Ovule

This article is about the plant structure. For animal ovules, see ovum.

"Micropyle" and "Ovular" redirect here. For the structure in animals, see Micropyle (zoology). For the shape, see oval.

Location of ovules inside a Helleborus foetidus flower

Ovule means "small egg". 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(s) forming its outer layer(s), the nucellus (or megasporangium), and the megaspore-derived female gametophyte (or megagametophyte) in its center. The megagametophyte (also called embryo sac in flowering plants) produces an egg cell (or several egg cells in some groups) for fertilization. After fertilization, the ovule develops into a seed.

Location within the plant

In flowering plants, the ovule is typically located inside, the organ of the flower called the gynoecium. The ovary of the gynoecium produces the ovules and ultimately becomes the fruit wall. Flowers may have one or multiple ovules per ovary. The ovules are attached to placenta in the ovary through a structure known as the funiculus, the plant equivalent of an umbilical cord. Different patterns of ovule attachment, or placentation, can be found among plants.

In gymnosperms such as conifers and similar plants, the ovules are typically borne on the surface of an ovuliferous (ovule-bearing) scale, usually within an ovulate cone (also called megastrobilus).

In some extinct plants (e.g. Pteridosperms), megasporangia and perhaps ovules were borne on the surface of leaves. In other extinct taxa, a cupule (a modified leaf or part of a leaf) surrounds the ovule (e.g. Caytonia or Glossopteris.

Ovule parts and development

Plant ovules: Gymnosperm ovule on left, angiosperm ovule (inside ovary) on right

The ovule appears to be a modified megasporangium with integuments surrounding it. Ovules are initially composed of diploid 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 the haploid female gametophyte or megagametophyte, which also remains inside the ovule. The remnants of the megasporangium tissue (the nucellus) typically remain inside the ovule. Megagametophytes produce archegonia (lost in some groups such as flowering plants), which produce egg cells. After fertilization, the ovule contains a diploid zygote and then embryo of the next sporophyte 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.

Integuments, chivai and chalaza

An integument is a protective cell layer surrounding the ovule. Gymnosperms typically have one integument (unitegmic) while angiosperms typically have two (bitegmic). The evolutionary origin of the integuments, especially the second integument typical of angiosperms remains obscure. The inner integument (which is integral to the formation of ovules from megasporangia) has been proposed to have formed from sterile branches surrounding a terminal megasporangium.^[1] 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.^[2]

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 Caytoniales or Glossopteridales cupules may have produced the outer integument of angiosperms.^[3]

The integuments develop into the seed coat when the ovule matures after fertilization.

The integuments do not enclose the nucellus completely but leave an opening at its apex referred to as the micropyle. The micropyle opening allows pollen to enter the ovule for fertilization. In gymnosperms (e.g., conifers), the pollen itself is drawn into the ovule and the micropyle opening closes after pollination. In angiosperms, only a pollen tube enters the micropyle. During germination, the seedling's radicle emerges through the micropyle.

Located opposite from the micropyle is the chalaza where the nucellus is joined to the integuments. Nutrients from the plant travel through the phloem of the vascular system to the funiculus and outer integument and from there apoplastically and symplastically 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.

Nucellus, megaspore and perisperm

The nucellus (plural: nucelli) is the central portion of the ovule inside the integuments. It functions as a megasporangium. In immature ovules, the nucellus contains a megasporocyte (megaspore mother cell), which undergoes sporogenesis via meiosis. Three of the four haploid spores produced in meiosis typically degenerate, leaving one surviving megaspore inside the nucellus, but in some cases more survive. After fertilization, the nucellus may develop into the perisperm that feeds the embryo. In some plants, the diploid tissue of the nucellus can give rise to a seed through a mechanism of asexual reproduction called nucellar embryony.

Megagametophyte

Megagametophyte formation of the genera Polygonum and Lilium. Triploid nuclei are shown as ellipses with three white dots. The first three columns show the meiosis of the megaspore, followed by 1-2 mitoses.

Ovule with megagametophyte: egg cell (yellow), synergids (orange), central cell with two polar nuclei (bright green), and antipodals (dark green)

The haploid megaspore inside the nucellus gives rise to the female gametophyte, called the megagametophyte.

In gymnosperms, the megagametophyte consists of around 2000 nuclei and forms archegonia, which produce egg cells for fertilization.

In flowering plants, the megagametophyte (also referred to as the embryo 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 of mitotic divisions. The cell closest to the micropyle opening of the integuments differentiates into the egg cell, with two synergid cells by its side that are involved in the production of signals that guide the pollen tube. Three antipodal cells form on the opposite (chalazal) end of the ovule and later degenerate. The large central cell of the embryo sac contains two polar nuclei.

Zygote, embryo and endosperm

The pollen tube releases two sperm 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 one zygote will develop into a mature embryo as the resources within the seed are limited.

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 as starch, proteins, and oils in the endosperm as a food source for the developing embryo and seedling, serving a similar function to the yolk of animal eggs. The endosperm is also called the albumen of the seed.

References

1. ^ Herr, J.M. Jr., 1995. The orign of the ovule. Am. J. Bot. 82(4):547-64
2. ^ Stewart and Rothwell, 1993. Paleobotany and the evolution of plants. Cambridge University Press.
3. ^ Frohlich and Chase, 2007. After a dozen years of progress the origin of angiosperms is still a great mystery. Nature 450:1184-1189 (20 December 2007) | doi:10.1038/nature06393;

• P.H. Raven, R.F. Evert, S.E. Eichhorn (2005): Biology of Plants, 7th Edition, W.H. Freeman and Company Publishers, New York, ISBN 0-7167-1007-2

See also

• Gynoecium
• Ovum
• Alternation of generations