While sitting here in front of my writing machine looking out of the window, wondering what sort of thing I could possibly pen a few words about, my eyes kept falling on the bare tree on the verge outside. And I asked myself a question - why do trees shed their leaves? It is a process which occurs every year in deciduous trees, and every few years in evergreens and conifers (not all the leaves at the same time of course, or they wouldn't be evergreen). After all, a leaf is one of nature's miracles. Some leaves are beautiful to look at; some are pleasing to touch; all are complex masterpieces of natural engineering which have evolved over millions of years to manufacture food for the plants that bear them - and plants, in turn, ultimately nourish and sustain all land animals, including you and me. So why throw them away?
Leaves are actually an integral part of a plant's stem, just as your fingers are part of your hand. They possess the same fibres and tissues as the stem. A typical leaf consists of a broad, flattened blade attached to the stem by a stalk. From the latter, veins radiate outwards through the leaf; as well as transporting nourishing materials to and from the leaf tissue, they act as supports for the leaf, rather like the ribs of an umbrella.
But what the leaves are really clever at is photosynthesis, the key to a leaf's food manufacturing ability. All plants contain chlorophyll, the green pigment which absorbs energy from sunlight enabling the plant to build up carbohydrates from water and carbon dioxide which it extracts from the air. All this chemistry goes on within the leaf's internal structure, which is protected by the leaf's skin: this joins the stem's skin without a break that might allow harmful agents from outside to get in. The central part of a leaf consists of soft-walled cells. About one fifth of these cells contain the chlorophyll that absorbs sunlight. The cells also produce enzymes - the proteins produced by all living cells - which act as catalysts in the chemical reactions on which life depends. In the process of photosynthesis, these enzymes act in conjunction with sunlight's radiant energy to break down the water in its system into its twin elements, hydrogen and oxygen. The oxygen passes into the atmosphere through pores in the leaf's surface to replace the oxygen used up by animals when they breath, and also by things burning. At the same time, the enzymes cause the hydrogen to combine with the absorbed carbon dioxide to form the carbohydrates without which plants could not exist. As a matter of fact, neither could we.
The chemical reactions that take place inside leaves, therefore, are vital to life on earth ˜ which brings us to the original question: why does nature permit such an essential piece of itself to drop off? It is all a question of what the leaf is really for. Although we couldn't get on without them, their main duty is to sustain their parent plant, particularly during the latter's initial phase of growth, when it may not be able to derive sufficient nutrients from the soil through its root system. Although the process of producing food continues as long as the plant exists, it is never so vital as during the first season's growth.
In the autumn, as the days grow shorter and the process of photosynthesis slows down because less light reaches the leaves, the leaves cease to be of any use to the plant. In fact they could be a hazard to its well-being, allowing water to escape into the atmosphere. So the plant causes a band of soft cells to form across the base of the stalk and the leaf eventually breaks off at this weak layer, leaving an open wound. However, scar tissue soon forms across the wound, sealing off the stem and preventing undue loss of needed moisture.
So there it is. It is all a part of the plant's fight for survival.
Happy gardening.
written by Bill Hutchings
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