Pistil, Ovule and Megasporogenesis

The gynoecium houses the female reproductive tissues

The gynoecium is the collection of carpels forming the female reproductive region of a flower. A flower may possess one carpel or several. When several carpels remain separate, the condition is apocarpous; when they are fused, it is syncarpous. These arrangements influence the organisation of the ovary and its chambers but do not change the underlying role of the ovule. A carpel or pistil typically has three main visible parts: stigma, style and ovary. The stigma provides a receptive surface for pollen grains, the style links stigma to ovary and provides a pathway for compatible pollen tubes, and the ovary encloses the ovules.

Within the ovary is the ovarian cavity or locule, and the ovules are attached to placenta tissue. Different species arrange ovules differently along the placenta, a pattern called placentation. Some fruits develop from ovaries containing only one ovule, such as mango in the typical example, while other ovaries contain many ovules, such as papaya and orchids. The presence of many ovules creates the potential for numerous seeds, although successful seed formation depends on fertilisation and later development of each ovule.

An ovule is a structured megasporangium

The ovule is attached to the placenta by a stalk known as the funicle. The junction between the stalk and the body of the ovule is the hilum. In the common inverted, or anatropous, ovule arrangement the ovule body curves so that the micropyle lies near the funicle. One or two integuments wrap around internal tissues, leaving a small opening at one end, the micropyle. The opposite basal region is the chalaza. The mass of tissue inside is the nucellus, which provides a supportive and nutritive environment for megaspore formation and embryo-sac development.

An embryo sac, the developing female gametophyte, forms within the nucellus, not within the integuments. The integuments later contribute to the seed coat. The micropyle will serve as an entry route for the pollen tube and may remain as a pore in the seed. This illustrates a recurring principle of biological structure: a layer that protects the pre-fertilisation ovule can later become a protective structure around the seed, while internal cells change through development.

The megaspore mother cell and megasporogenesis

Inside the nucellus, usually toward the micropylar region, a large diploid megaspore mother cell (MMC) differentiates. It has dense cytoplasm and a prominent nucleus. The MMC undergoes meiosis, producing four haploid megaspores. This reduction division is megasporogenesis. Its purpose is the same genetic principle encountered in the anther: meiosis reduces 2n to n, so later gamete fusion can restore a diploid zygote. The ovule's maternal somatic tissues, such as nucellus and integuments, remain diploid during this process.

In the common developmental pathway described here, the four megaspores form a tetrad and three subsequently degenerate; one functional megaspore survives and forms the embryo sac through mitotic nuclear divisions. This is called monosporic development because a single megaspore contributes to the gametophyte. There are other types of embryo-sac development among flowering plants, so the common pathway should not be mistaken for the only possible one. The next stage is gametophyte development: meiosis has already happened when the functional megaspore begins its mitoses.

Comparing the two kinds of sporogenesis

Microsporogenesis occurs in the anther's microsporangia and usually generates four functional haploid microspores per mother cell. Megasporogenesis occurs within the ovule's nucellus and also begins with one diploid mother cell producing four haploid products, but typically only one megaspore remains functional in the common monosporic route. Neither process directly equals fertilisation. They are early preparatory events that produce the haploid spores from which male and female gametophytes develop.

Quick Quiz