Reproduction in plants: flower structure and pollination

Learning objectives

By the end of this lesson, you should be able to:

• distinguish sexual from asexual reproduction
• identify the main parts of a typical flowering plant
• describe the functions of the stamen and carpel
• explain pollination and distinguish its two main types
• relate flower structure to the method of pollination

Reproduction in flowering plants

Reproduction is the process by which organisms produce new individuals. In flowering plants, sexual reproduction involves the fusion of male and female gametes. It produces seeds and usually involves flowers. Asexual reproduction involves one parent and does not involve fusion of gametes; it can produce new plants from vegetative parts such as stems, roots or leaves.

Sexual reproduction creates genetic variation among offspring because genetic material from two gametes combines. Asexual reproduction generally produces offspring genetically very similar to the parent, except when mutations occur.

Structure of a typical flower

A flower is a reproductive shoot. A typical complete flower has four main whorls:

• Sepals: usually green structures that protect the flower while it is in the bud.
• Petals: often coloured or scented and may attract animal pollinators.
• Stamens: the male reproductive organs. Each stamen usually consists of a filament supporting an anther.
• Carpel or pistil: the female reproductive organ. It includes the stigma, style and ovary.

The anther produces pollen grains. Pollen carries the male gametophyte and ultimately the male gametes. The stigma receives pollen. The style connects the stigma to the ovary and provides a route for pollen tube growth. The ovary contains one or more ovules. Each ovule contains the female gametophyte, including the egg cell.

A flower may be bisexual, with both stamens and carpels, or unisexual, with only one of these reproductive structures. A plant species may have male and female flowers on the same plant or on separate plants.

Pollination

Pollination is the transfer of pollen grains from an anther to a stigma of a flower of the same species. Pollination is not fertilisation. Pollination places pollen on the stigma; fertilisation occurs later when male and female gamete nuclei fuse.

There are two main types:

Self-pollination occurs when pollen reaches the stigma of the same flower or another flower on the same plant. It can be reliable when pollinators are scarce and helps preserve successful parental characteristics. However, it usually produces less genetic variation than cross-pollination.

Cross-pollination occurs when pollen is transferred from a flower on one plant to a flower on a different plant of the same species. It often increases genetic variation, which may help a population respond to environmental change. It depends on pollen being carried between plants by an agent.

Agents and adaptations for pollination

Common pollinating agents include insects, birds, bats, wind and, in some species, water. Flowers have features that increase the chance that pollen reaches a suitable stigma.

Insect-pollinated flowers are often conspicuous, scented and supplied with nectar. Their pollen grains may be relatively large, sticky or spiny so that they attach to visiting insects. Their anthers and stigmas are often positioned so that a visitor brushes against them.

Wind-pollinated flowers usually do not need showy petals or nectar. Their anthers may hang outside the flower so that pollen is released into moving air. Their stigmas are often large, feathery and exposed, increasing the chance of trapping airborne pollen. Wind-pollinated plants commonly produce large quantities of small, light pollen grains.

These are general adaptations, not rules that apply to every species. A flower's structure should be interpreted in relation to its pollination method.

Comparing insect and wind pollination

Why pollination matters

Pollination makes fertilisation possible in flowering plants. Without suitable pollen reaching a stigma, the ovules may not be fertilised and seeds will not develop normally. In crop production, successful pollination can therefore influence the formation of fruits and seeds.

Farmers and gardeners may encourage pollinators by maintaining flowering plants and avoiding unnecessary destruction of pollinator habitats. Some crops can also be pollinated by hand when natural pollination is inadequate.

WASSCE examination focus

Define pollination as the transfer of pollen from anther to stigma. Do not define it as the fusion of gametes; that is fertilisation. In comparison questions, give paired differences between insect- and wind-pollinated flowers and link each feature to its function.

For a labelled flower diagram, be able to identify anther, filament, stigma, style, ovary, ovule, petal and sepal. Draw label lines clearly and avoid crossing them.

Quick check

1. Name the male and female reproductive organs of a typical flower.
2. Define pollination.
3. State one advantage of cross-pollination.
4. Give two adaptations of wind-pollinated flowers.

Answers: 1. Stamen and carpel. 2. Transfer of pollen from anther to stigma of a flower of the same species. 3. It usually increases genetic variation. 4. Exposed dangling anthers and large feathery stigmas; small, light pollen is another adaptation.