, the ovules are born 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 within Angiosperms.
The female gametophyte is a 7-celled, 8 nucleate organism in the haploid phase of the plant life cycle. The gametophyte is housed within the nucellus, a tissue that is of sporophytic (parental) origin, which in turn is encased in one or two sporophytic layers known as integuments, structures that become the seed coat. Together, these structures comprise the ovule. The female gametophyte is formed through a meiotic division of a nucellar cell followed by three mitotic divisions of one of these meiotic products. It is through this process that the female gamete or egg cell is formed. In addition to the egg cell, the female gametophyte consists of two synergid cells, three antipodal cells and two polar nuclei within a central cell. The role of the synergid cells appear to be involved in guidance of the pollen tube which contain two sperm nuclei prior to fertilization. The role of the antipodals has yet to be determined. The role of the central cell with polar nuclei is discussed below.
Unique to Angiosperms, flowering plants exhibit double fertilization whereby two sperm nuclei are deposited in the female gametophyte with each undergoing a fertilization event. The first sperm nucleus undergoes fusion with the egg cell nucleus forming the zygote while the second sperm nucleus undergoes fusion with the two polar nuclei of the central cell resulting in a triploid cell that proliferates through mitosis giving rise to the endosperm. The endosperm is often 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 lifecycle are multicellular organisms. In land plant evolution, there has been an increased reduction in the female gametophyte such that it is entirely dependent upon the parent sporophyte in Angiosperms. The sporophyte is thus termed the dominant phase in the lifecycle, in contrast with non-vascular plants such as mosses where the gametophyte is the dominant phase in the life cycle.