0610 · 16.3Sexual reproduction in plants

Flower structure and reproductive parts

0:00A flower cannot walk towards another flower, yet flowering plants can reproduce sexually. Somehow, the male reproductive material must reach the female reproductive material, even when they belong to different plants. Flowers solve this problem using structures that attract insects or make use of the wind. Today, we will follow that journey, from pollen leaving an anther to fertilisation inside an ovule, and finally to the conditions that allow a seed to germinate.

0:23The starting idea is that sexual reproduction involves the fusion of nuclei from male and female gametes. A gamete is a sex cell, and its nucleus contains genetic information. In flowering plants, pollen grains carry the male reproductive nuclei, while ovules contain the female gametes. Keep those locations in mind, because the flower’s structures help bring these nuclei together without the plant itself needing to move. Let us begin with the flower’s structure, working from the outside towards the centre.

0:50The sepals are the outer structures that protect the flower when it is still a bud. Inside them are the petals, which are often large and brightly coloured in insect-pollinated flowers. Their colour helps attract insects, while scent and nectar also encourage insects to visit the flower. Nectar is a sugary liquid that provides food for visiting insects. It is produced by structures called nectaries, rather than by the pollen-producing anthers.

1:13The important connection is that an insect arrives looking for food, but its visit can also transport pollen. The flower does not need the insect to understand this job. The position of the reproductive parts makes contact happen during feeding. The male parts of the flower are the stamens. Each stamen consists of an anther and a filament, so a stamen is not simply another name for an anther. The anther makes pollen grains, and the filament holds the anther in position.

1:39In an insect-pollinated flower, this places the pollen where it can brush against a visiting insect. The female part is the carpel, which consists of the stigma, style and ovary. The stigma receives pollen, and the style holds up the stigma above the ovary. The ovary contains ovules, and each ovule contains a female gamete. After fertilisation, an ovule can develop into a seed, so an ovule and an ovary are different structures with different roles.

2:05When identifying these parts, use their relationships rather than memorising one particular picture. Find the swollen ovary containing ovules, then follow the style upwards to the stigma. Find an anther, then trace its supporting filament to identify the complete stamen. Flowers vary in shape, and some have several carpels, but these structural relationships help you recognise the parts in unfamiliar diagrams and images. For a drawing, start with a clear outline of the flower in longitudinal section, meaning a cut down its length.

2:33Show the ovules inside the ovary, not floating beside it, and connect the stigma to the ovary through the style. Draw each anther attached to its filament, with petals around the reproductive structures and sepals outside and below them. Use clear single lines and straight label lines that touch the correct structures. Now imagine a bee feeding from a flower. As it moves past an anther, pollen grains stick to its body, and some may later rub onto a stigma.

2:59Pollination is the transfer of pollen grains from an anther to a stigma. Notice that this definition describes a transfer between two flower structures, not the fusion of nuclei and not the formation of a seed. Insect-pollinated flowers usually have large, brightly coloured petals, scent and nectar to attract visitors. Their anthers and stigmas are usually inside the flower, where an insect is likely to touch them. Their pollen grains are relatively large and sticky or spiky, helping them attach to insects.

3:25They generally produce smaller amounts of pollen than wind-pollinated flowers, because insects provide a more directed means of transport. Wind-pollinated flowers solve the same transport problem without attracting an animal. Their anthers hang outside the flower on long filaments, exposing the pollen to moving air. Their stigmas are feathery and project outside the flower, providing a large surface area to catch airborne pollen. Grasses are useful examples because their flowers often look very different from the large, colourful flowers that attract insects.

3:53Wind-pollinated flowers have small, dull petals and do not provide scent or nectar to attract insects. Their pollen grains are small, light and smooth, making them easier to carry through the air. They produce large quantities because much of that pollen will never reach a suitable stigma. The contrast makes sense. Insect-pollinated pollen needs to attach to a carrier, whereas wind-pollinated pollen needs to travel readily in air. Here is a worked example using an unfamiliar flower.

Pollination by insects and wind

4:20Its anthers hang outside on long filaments, its stigma is feathery, and its petals are small and dull. First, ask what each feature allows the flower to do, such as releasing pollen into air, intercepting airborne pollen, and reproducing without attracting insects. Together, these observations identify a wind-pollinated flower, and they let us predict small, light, smooth pollen produced in large quantities. There is another distinction that does not depend on whether pollen travels by insect or wind.

4:48Self-pollination is the transfer of pollen grains from the anther of a flower to the stigma of the same flower or a different flower on the same plant. Cross-pollination is the transfer of pollen grains from the anther of a flower to the stigma of a flower on a different plant of the same species. The key question is whether the pollen moves between different plants. Suppose a bee carries pollen between two flowers growing on the same plant.

5:11Although the flowers are different, this is still self-pollination because they belong to one plant. If the bee then carries pollen to a flower on a second plant of the same species, that is cross-pollination. Counting flowers is therefore not enough. You must identify which plant each flower belongs to. Self-pollination generally produces less genetic variation than cross-pollination, because the reproductive material comes from the same plant. It does not mean that the offspring are necessarily genetically identical, so do not confuse it with asexual reproduction.

