Sexual Reproduction in Flowering Plants

The flower is the reproductive organ of a flowering plant (angiosperm). A typical flower has four whorls: the calyx (sepals), the corolla (petals), the androecium (the male part, made of stamens) and the gynoecium (the female part, the pistil). Sexual reproduction in plants involves the formation of male and female gametes and their fusion.

The stamen is the male reproductive part. Each stamen has a stalk (the filament) and a knob-like anther at the top. The anther usually has four pollen sacs in which the pollen grains develop by meiosis from pollen mother cells — this is microsporogenesis. Each pollen grain is the male gametophyte.

A mature pollen grain has a tough two-layered wall: the hard outer exine (made of a very resistant substance, sporopollenin, which protects it) and the inner intine. Inside, the pollen grain has two cells: a larger vegetative cell and a smaller generative cell. The generative cell later divides to form the two male gametes. Pollen grains are extremely well preserved (because of sporopollenin) and some people are allergic to them (causing pollen allergy).

The pistil (gynoecium) is the female reproductive part of the flower. A pistil has three regions: a sticky top called the stigma (which receives pollen), a stalk called the style, and a swollen base called the ovary. Inside the ovary are one or more ovules, which after fertilisation become seeds.

Each ovule is attached to the ovary wall by a stalk (the funicle) and is covered by protective coats (integuments) that leave a small opening, the micropyle. Inside the ovule is a mass of tissue (the nucellus) containing the embryo sac — the female gametophyte.

The embryo sac develops by meiosis (megasporogenesis) and is usually 7-celled and 8-nucleate. Its important parts are:

• The egg cell (female gamete), together with two helper cells (synergids), at the micropylar end — this group is the egg apparatus.
• Three antipodal cells at the opposite (chalazal) end.
• A large central cell with two polar nuclei in the middle.

So the female gametophyte is small but precisely organised, with the egg ready for fertilisation and the polar nuclei ready to form the food tissue of the seed.

Pollination is the transfer of pollen grains from the anther to the stigma. When pollen lands on the stigma of the same flower (or same plant) it is self-pollination; when it is carried to the stigma of a different plant of the same kind it is cross-pollination. Cross-pollination is carried out by agents such as wind, water, insects and birds; flowers have many adaptations (bright petals, scent, nectar) to attract pollinators.

After pollination, the pollen grain absorbs water and germinates, growing a pollen tube down through the style to the ovule. The pollen tube carries the two male gametes and enters the embryo sac. Then a special event called double fertilisation takes place (unique to flowering plants):

• One male gamete fuses with the egg cell to form the zygote (this is syngamy). The zygote becomes the embryo.
• The other male gamete fuses with the two polar nuclei (the central cell) to form a triploid (3n) cell that becomes the endosperm — the food store for the embryo. This is triple fusion.

After fertilisation, the ovule becomes the seed and the ovary becomes the fruit. The seed contains the embryo (with the future root and shoot) and a food supply, protected by a seed coat. The fruit protects the seeds and often helps in their dispersal. In this way one fertilised flower gives rise to seeds that can grow into new plants.

Many plants actively avoid self-pollination because constant self-fertilisation reduces variation and vigour. To encourage cross-pollination, flowers use special arrangements called outbreeding devices:

• Dichogamy — the anther and the stigma of a flower mature at different times, so self-pollen is not ready when the stigma is receptive.
• Herkogamy — a physical or spatial barrier between anther and stigma (for example, they are placed apart) that prevents self-pollen from reaching the stigma.
• Self-incompatibility — a genetic mechanism that prevents the pollen of a flower from fertilising the egg of the same plant, even if it lands on the stigma.

Some flowers are even unisexual, which makes self-pollination impossible. Cross-pollination is carried out by agents such as wind, water, insects, birds and even bats (large night-flowering blooms are often pollinated by bats).

A few plants reproduce in special ways that do not follow the normal seed-from-fertilisation route:

• Apomixis — the formation of seeds without fertilisation. The seeds are genetically identical to the parent (useful for keeping a hybrid's good qualities).
• Polyembryony — the occurrence of more than one embryo in a single seed (commonly seen in citrus fruits like orange).
• Parthenocarpy — the formation of fruit without fertilisation; such fruits are seedless (for example, banana).