Animals eat plant fruits, the fleshy wrappings around seeds. Seeds spread in this way can resist the digestive processes of the animals that feed on the fruit. They pass right through the animal’s gut without injury. The seeds start to grow when they are deposited in the animal’s dung, which often acts as a fertilizer.
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The biggest seed is the coco de mer, a kind of coconut that grows in the Seychelles, a group of islands in the Indian Ocean. The coco de mer seeds weigh 20 kg each, so it can be dangerous to walk beneath the parent tree. These giant seeds take 10 years to develop on the tree.
Strangely, the coco de mer does not grow close to the beach, so how the giant seeds reach the sea is still not understood.
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Many seeds are carried by the wind. Some have a fluffy ‘umbrella’, which is carried for long distances. Others have wings that allow the seed to glide or spin around like a helicopter blade. The wings slow the seed’s descent so that the wind can carry it further away. One particular seed, from a tropical tree called Zanonia, glides so effectively it was used as the model for the wings of early planes.
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After fertilization takes place, the egg cell develops into a seed from which a new plant can develop. The seed contains an embryo from which the new plant will grow. It also contains a food store to nourish the embryo until it has developed roots and leaves. The seed is enclosed in a tough coat to protect it from drying out.
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Pollination takes place when a pollen grain is deposited on the tip of a pistil. It then grows a long tube down inside the pistil that fuses with the egg cell and completes the process of fertilization.
Sometimes pollen is blown by the wind, and a single pollen grain is accidentally deposited on the pistil. In other flowers, insects do the same job, carrying pollen stuck to their bodies as they travel from flower to flower. In some plants, the flower is modified to force the insects feeding on nectar to brush past the stamens, collecting a large amount of pollen on their bodies as they pass by.
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Pollen is the plant’s equivalent of an animal’s sperms. It carries the male reproductive genes.
Pollen consists of tiny grains, each with a tough coat that is often patterned with characteristic ridges and spikes. When inhaled, the fine pollen causes allergies such as hay fever in some susceptible people. Pollen can be found in fossil deposits, making it possible to identify the plants that were living then – even though no actual plant fossils may be found.
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