The Pistil, Megasporangium (Ovule) and the Female Gametophyte

Just as the androecium houses the entire developmental sequence leading to the male gametes, the

gynoecium's ovules house the parallel sequence on the female side: ovule structure, megasporogenesis,

and the maturation of the female gametophyte, or embryo sac.

An ovule begins as a small outgrowth, integumented, borne on the placenta inside the ovary and

attached to it by a stalk called the funicle. The precise point where the funicle joins the main

body of the ovule is called the hilum. The main body of the ovule -- the nucellus, a mass of

parenchymatous tissue -- is enclosed by one or two protective coverings, the integuments, which

grow up around it but do not close completely at one end, leaving a small opening called the

micropyle. It is through this opening that a pollen tube commonly enters the ovule during

fertilisation, and later, in the mature seed, the micropyle serves as a passage for water and

oxygen uptake during germination. The region at the base of the ovule, where the integuments, the

nucellus and the funicle all merge together, is called the chalaza.

While several ovule shapes occur in flowering plants, by far the commonest is the anatropous type,

in which the whole body of the ovule becomes inverted, or bent through roughly 180 degrees, during

early development, so that the funicle comes to lie fused flat along one side of the ovule's body.

As a result, in the mature anatropous ovule the micropyle ends up positioned close to the funicle

and hilum, with the chalaza at the opposite pole -- an arrangement that, among other things, places

the ovule's most vulnerable opening, the micropyle, in a relatively protected position close to its

point of attachment.

Deep within the nucellus, close to the micropylar end, one particular cell differentiates as the

megaspore mother cell (MMC) -- a conspicuously large cell with dense cytoplasm and a prominent

nucleus. The diploid MMC undergoes meiosis (megasporogenesis), producing four haploid megaspores,

typically arranged in a single linear row. In the great majority of flowering plants, only one of

these four megaspores -- usually the one situated at the chalazal end of the row -- survives as the

functional megaspore, while the other three progressively degenerate; because only a single

surviving product is retained, this pattern is described as monosporic development.

The functional megaspore then embarks on the construction of the mature female gametophyte, the

embryo sac. Its nucleus divides mitotically, without any accompanying wall formation, in three

successive rounds, so that a single cell comes to contain eight free nuclei. These eight nuclei

then organise themselves into their final positions, and cell walls form around most, though not

all, of them, giving the mature embryo sac its distinctive seven-celled, eight-nucleate

organisation -- the pattern first fully described in the genus Polygonum and often called the

Polygonum type, and it is the pattern found in most flowering plants.

Three of the eight nuclei group together, each within its own cell wall, at the micropylar end of

the embryo sac, forming what is called the egg apparatus: a single egg cell flanked on either side

by two synergid cells. Each synergid bears, at its micropylar tip, a specialised wall structure

called the filiform apparatus -- a set of finger-like ingrowths that increase the local surface

area and are believed to help guide the growing pollen tube toward the embryo sac. At the opposite, …