How Pollination Works

It is usually pollination requirements that cause a flower to be a particular size, shape or colour, or to have the scent it does.

Plants can reproduce either non-sexually or sexually. Non-sexual, or vegetative, reproduction involves only one plant, and the new plant is exactly like its parent. Division, cuttings and layering are all types of vegetative propagation. And with many house plants these methods give quicker, easier and more predictable results than raising plants from seed.

Sexual reproduction

In the wild, most plants reproduce from seed, and this is why they flower. Plant breeders take advantage of this to develop new varieties. Since seedlings differ from their ‘parents’, by careful breeding and selection, plants with extra vigour, larger flowers or new colours are developed.

How pollination works Dust-like pollen is in fact male cells, produced by the male organs, or stamens, of a flower. A single grain of pollen is too small to see but pollen is produced in vast quantity.

In pollination ripe pollen is transferred from the anthers of a flower’s stamens to the female organ, or stigma, of a flower of the same species. This has to take place at exactly the right time, and the stigma usually becomes sticky then, so the pollen adheres.

When the pollen lands on the stigma, it sends a thread down the tube-like style to the ovule at the stigma’s base. This contains the egg, which is then fertilized, and a new seed develops.

Cross pollination

In self-fertile plants, an egg can be fertilized by pollen from its own flower. Victoria plums, tomatoes and strawberries are examples.

Self-sterile plants need pollen from another plant of the same species to fertilize their eggs. This is called cross-pollination, and is why you need to grow two compatible apple trees to get fruit.

Insect pollinatorsHow Pollination Works

  • Most self-sterile plants need insects — often bees — to transfer pollen. As well as fertilizing seed, pollen is a highly nourishing food for insects. (Nectar is an additional, sweeter bribe, for birds and insects.)
  • As insects feed on pollen or nectar, their bodies, legs or wings collect ripe, sticky pollen from the anthers of one flower, and this rubs off onto the stigma of another flower.
  • A fine paintbrush can be used to transfer pollen from the anthers to the stigmas of flowers. This can result in beautiful hybrids such as the miniature orchid ‘Princess Rose’.
  • Other animal pollinators In the Tropics, birds are as important as insects for pollination.
  • Bats are pollinators too, and go for sturdy, wide-mouthed, bell-shaped flowers. Bat-pollinated flowers are usually white or drab, as bats are colour blind. Large flowers, such as Proteas, are pollinated by possums and even rats.

Cheats

Some flowers go to extreme lengths to attract their pollinators. The orchid mimics the appearance of the particular female wasp or bee it needs so the male tries to mate with the flowers, and pollination occurs.

How flowers attract pollinators

  • A flower has to attract the right pollinator at just the right time, and over thousands of years, flowers and their pollinators have evolved together.
  • Dots or stripes on the petals work like runway markings for aeroplanes, guiding particular insects into the centre of suitable flowers.
  • Butterflies prefer flat platforms, and pollinate flowers such as Lantana, on which they can rest while probing for nectar, while bees prefer a landing platform in the form of a wide lower lip.
  • Night-scented flowers, such as Night-Scented Stock and Datura, attract night-flying pollinators, especially moths. (Most night-pollinated flowers are white or pale coloured, to be easily seen.) And Stapelia flowers look and smell like rotting flesh, to attract blowflies.
  • Flowers that close at night or when the sun goes in do so because their pollinators are on the wing only in sunny weather.

Wind and water

Some flowers are pollinated by wind and have no scent, nectar or petals, because they don’t need them. Grasses and sedges, such as the Umbrella Plant (Cyperus altemifollius) and the catkin-bearing plants are good examples. Such plants produce enormous quantities of pollen as so much of it is lost on the wind.

Self-fertile plants can be pollinated by rain or dew, while some water plants, such as Sea Grasses, send clouds of pollen into the water to pollinate nearby plants.

In the home garden only one parent of the seed is known : the pollen that fertilized the seed ovaries may have come from any of the garden plants or from the hedgerow or field beyond. In the seed grounds special precautions are taken to exclude other types of the same flower, or to prevent insects from carrying pollen from one type to another; and so the resulting seedlings are uniform in colour and habit. Plants raised from home-saved seed often vary to a remarkable degree, and seedlings so raised may be nearly worthless.

It has already been suggested that seed raising is easy. This is true, since seeds have a wonderful vitality and Nature is very resourceful. Extremely interesting photographs have been taken which illustrate how obstacles are overcome by seeds that do not happen to fall in with ideal conditions. Some of these show how a seed will turn itself over completely as it grows, if it has started upside down. Some show how a rootlet will gradually draw the seed under the soil as it grows. Some illustrate how a growing seedling will move out of its way quite a large stone or other obstruction.

Seedlings will, in fact, win through and become mature plants, even when badly treated. But with seeds, as with human beings, it does not make for good health to be constantly fighting against adversity, nor does it make for health to be too much coddled. The gardener’s business is to see that the conditions are somewhere between these two extremes.

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