Everything about Ovule totally explained
Ovule literally 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
integuments forming its outer layer, 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 the
egg cell for
fertilization. After fertilization, the ovule develops into a
seed.
Location within the plant
In
flowering plants, the ovule is located within the actual
flower, the part of the
carpel known as the
ovary, which ultimately becomes the
fruit. Depending on the plant, flowers may have one or multiple ovules per ovary. The ovule is attached to the placental wall of 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 parietal placentation, the ovules are attached to the outer ovary wall, whereas in free central placentation, they're attached to a central column within the ovary. In axile placentation, they're attached to radial spokes within the ovary.
In
gymnosperms such as conifers and similar plants, the ovules are borne unenclosed on the surface of an ovuliferous (ovule-bearing) scale, usually within an ovulate
cone (also called
megastrobilus).
Ovule parts and development
The ovule is composed of
diploid maternal tissue that gives rise to the
haploid tissue of the female gametophyte. The maternal tissues of the ovule include the integuments and the nucellus. The next "generation" formed within the ovule are the haploid megaspore and megagametophyte, or embryo sac. After fertilization of the egg cell and formation of a
zygote, the ovule contains the
embryo of the next
sporophyte generation and, in flowering plants, the triploid
endosperm.
Integuments, micropyle and chalaza
The integuments are the outer cell layers of the ovule enclosing the nucellus. Gymnosperms typically have one integument layer while angiosperms typically have two. The integuments develop into the seed coat when the ovule matures after fertilization.
The integuments don't enclose the nucellus completely but leave an opening at its apex referred to as the
micropyle. The micropyle opening allows the
pollen tube to enter the ovule for fertilization. In gymnosperms (for example conifers), the
pollen itself is drawn into the ovule and the micropyle opening closes after pollination. 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 consists of
diploid maternal tissue and has the function of a
megasporangium. In immature ovules, it contains a megasporocyte (megaspore mother cell), which undergoes
sporogenesis via
meiosis. Three of the four
haploid cells produced in meiosis degenerate, leaving one surviving megaspore inside the nucellus. After fertilization, the nucellus develops 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 and embryo sac
The haploid megaspore inside the nucellus gives rise to the female
gametophyte (megagametophyte). In gymnosperms, the female gametophyte consists of around 2000 nuclei and forms
archegonia which produce the egg cells for fertilization. In flowering plants, the megagametophyte, also referred to as
embryo sac, is much smaller and typically consists of only seven cells and eight nuclei. The embryo sac 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 may be involved in the production of signals that guide the pollen tube. Three antipodal cells form on the opposite (chalazal) end of ovule and later degenerate, serving no obvious function. 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's 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 into a zygote, the other fuses with the two polar nuclei of the central cell to give rise to the triploid
endosperm. This double fertilization is unique to flowering plants. 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.
Further Information
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