Sexual reproduction in flowering plants

A plant's leaves make food, its roots take in water and minerals, and its stem holds it up. Flowers have a different job: in angiosperms, the flowering plants, the flower is the reproductive organ. Plants without flowers, such as pines, also reproduce sexually; you will meet them in the next chapter.

Start with an unopened bud. Its outside is a wrapping of thin, flat, green parts, the sepals. They form the outermost ring, or whorl, and shield the bud. Open the bud and the coloured petals unfold inside. Activity 11.5 has you examine the rings one at a time, working inwards. Try it here: peel the flower one whorl at a time, tap any part, and cut the ovary open.

Count the rings and you get four kinds of part. Find all four in one flower and botanists call it a complete flower. In some flowers the sepals, or the petals, grow joined together (fused). The bright petals do the advertising, drawing in the animals that carry pollen.

Male part, the stamen: a stalk called the filament carrying an anther at its tip. Anthers make pollen grains, and inside each grain are male gametes. Female part, the pistil, in three sections from the top: the stigma, often flat or sticky, which receives pollen; the style, a slender tube; and the swollen ovary at the base. Tucked inside the ovary are the ovules; each one contains a single egg cell, the female gamete (Fig. 11.11).

Not every flower has every part. Fill your own Table 11.1 for the flowers below, then try the real ones around you.

Is moving pollen from stamen to stigma really needed for a fruit? Activity 11.6 tests it on a pea (matar) plant. On one plant, choose two young buds and three newly opened flowers. Take the stamens out of one bud and one open flower. Cover four of them loosely with muslin cloth bags and leave one open flower uncovered.

Muslin lets air through but stops insects and stray pollen. Predict what each flower will give, then unwrap the bags.

Only one treatment fails: the bud whose stamens were removed and which was then bagged. It had no pollen of its own and the bag kept outside pollen away. Every other flower made a pod.

What the bags show: when pollen is kept off the stigma, the pod never forms. The step the bag blocked has a name: pollination, pollen arriving on the stigma from an anther.

Pollination comes in two kinds, decided by where the pollen started out. Switch between them below.

Self-pollination keeps the pollen within one plant: it lands on the stigma of the flower that made it, or of another flower on that same plant. Cross-pollination moves it between two plants of the same kind: from a flower on one plant to a stigma on the other.

Both kinds end the same way, with pollen on a stigma, and both can lead to fruits and seeds. Without pollination, a flower gives neither, which is why plants have so many ways of making sure it happens.

A plant cannot walk its pollen across to another flower. It relies on outside carriers, the pollinators: wind, water, insects such as bees and butterflies, or birds. A flower's shape, colour and pollen reveal which carrier it is built for. Pick a pollinator to see it at work.

Pollination only delivers pollen to the doorstep. The real event happens deep inside the pistil, once a pollen grain lands on a compatible stigma. Step through it.

The joining of male gamete and egg cell is fertilisation, the point where a new life begins. What forms is one cell, the zygote; dividing again and again, it builds an embryo. While that happens, the ovary swells into the fruit and each fertilised ovule inside it turns into a seed.

Wind pollination and insect pollination are both common strategies, but how efficient is each? Activity 11.7 gives approximate figures (Table 11.3): a wind-pollinated grass such as maize or wheat releases about 5,00,000 to 10,00,000 pollen grains per flower and forms about 50 to 200 seeds. An insect-pollinated plant such as sunflower releases about 20,000 to 40,000 and forms about 800 to 1,000 seeds. Move the sliders to work out the pollen to seed ratio.

Wind is wasteful: almost every grain falls somewhere useless, so it can take thousands of grains for one seed. Insects deliver pollen almost to the door, so a few dozen grains go a long way. Even so, flooding the air with pollen works for grasses: the wind costs the plant nothing, it spends nothing on colour, scent or nectar, and with so many grains in the air a few are very likely to land on a stigma.