The functions of leaves
Leaves are not just a decorative part of houseplants. For plants they are almost as essential as lungs are for people. They take carbon dioxide from the air and receive water from the roots.
From these two raw materials they produce organic compounds, mainly sugar. The energy for this production comes from sunlight. The whole process is called photosynthesis. In the dark it does not take place, and in winter it happens only to a very limited extent, because the days are shorter and there is not enough sunlight.
When a plant produces sugar, it also releases oxygen – and oxygen is indispensable for all living beings.
How can sunlight reach the lowest leaves?
If you look closely, you can see that the leaves at the top of a tree are positioned vertically so that some light passes through to those growing below. The lower leaves, held horizontally, catch every ray of light that gets through.
There is a tree in Australia called Eucalyptus globulus in which you can see the leaves turn exactly edge-on to the direction of the sun's rays. Protecting itself in this way against the scorching sun, the tree casts almost no shade.
The upper and lower surfaces of the leaves are covered with a layer of epidermal cells. Especially on the underside, this layer contains countless tiny openings. These are stomata, which can open or close as needed.
When a plant gets too little water, it protects its last reserves and closes its pores. It then stops producing energy and stops growing. Some plants have leaves covered with a waxy layer or hairs, which reduce moisture loss. This layer can be destroyed, for example, by acidic compounds in air pollution, especially acid rain. The plant then suffocates and dies.
The veins of the leaves are visible in every plant, especially when you hold a leaf up to the light. The veins form a system of small tubes and channels that carry the sugar produced in the leaves to all parts of the plant. Through this system the plant also takes up water and minerals from the roots. The upper parts of the veins contain tissue that strengthens the leaf, while sap flows through the lower parts.
Chlorophyll is found in all green parts of plants, most of all in the leaves. Its task is to absorb solar energy, combine it with water and carbon dioxide and turn it into sugar. This sugar is then transported to the parts of the plant that need energy. Chlorophyll absorbs mainly the red part of the solar spectrum and reflects green. Thanks to this, photosynthesis can also take place under artificial light.
When we think of leaves, we mean the plant's true leaves. But there are other kinds of leaves too.
Cotyledons are the first leaves of a plant and are still part of the embryo. They feed the germinating plant, giving it a good start in life.
Scale leaves are small, scaly leaves that protect the bases of shoots and buds.
Bracts are often large and beautifully coloured. They are arranged around the inflorescence, either for protection or to attract insects.
The flower itself often has two kinds of leaves: sepals and petals. Flowers sometimes grow between two leaves.
Where there is carbon dioxide, plants grow faster. Gardeners often make use of this in greenhouses by enriching the air with carbon dioxide. Of course, this cannot be done at home.
Houseplants need feeding. They can also take in additional nutrients through their leaves.
If, for example, they turn yellow because of a nitrogen deficiency, they can be sprayed with a urea solution. The concentration must not exceed 0.2%, otherwise it may scorch the leaves.
Some plants even feed by catching insects with their leaves and digesting them.
Smooth, dark green leaves sometimes become spotted with dust. Spraying with water can also leave lime marks on the leaves. In such cases, after washing the plant thoroughly, you can spray it with a leaf-shine product.
Be careful when restoring the shine, though, and clean only the upper surfaces of the leaves. Some plants cannot tolerate such treatment at all.
Plant leaves vary in size. Some are only a few millimetres long. The largest leaves belong to the South American giant water lily, Victoria amazonica. Its leaves reach a diameter of a few metres and can bear the weight of a child.