• 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.

  • [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.

  • 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.

  • 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.

  • Different patterns of ovule attachment, or placentation, can be found among plant species, these include:[1] • Apical placentation: The placenta is at the apex (top) of the

  • In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells.

  • Nucellus, megaspore and perisperm[edit] The nucellus (plural: nucelli) is part of the inner structure of the ovule, forming a layer of diploid (sporophytic) cells immediately
    inside the integuments.

  • In some cases all four megaspores survive, for example in the Fritillaria type of development (illustrated by Lilium 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 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.

  • In some plants, the diploid tissue of the nucellus can give rise to the embryo within the seed through a mechanism of asexual reproduction called nucellar embryony.

  • A common pattern of embryo sac development (the Polygonum type maturation pattern) includes a single functional megaspore followed by three rounds of mitosis.

  • 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.

  • 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.

  • The large central cell of the embryo sac contains two polar nuclei.

  • The ovule is a small structure present in the ovary.

  • Three antipodal cells form on the opposite (chalazal) end of the ovule and later degenerate.

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

  • There is only one elongated placenta on one side of the ovary, as ovules are attached at the fusion line of the carpel’s margins .

  • 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.

  • [7] Embryos may be described by a number of terms including Linear (embryos have axile placentation and are longer than broad), or rudimentary (embryos are basal in which
    the embryo is tiny in relation to the endosperm).

  • It consists of three parts: the integument, forming its outer layer, the nucellus (or remnant of the megasporangium), and the female gametophyte (formed from a haploid megaspore)
    in its center.

  • Megaspores remain inside the ovule and divide by mitosis to produce the haploid female gametophyte or megagametophyte, which also remains inside the ovule.

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

  • Ovules are initially composed of diploid maternal tissue, which includes a megasporocyte (a cell that will undergo meiosis to produce megaspores).


Works Cited

[‘1. Kotpal, Tyagi, Bendre, & Pande. Concepts of Biology XI. Rastogi Publications, 2nd ed. New Delhi 2007. ISBN 8171338968. Fig. 38 Types of placentation, page 2-127
2. ^ Herr, J.M. Jr., 1995. The origin of the ovule. Am. J. Bot. 82(4):547-64
3. ^
Jump up to:a b Stewart, W.N.; Rothwell, G.W. (1993). Paleobotany and the evolution of plants. Cambridge University Press. ISBN 0521382947.
4. ^ 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;
5. ^ Seeliger K, Dukowic-Schulze S, Wurz-Wildersinn R, Pacher M, Puchta H (2012). “BRCA2 is a mediator of RAD51- and DMC1-facilitated homologous recombination in Arabidopsis
thaliana”. New Phytol. 193 (2): 364–75. doi:10.1111/j.1469-8137.2011.03947.x. PMID 22077663.
6. ^ Gifford, E.M.; Foster, A.S. (1989), Morphology and evolution of vascular plants, New York: W. H. Freeman and Company
7. ^ Tsou 1994.
8. ^ The Seed
Biology Place:Structural seed types based on comparative internal morphology
Photo credit: https://www.flickr.com/photos/ferrucciozanone/8394286483/’]