- "Micropyle" redirects here. For the structure in animals, see Micropyle (zoology).
Ovule literally means "small
eggAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
." 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
megasporangiumA sporangium is a plant, fungal, or algal structure producing and containing spores. Sporangia occur in angiosperms, gymnosperms, ferns, fern allies, bryophytes, algae, and fungi...
), and the megaspore-derived female gametophyte (or
megagametophyte) in its center. The megagametophyte (also called
embryo sac in flowering plants) produces the
egg cellAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
for fertilization. After fertilization, the ovule develops into a
seedA seed , referred to as a kernel in some plants, is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
.
Location within the plant
In flowering plants, the ovule is located within the actual
flowerA flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to mediate the union of male sperm with female ovum in order to produce seeds...
, the part of the carpel known as the
ovaryIn the flowering plants, an ovary is a part of the female reproductive organ of the flower or gynoecium. Specifically, it is the part of the carpel which holds the ovule and is located above or below or at the point of connection with the base of the petals and sepals...
, which ultimately becomes the
fruitThe term fruit has different meanings dependent on context, and the term is not synonymous in food preparation and biology. Fruits are the means by which flowering plants disseminate seeds, and the presence of seeds indicates that a structure is most likely a fruit, though not all seeds come from...
. Depending on the plant, flowers may have one or multiple ovules per ovary. The ovules are attached to the placental wall of the ovary through a structure known as the
funiculusThe cerebrospinal nerves consist of numerous nerve fibers collected together and enclosed in membranous sheaths. A small bundle of fibers, enclosed in a tubular sheath, is called a funiculus; if the nerve is of small size, it may consist only of a single funiculus; but if large, the funiculi are...
, the plant equivalent of an
umbilical cordIn placental mammals, the umbilical cord is the connecting cord from the developing embryo or fetus to the placenta...
. Different patterns of ovule attachment, or
placentationIn biology, placentation refers to the formation, type and structure, or arrangement of placentas. The function of placentation is to transfers nutrients from maternal tissue to a growing embryo...
, can be found among plants: In parietal placentation, the ovules are attached to the outer ovary wall, whereas in free central placentation, they are attached to a central column within the ovary. In axile placentation, they are attached to radial spokes within the ovary.
In
gymnospermGymnosperm is a group of spermatophyte seed-bearing plants with ovules on scales, which are usually arranged in cone-like structures....
s such as conifers and similar plants, the ovules are borne unenclosed on the surface of an ovuliferous (ovule-bearing) scale, usually within an ovulate
coneA cone is an organ on plants in the division Pinophyta that contains the reproductive structures. The familiar woody cone is the female cone, which produces seeds. The male cones, which produce pollen, are usually herbaceous and much less conspicuous even at full maturity...
(also called
megastrobilusA strobilus is an organ of many plants that contains the reproductive structures. Strobili are ordinarily called cones in many of these groups...
).
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
zygoteA zygote , or zygocyte, is the initial cell formed when a new organism is produced by means of sexual reproduction. A zygote is synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development...
, the ovule contains the
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
of the next
sporophyteAll land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or alga that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...
generation and, in flowering plants, the triploid
endospermEndosperm is the tissue produced in the seeds of most flowering plants around the time of fertilization. It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. This makes endosperm an important source of nutrition in human diet...
.
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 do not enclose the nucellus completely but leave an opening at its apex referred to as the
micropyle. The micropyle opening allows the
pollen tubeThe pollen tube of most seed plants acts as a conduit to transport sperm cells from the pollen grain, either from the stigma to the ovules at the base of the pistil, or directly through ovule tissue in some gymnosperms...
to enter the ovule for fertilization. In gymnosperms (e.g. conifers), the
pollenPollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement between the stamens to the pistil of flowering plants or from the male cone to the...
itself is drawn into the ovule and the micropyle opening closes after pollination. During
germinationGermination is the process in which a seed or spore emerges from a period of dormancy. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However, the growth of a sporeling from a spore, for example the growth of hyphae from fungal...
, the
seedlingthumb|Monocot and dicot seedlingsA seedling is a young plant sporophyte developing out of a plant embryo from a seed. Seedling development starts with germination of the seed. A typical young seedling consists of three main parts: the radicle , the hypocotyl , and the cotyledons...
's
radicleIn botany, the radicle is the first part of a seedling to emerge from the seed during the process of germination. The radicle is the embryonic root of the plant, and grows downward in the soil...
emerges through the micropyle.
Located opposite from the micropyle is the
chalazaThe chalaza is a structure inside animal eggs and plant ovules. It attaches or suspends the yolk or nucellus within the larger structure.-In animals:...
where the nucellus is joined to the integuments. Nutrients from the plant travel through the
phloemIn vascular plants, phloem is the living tissue that carries organic nutrients , particularly sucrose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
of the vascular system to the funiculus and outer integument and from there
apoplasticallyWithin a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the Casparian strip in roots, air spaces between plant cells and the cuticula of the plant....
and
symplasticallyThe symplast of a plant is the inner side of the plasma membrane in which water can freely diffuse.The plasmodesmata allow the direct flow of small molecules such as sugars, amino acids, and ions between cells...
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
megasporangiumA sporangium is a plant, fungal, or algal structure producing and containing spores. Sporangia occur in angiosperms, gymnosperms, ferns, fern allies, bryophytes, algae, and fungi...
. In immature ovules, it contains a megasporocyte (megaspore mother cell), which undergoes
sporogenesisSporogenesis is the production of spores in biology. The term is also used to refer to the process of reproduction via spores. Reproductive spores are formed in many eukaryotic organisms, such as plants, algae and fungi, during their normal reproductive life cycle...
via
meiosisIn biology, meiosis is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original...
. 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 reproductionAsexual reproduction is reproduction which does not involve meiosis, ploidy reduction, or fertilization. Only one parent is involved in asexual reproduction. A more stringent definition is agamogenesis which refers to reproduction without the fusion of gametes...
called
nucellar embryonyNucellar embryony is a form of seed reproduction that occurs in certain plant species, including many citrus varieties. During the germination of seeds from plants that possess this genetic trait, the nucellar tissue which surrounds the embryo sac can produce additional embryos which are...
.
Megagametophyte and embryo sac
The haploid megaspore inside the nucellus gives rise to the female
gametophyteIn plants and algae that undergo alternation of generations, a gametophyte is the multicellular structure, or phase, that is haploid, containing a single set of chromosomes:...
(megagametophyte). In gymnosperms, the female gametophyte consists of around 2000 nuclei and forms
archegoniaAn archegonium , from the ancient Greek ἀρχή and γόνος , is a multicellular structure or organ of the gametophyte phase of certain plants producing and containing the ovum or female gamete. The archegonium has a long neck and a swollen base...
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
mitoticMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing...
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
spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
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
zygoteA zygote , or zygocyte, is the initial cell formed when a new organism is produced by means of sexual reproduction. A zygote is synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development...
will develop into a mature
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
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
endospermEndosperm is the tissue produced in the seeds of most flowering plants around the time of fertilization. It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. This makes endosperm an important source of nutrition in human diet...
. This double fertilization is unique to flowering plants. The plant stores nutrients such as
starchStarch or amylum is a polysaccharide carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds.Starch is produced by all green plants as an energy store and is a major food source for humans....
,
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
s and
oilAn oil is any substance that is liquid at ambient temperatures and is hydrophobic but soluble in organic solvents. Oils have a high carbon and hydrogen content and are nonpolar substances. The general definition above includes compound classes with otherwise unrelated chemical structures,...
s 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.