From the Ground Up

by Derek Burch


Leaves -Part B



A few more things about leaves and what they do - and how they do it, and what might go wrong


The loss of water from a leaf is a simple physical process of evaporation from the cell surfaces, which have to stay wet so that gases can dissolve and pass in and out. It is given the name transpiration because the plant has some control over it.

The holes through which air enters the leaf, thousands of them per square inch and mostly in the lower surface, are called stomata, and they can close in response to too much water loss by the leaf. They don't always stop in time, and the leaf wilts, usually closing the stomata more tightly. This is a mixed blessing: it stops most of the water loss, but with the stomata tightly closed the exchange of carbon dioxide and oxygen with the outside air cannot take place. Without this, some of the photosynthesis reactions cannot go on, so the energy-trapping function of the leaf is lost.

How should you, as a grower, respond if your plants are wilting? First of all, don't automatically reach for the hose, particularly in the middle of the day. Or, if you do go for the hose, use it to mist the leaves rather than adding water to the root area. What you are probably seeing is water loss too fast for the roots to take in enough to meet the demand, rather than dry soil. Of course, the soil may also be dry, and that can be remedied, but do check first before watering. You don't want to drown the roots.

Wilting happens in nature, just as it does in our nurseries and gardens. The rate of water loss depends on air movement over the surface of the leaf, and on the temperature (is anyone else old enough to remember what made a good drying day when we still hung clothes outside on a clothesline?). Many plants have evolved ways of changing the amount of sunlight that reaches their leaves, which, of course, affects the temperature of the inside of the leaf. Some of the most dramatic happen in the bananas, and if you are ever around a clump in hot sunshine, you will see that in the middle of the day the blade of the leaf folds down along the centre part. This means that the sun's rays strike it at an angle, a glancing blow if you like, which stops the leaf from overheating, and keeps the energy reaching the chloroplasts to a level which they can handle.

I know a load of trivia about leaves, and could do pages of unrelated facts for you. How is it that I am never invited onto a quiz show that would let me turn this clutter of information into real money!

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