The Function of Leaves

Leaves are vital to the living world, for lonely they can convert the sun’s energy into food for plants and, subsequently, for animals and people.

Leaf shapes and patterns

Plants are classified according to their leaves. It is useful to know the proper names for the different shapes so that you can identify species from written descriptions. Many plants also have distinguishing leaf patterns.

leaf shapes and patterns

Leaves feed the plant

Leaves are an essential part of a plant because it is through their pores that the plant ‘breathes’, or absorbs, the gas carbon dioxide from the atmosphere. Leaves and stems contain a coloured pigment called chlorophyll which makes them green. This pigment absorbs energy from sunlight and enables the leaves to convert the carbon dioxide and water, drawn up from the roots, into carbohydrates and oxygen. The plant uses the carbohydrates as food to make it grow, and releases the oxygen back into the atmosphere through the leaves. This food-making process is called photosynthesis and only takes place during daylight. This is why it is essential to give a house plant plenty of light and air or it will not grow.

Nature’s life cycle

The oxygen given off by the leaves of plants is the very ‘air’ humans and animals need in order to live and breathe. After using the sun’s energy to make their own food, plants in turn become food for animals and, subsequently, humans. Without plants we would not be able to use the sun’s life-giving energy.

Talk to your plants

Some people think ‘talking to your plants’ is just a joke. But if you talk to them as you lean over to tend them, you breathe out carbon dioxide which is the gas that helps them grow.

Changes with light and shade

Leaves position themselves to obtain the maximum sunlight. Tolmiea leaves, for example, fan themselves out in alternating layers. Large trees lift their upper leaves ‘up on edge’ so that light can filter through to the lower branches. Eucalyptus does this so effectively that it casts almost no shadow. Some plants, such as the Swiss Cheese Plant alter their leaves to make best use of the light available.

Leaf pests and diseases

Leaves turning yellow, curling, wilting or turning brown at the edges can all be caused by the plant’s treatment. Check the ‘Secrets of Success’ notes for each plant and be sure they are getting the right amount of light, water and food.

Small brown discs on the underside of leaves are scale. Treat by wiping off with a damp cloth and spraying the plant with a suitable insecticide.

Sticky leaves with minute white specks are caused by an attack of tiny white moths called whitefly. Treatment is difficult but regular spraying with a suitable insecticide may get rid of them.

Holes in the leaves may be caused by caterpillars or earwigs. Pick them off and no further treatment should be needed.

A white powdery deposit on leaves is the fungus disease mildew. Remove badly affected leaves and improve ventilation around plant.

Brown moist spots on leaves can be caused by bacteria or fungus. Treat by removing and burning infected leaves, spraying with Benlate and keeping the plant fairly dry for about 3 weeks.

A hard corky growth on the underside of leaves is caused by the plant having waterlogged compost. Treat by removing badly affected leaves, moving plant to a better place and improving drainage in pot.

The function of pores

There are pores in the top and bottom surfaces of a leaf. The pores have lips that can open and close to allow the plant to breathe in carbon dioxide and exhale oxygen. The pores also take in water vapour and allow any excess moisture to evaporate through them.

If a plant does not have enough water, it closes its pores to prevent losing any more moisture. Once the pores are closed it can no longer absorb anything to make food, so the plant stops growing. Some plants have a hairy covering on the leaves which helps to reduce the amount of water evaporating from them. Some plants with long narrow leaves can roll them inwards to reduce evaporation. This is a useful warning sign and tells you it’s time to water the plant.

The function of veins

The veins in leaves are quite easy to see, especially on the underside. These are the pipelines the leaf uses to transport food produced by the leaf cells to other parts of the plant. They also carry minerals and water that come up from the roots.

Keeping the leaves clean

Indoor plants can become rather dusty and spotted with a lime deposit from spraying and watering. This can block the pores through which the plant takes in its vital supplies. You can blow off dust gently and spray the upper surfaces with leaf shine. Do not spray the undersides as the pores are larger there and could become clogged. Ensure that your plant is a variety that tolerates leaf shine and use it sparingly.

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