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The mechanism behind the structure
By autumn, for the majority of plants, the fertilised egg has developed into the plant embryo. The ovule of the flower which contained the egg has grown into the seed (which contains the embryo), and the ovaries that contained the ovules have grown into fruits. The next phase is usually dispersal.
Almost all plants have evolved a mechanism by which the mature seeds are not only shed from their container (the fruit), but are also scattered away from their parent plant. The advantage of this process of seed dispersal is that the new generation of seedlings germinates some distance from the parent, avoiding being shaded by it and not taking nutrients from the soil that would hinder the growth of both of them. Dispersal is also important because it becomes the mechanism through which plants locate new habitats and colonise new ground. Long term survival of a species may depend upon its ability to reach new environments.
Orchid seed is so fine that it is like dust, and can be picked up and blown around in the wind. There have been reports of orchids turning up 50-100 miles away from their parent plants. This process though is very wasteful, and with seeds so small the chances of survival are extremely slim; orchids have to produce them in their millions.
Wind carried fruits and seeds that are heavier need a ‘sail’. The hairs on the dandelion fruit or the fluffy strands of the wild clematis fruit (old man’s beard) create wind resistance, enabling the breeze to lift and transport the fruits to new locations.
For trees, shedding their fruits from high up, all that is necessary is a way of delaying the fall long enough for the wind to carry them some distance from the parent. An outgrowth of the fruit wall, and a seed asymmetrically placed inside makes the fruits of sycamore, field maple and ash spin as they fall, generating lift that delays descent long enough for the landing to be some way from the parent.
The wonderful display of red berries of black bryony or hawthorn, and other succulent fruits which provide food for our birds in autumn, are really a dispersal strategy, the animals being used as the mechanism to distribute the seeds. Although the fruit is consumed, the seed inside it has a hard resistant coat and can travel through the bird’s digestive system unharmed. Later it will probably be deposited some way from the parent plant (together with a small dose of manure).
If the plants are growing closer to the ground hitching a lift on the fur of ground moving animals becomes an effective strategy. The burs from burdock that can be a nuisance in the animals’ fur have an unseen sophistication in the way they operate. The hooked fruits are obvious enough, but initially a dog will largely ignore them, since they cause little irritation. Later though, as the bur dries out, small straight sharp fibres are released. These work their way through to the animal’s skin and set it scratching a day or so after the bur was picked up. Wood avens (Geum urbanum) have styles with an odd kink in. When the fruits mature these break off half way along the kink, leaving a very effective hook, for animal dispersal.
Some plants like vetches and other members of the Fabaceae (pea family) have pods which, unlike peas and beans, dry out to tough leathery husks. As the pod dries, fibres in the fruit wall shrink, tending to twist each half of the pod in opposite directions. For a while the tissue holding the sides together withstands the strain, but at some point the tension becomes too much and it breaks. The explosive curling of each side of the pod whips the small cannonball-like seed out, throwing them some distance from the parent.
All these mechanisms can be found easily in one walk down the line, but there are many more to find by observing the structure and working out how the dispersal happens.
August Nature Notes
September Nature Notes
October Nature Notes
November Nature Notes
December Nature Notes
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