Sexual Reproduction in Flowering Plants

The flower is the reproductive structure of angiosperms; its male part is the androecium, made of stamens. Each stamen has a stalk, the filament, and a terminal anther. A typical anther is dithecous (has two lobes) and tetrasporangiate (contains four microsporangia, two per lobe). The microsporangia develop into the pollen sacs that are packed with pollen grains.

In cross-section a young microsporangium has four wall layers — the epidermis, the endothecium, the middle layers and the innermost tapetum. The outer three protect and help the anther dehisce, while the tapetum nourishes the developing pollen grains and supplies materials (including sporopollenin) for the pollen wall — a heavily asked function. The centre of each microsporangium holds the sporogenous tissue.

The formation of pollen is microsporogenesis: the cells of the sporogenous tissue act as microspore mother cells (pollen mother cells) and undergo meiosis, each producing a tetrad of four haploid microspores. These separate and mature into pollen grains, the male gametophytes.

A mature pollen grain has a two-layered wall. The hard outer exine is made of sporopollenin, one of the most resistant organic materials known (it withstands high temperature, acids and enzymes, which is why pollen fossilises well); the exine has thin, pore-like areas called germ pores. The inner wall, the intine, is a thin layer of cellulose and pectin. When shed, most pollen grains are at the 2-celled stage — a large vegetative cell (with abundant food and a large nucleus) and a small generative cell (which later divides to form the two male gametes). For NEET, fix the anther structure (dithecous, tetrasporangiate), the tapetum's nourishing role, microsporogenesis by meiosis, and the pollen wall (exine = sporopollenin with germ pores; intine = cellulose) with the 2-celled condition.

The female part of the flower is the gynoecium, made of one or more pistils (carpels). Each pistil has three parts: the stigma (the landing platform for pollen), the style (the slender stalk) and the swollen basal ovary that contains the ovules. The number and arrangement of ovules vary, but each ovule is the structure that becomes a seed.

An ovule (megasporangium) is attached to the ovary by a stalk, the funicle; the point of attachment is the hilum. The body of the ovule is the nucellus, covered by one or two protective integuments that leave a small opening, the micropyle. The basal region is the chalaza. Inside the nucellus develops the embryo sac (female gametophyte).

The formation of the embryo sac begins with megasporogenesis: a cell of the nucellus becomes the megaspore mother cell and undergoes meiosis to give four haploid megaspores. Usually only one megaspore remains functional (the other three degenerate). The functional megaspore then enlarges and its nucleus divides mitotically three times to give eight nuclei, which organise into the mature embryo sac.

The mature embryo sac is famously 7-celled and 8-nucleate — a guaranteed NEET fact. At the micropylar end lies the egg apparatus: one egg cell and two synergids (the synergids bear a filiform apparatus that guides the pollen tube). At the opposite chalazal end are three antipodal cells. In the centre is a large central cell containing two polar nuclei. So there are eight nuclei but only seven cells, because the two polar nuclei share the single central cell. For NEET, fix the pistil parts, the ovule structure (funicle, integuments, micropyle, nucellus), megasporogenesis (meiosis → 4 megaspores, one functional), and the 7-celled/8-nucleate organisation (3 + 2 + 3 nuclei: egg apparatus, central cell, antipodals).

Pollination is the transfer of pollen grains from the anther to the stigma. When the pollen lands on the stigma of the same flower it is autogamy (self-pollination); transfer to another flower of the same plant is geitonogamy (genetically still self-pollination); and transfer to a flower of a different plant of the same species is xenogamy (cross-pollination), the only type that brings genetically different pollen.

Plants rely on external agents for pollination. Abiotic agents are wind (anemophily) — producing large amounts of light, dry pollen (e.g. grasses) — and water (hydrophily), which is rare (e.g. Vallisneria, Zostera). The most common biotic agent is insects (entomophily); insect-pollinated flowers are usually large, coloured, scented and offer nectar as a reward. Many plants have outbreeding devices (such as separate maturation times of anther and stigma, or self-incompatibility) to prevent self-pollination and promote cross-pollination.

After a compatible pollen grain lands, the pollen–pistil interaction allows it to germinate: the pollen puts out a pollen tube that grows through the style toward the ovule, carrying the two male gametes. The tube enters the ovule (usually through the micropyle), then enters the embryo sac through a synergid, guided by the filiform apparatus, and releases the two male gametes.

Then comes the event unique to flowering plants: double fertilisation. One male gamete fuses with the egg cell — this is syngamy — forming the diploid (2n) zygote. The other male gamete fuses with the two polar nuclei of the central cell — called triple fusion — forming the triploid (3n) primary endosperm nucleus (PEN), which develops into the endosperm. Because two fusion events (syngamy + triple fusion) occur in the same embryo sac, the process is called double fertilisation. For NEET, fix the pollination types/agents, the pollen-tube path, and the two fusions with their products (zygote 2n; PEN 3n).

After double fertilisation, a sequence of post-fertilisation events turns the fertilised ovule into a seed and the ovary into a fruit. The first to develop is usually the endosperm, formed from the primary endosperm nucleus (3n). The endosperm's role is to nourish the developing embryo; it develops before the embryo so that food is ready. In some seeds the endosperm is fully used up during development (non-endospermic / exalbuminous seeds, e.g. pea, bean), while in others it persists in the mature seed (endospermic / albuminous seeds, e.g. castor, maize).

The zygote develops into the embryo, usually after some endosperm has formed. A dicot embryo has an embryonal axis and two cotyledons; it passes through proembryo, globular and heart-shaped stages. A monocot embryo has a single cotyledon (the scutellum), with the plumule covered by a coleoptile and the radicle by a coleorhiza.

As the embryo matures, the ovule develops into the seed: the integuments harden into the seed coat (testa and tegmen), and the micropyle may persist. The ovary develops into the fruit, its wall becoming the pericarp. A fruit formed only from the ovary is a true fruit; when other floral parts (especially the thalamus) also form part of the fruit it is a false fruit — the classic example is the apple. In some plants fruits develop without fertilisation, giving seedless fruits — this is parthenocarpy (e.g. banana).

Two related phenomena are often tested. Apomixis is the formation of seeds without fertilisation — an asexual method that mimics sexual reproduction (seen in some grasses and Citrus). Polyembryony is the occurrence of more than one embryo in a single seed (e.g. Citrus, mango). For NEET, fix the order (endosperm before embryo), the dicot vs monocot embryo, ovule → seed and ovary → fruit, the true-vs-false fruit distinction (apple = false), and the definitions of parthenocarpy, apomixis and polyembryony.