Why trees change colour in Autumn Every year with the approach of autumn the trees come to their period of rest. Little by little the leaves turn yellow and then curl up and fall to the ground. The tree then remains almost completely inactive until the return of good weather in the spring. It is a sad sight but at the same time a wonderful one, because before dropping the leaves turn to shades of yellow, brown and red. The explanation of this is quite simple. Plants are living organisms and must have food, and this they obtain by utilizing the organic substances provided by the leaves. At the same time, like animals, they also produce waste matter. Animals are able to get rid of the waste materials from their food but the plant has to retain them in its tissues until autumn. When the time comes for leaves to fall, the plant extracts from them all the products which can be used, leaving behind the waste materials. This is what gives the foliage its yellow, brown and red colours. Why trees loose their leaves in Autumn In autumn the leaves of many trees change colour and then gradually fall and the branches become bare. Other trees like the pine, the fir, the laurel and the holy keep their green foliage even in the winter, and you may therefore think that they do not need to change their leaves every year. This is not so; even the ever-green plants change their old leaves for new ones, but they do it a little at a time throughout the year. This can be seen from the layer of dry needles which are found under the fir trees. All trees change their leaves, but why? There are many reasons. In the case of broad-leaved plants there is a problem of defence against the cold. If they kept their foliage during the winter they would expose an enormous area to the frost, equal to that of all the leaves put together side by side. In addition the plant needs to rest and therefore discards all those tiny chemical workshops which evaporate water in large amounts and call on the roots to supply more and more. But the main reason for the change, even for the ever-greens, is that eventually the chemical laboratories in the leaves get old and need to be replaced by new and efficient ones. The shorter days of autumn hastens this change. When the old leaf has fallen, a healing layer forms on the stem and closes he wound, leaving the leaf scar. This can be seen clearly on many twigs in winter and is one of the marks by which trees are identified |
