3: funiculus 4: raphe Ovule orientation may be
anatropous, such that when inverted the micropyle faces the placenta (this is the most common ovule orientation in flowering plants),
amphitropous,
campylotropous, or
orthotropous (
anatropous are common and micropyle is in downward position and chalazal end in on the upper position hence, in
amphitropous the anatropous arrangement is tilted 90 degrees and in
orthotropous it is completely inverted) . The ovule appears to be a 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) surround the megagametophyte. 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, after cell division begins, an
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, micropyle, chalaza and hilum An
integument is a protective layer of cells surrounding the ovule. Gymnosperms typically have one integument (unitegmic) while angiosperms typically have two integuments (bitegmic). The evolutionary origin of the inner integument (which is integral to the formation of ovules from megasporangia) has been proposed to be by enclosure of a megasporangium by sterile branches (telomes).
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. 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 cupules of the kind produced by the
Caytoniales or
Glossopteridales may have evolved into the outer integument of angiosperms. The integuments develop into the seed coat when the ovule matures after fertilization. The integuments do not enclose the nucellus completely but retain an opening at the apex referred to as the
micropyle. The micropyle opening allows the pollen (a male
gametophyte) to enter the ovule for fertilization. In gymnosperms (e.g., conifers), the pollen is drawn into the ovule on a drop of fluid that exudes out of the micropyle, the so-called pollination drop mechanism. In gymnosperms, three of the four
haploid spores produced in meiosis typically degenerate, leaving one surviving megaspore inside the nucellus. Among angiosperms, however, a wide range of variation exists in what happens next. The number (and position) of surviving megaspores, the total number of cell divisions, whether nuclear fusions occur, and the final number, position and ploidy of the cells or nuclei all vary. A common pattern of embryo sac development (the
Polygonum type maturation pattern) includes a single functional megaspore followed by three rounds of mitosis. In some cases, however, two megaspores survive (for example, in
Allium and
Endymion). 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 subsequent arrangement of cells is similar to the
Polygonum pattern, but the ploidy of the nuclei is different. 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 the embryo within the seed through a mechanism of
asexual reproduction called
nucellar embryony.
Megagametophyte 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. The zygote then develops into a megasporophyte, which in turn produces one or more megasporangia. The ovule, with the developing megasporophyte, may be described as either tenuinucellate or crassinucellate. 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. 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).
Types of gametophytes Megagametophytes of flowering plants may be described according to the number of megaspores developing, as either
monosporic,
bisporic, or
tetrasporic. (RF) == See also ==