5:40Cross-pollination brings together genetic information from different plants of the same species. This generally increases variation among offspring, creating a wider range of inherited characteristics within the population. Why does that variation matter to a population? If conditions change, such as a new disease becoming common, some individuals may have characteristics that help them survive and reproduce. A more varied population has a greater chance of containing such individuals, so cross-pollination can improve its capacity to respond to environmental change.

6:10Less variation following repeated self-pollination can reduce that capacity, although greater variation does not guarantee survival. Self-pollination can also make reproduction less dependent on pollen arriving from another plant. In plants able to transfer their own pollen without an animal, reproduction may continue even when insect pollinators are scarce. Insect-mediated cross-pollination depends more strongly on pollinators moving between plants, while wind-pollinated plants rely on wind instead. Some self-pollination still involves insects, so it is too absolute to say that self-pollination never needs a pollinator.

Fertilisation and germination requirements

6:44Pollination has delivered a pollen grain to the stigma, but the female gamete is still inside an ovule. A suitable pollen grain grows a pollen tube, which extends down through the style towards the ovary. The tube enters an ovule through a small opening called the micropyle. This provides a route by which the pollen nucleus can reach the female gamete inside the ovule. Fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule.

7:07Fusion means that the nuclei join, bringing together male and female genetic information. The pollen tube provides the route, but the fusion of nuclei is the actual fertilisation event. Remember the sequence as a connected journey: pollen reaches the stigma, the tube grows towards an ovule, and then the nuclei fuse inside that ovule. This explains several common mistakes at once. A pollen grain landing on a stigma is pollination, not fertilisation, because the nuclei have not yet fused.

7:35The whole pollen grain does not travel down the style. A pollen tube grows from it. The ovary contains the ovules, and the tube enters an ovule, rather than fertilisation occurring on the stigma or somewhere along the style. Following fertilisation, an ovule can develop into a seed containing a young plant. Germination is the beginning of growth of that young plant from the seed. Before it can grow successfully, the seed needs water, oxygen and a suitable temperature.

8:00These requirements are not an arbitrary list. Each allows an essential part of the seed’s living processes to happen. Water is absorbed by the seed, allowing its cells to become active and helping the seed coat split. It also allows enzymes to act on stored food, making nutrients available for growth. Oxygen is needed for aerobic respiration, which releases energy from that food. Germination therefore depends on access to both water and oxygen, which explains why completely waterlogged conditions can prevent successful germination.

8:29A suitable temperature allows the seed’s enzymes to work effectively. At low temperatures, enzyme-controlled reactions are too slow for rapid germination, while very high temperatures can damage enzymes. The suitable temperature depends on the type of seed, so there is no single temperature that is best for every species. Light is not a requirement for the seeds in our investigation. Stored food supports early growth before the seedling depends on photosynthesis.

Investigating seed germination

8:53We can investigate these requirements by changing one condition at a time. Begin with equal numbers of the same kind of seed from the same batch, using several seeds in each group. The control group has moist cotton wool, access to air and a suitable temperature. This provides a comparison in which all three requirements are available, helping us judge the effect of removing one of them. For the water comparison, place seeds on dry cotton wool while keeping temperature and access to air the same.

9:18For the temperature comparison, place seeds on moist cotton wool in a cold environment, with the same access to air. For the oxygen comparison, place seeds in boiled water that has cooled, with a layer of oil preventing oxygen from re-entering. Boiling removes dissolved oxygen, and cooling avoids damaging the seeds with hot water. Observe all groups over the same period and record how many seeds germinate, using the appearance of a root as the same criterion throughout.

9:44Keep the seed type, number of seeds and other relevant conditions consistent, and repeat the investigation to check reliability. We would expect the control group to germinate more successfully than groups lacking a required condition. These are predictions to test, not results that we should invent before collecting observations. To investigate whether light is needed, compare two further groups with water, oxygen and the same suitable temperature. Keep one in light and the other in darkness, without allowing their temperatures to differ.

10:10Germination in darkness demonstrates that light is not required for those seeds. However, a seedling later needs light for photosynthesis, so conditions for starting germination must not be confused with conditions for sustained plant growth.

Exam answers and recap

10:23Let us connect this understanding to an exam-style question. A diagram shows hanging anthers and an external feathery stigma, and the question asks you to identify the pollination method and describe two structural adaptations. First, identify the method from the combined evidence, rather than guessing from the flower’s size alone. Then give separate, precise points, linking each structure to how it helps pollen transfer. A suitable answer would be this.

10:48The flower is wind-pollinated. Its anthers hang outside the flower on long filaments, allowing pollen to be released into the wind. Its feathery stigma projects outside the flower, providing a large surface area to catch airborne pollen grains. Each sentence contributes a clear point, and the descriptions explain why those structures are useful. If a question asks you to describe fertilisation, do not spend the answer describing insects visiting flowers.

11:12State that fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule. If it asks you to describe the process leading to fertilisation, include the pollen tube growing down through the style to the ovary and entering the ovule through the micropyle. The command word and the question’s focus tell you how much of the journey to include. Let us bring everything together. Flower structures protect the reproductive parts, attract insects where needed, produce pollen and receive it on the stigma.

11:39Pollination transfers pollen, while fertilisation joins nuclei inside an ovule after pollen tube growth. Self-pollination stays within one plant, whereas cross-pollination occurs between different plants of the same species and generally increases variation. Finally, a seed needs water, oxygen and a suitable temperature to begin growing into a new plant.