Pistil, ovule and megasporogenesis

The gynoecium may be a single pistil (monocarpellary) or several (multicarpellary). Several pistils may be fused (syncarpous) or free (apocarpous).

Each pistil has a stigma, the landing platform for pollen; a style, the long slender part below it; and an ovary, the swollen base. The ovary encloses a cavity, the locule, in which the placenta bears the ovules (megasporangia).

An ovary may hold one ovule (wheat, paddy, mango) or many (papaya, watermelon, orchids).

The ovule hangs from the placenta by a stalk, the funicle. The point where the body of the ovule joins the funicle is the hilum.

One or two integuments wrap the ovule except at the tip, where a small opening, the micropyle, is left. The chalaza lies at the opposite, basal end.

Inside the integuments is the nucellus, a mass of cells with plenty of stored food. The embryo sac (female gametophyte) sits in the nucellus; an ovule usually has one embryo sac, formed from a megaspore.

Megasporogenesis is the formation of megaspores from the megaspore mother cell (MMC). Usually one MMC differentiates in the micropylar region of the nucellus; it is a large cell with dense cytoplasm and a prominent nucleus.

The MMC divides by meiosis and gives four haploid megaspores.