The pistil, ovule and embryo sac

The female part of the flower is the pistil. It has three parts: a sticky stigma on top that receives pollen, a long style, and the swollen ovary at the base. A flower may have one pistil or several, free or fused. Inside the ovary, on a cushion called the placenta, are the ovules: these will become seeds.

The parts of an ovule:

• The funicle: the stalk that joins the ovule to the placenta. The point where it joins the body of the ovule is the hilum.
• One or two protective coats, the integuments, which leave a small opening at the tip, the micropyle.
• The chalaza: the base of the ovule, opposite the micropyle.
• The nucellus: the mass of cells inside the integuments, which holds the embryo sac.

Near the micropyle, one cell of the nucellus grows large: the megaspore mother cell (MMC). It is diploid. It divides by meiosis into four megaspores; this is megasporogenesis. In most flowering plants, three of them die and only one stays functional. Because the embryo sac grows from a single megaspore, this is called monosporic development.

The functional megaspore becomes the female gametophyte, the embryo sac. Its nucleus divides three times by mitosis, without making cell walls (free-nuclear divisions), giving 8 nuclei: four at each end. Then walls form:

• At the micropylar end, three cells form the egg apparatus: one egg cell and two synergids.
• At the chalazal end, three cells form the antipodals.
• One nucleus from each end moves to the middle. These two polar nuclei share the large central cell.

So the embryo sac has 8 nuclei but only 7 cells.

The synergids have special finger-like thickenings at their tip, the filiform apparatus, which guides the pollen tube into the synergid.