The childhood image of what a plant looks like stays with us all our lives. As in a child's drawing, a plant has a stem sticking out of the ground, and this stem has leaves on it and it has a flower at the top. This image is true for most plants, though as we grew older we found out that below the ground there are roots. And if our plants are to look their best we have to look after all these parts. And to do that properly we really ought to know a little about what each part is for - what it does and how it does it.
So let's start with the leaves. Leaves are remarkable things. They breathe in air, extract the carbon dioxide from it and, with light as a source of energy, combine it with water to convert it into food. They breathe in air and extract the oxygen which is used to convert the food back into energy for the plant's growth. They breathe out waste gases from these processes, as well as excess water. They can even take over some of the normal functions of the root and absorb water and mineral salts directly. Some can miraculously propagate themselves. However their main role is to manufacture food.
The raw materials of a plant's food are air, water and light. By the process of photosynthesis these are converted into food. On a leaf, largely on the underside, are numerous pores, or stomata, which can open to admit air. The small amount of carbon dioxide present in the air is extracted. Water is drawn up from the roots to provide hydrogen and this is combined with the carbon dioxide to make a simple sugar. The energy necessary to do this conversion is provided by light absorbed by leaf cells containing chlorophyll - the pigment which makes leaves green. The oxygen released in this process is given off as waste through the stomata, and the sugar is stored in the leaves or other parts of the plant. Photosynthesis can take place only in light, but the light levels needed for efficient food conversion vary from plant to plant, primarily depending on their original habitat. Plants with plenty of chlorophyll need less light than those with little. Generally, plants with dark green leaves have more chlorophyll in them than those with light green leaves. Hence, plants with dark green leaves need less light to survive than those with light green leaves.
The sugar manufactured and stored during the hours of light is used to create energy to make the plant grow. Oxygen is needed. The stomata admit air and the oxygen in it combines with the sugar to produce energy, leaving carbon dioxide and water as waste, and these are given off through the stomata, a process known as respiration. Unlike photosynthesis respiration does not need light, and most of it takes place at night. Anything which reduces the amount of oxygen in the air and increases the amount of carbon dioxide could be regarded as antisocial as far as human beings are concerned. And this is the reason why flowers and plants used to be taken out of hospital wards at night.
Giving off water - transpiration - is another important role for the leaf. Much of the water vapour escapes through the stomata, but it also escapes by evaporation through the surface of the leaf and stems. When water escapes a kind of pull is exerted through the plant and water is sucked up from the roots to replace that which has been lost, and so water taken up by the roots is circulated to all parts of the plant. But evaporation can be dangerous to a plant. In hot dry rooms the plant may lose water faster than it can be replaced, and the plant wilts, often dramatically. It is the dryness of the air rather than the warmth that causes the trouble; in a humid jungle with a high relative humidity there is no such problem. Wilting can happen at any time of the year, not just in summer, but also in winter in centrally heated rooms where the air is desert dry - just when you don't expect it. This danger explains the emphasis on the need for providing extra humidity in such conditions. The roots supply water and the elements which the plant cannot extract from the air. The water and the mineral salts from the compost which have dissolved in it are drawn into the plant through the fine hairs of the roots by a process known as osmosis. Water can move through the semi-permeable wall of the root hair if the concentration of minerals in the water is weaker than the concentration of minerals in the sap inside the root hair. However, if strong doses of fertilizer are applied to the compost so that the concentration in the compost is stronger than that of the sap, the process of osmosis may be reversed, causing water to flow from the roots into the compost so that the plant becomes dehydrated and wilts. Since plants in pots need only minute amounts of minerals they must be fed sparingly. Never apply a stronger dose of fertilizer than that recommended by the makers - indeed a weaker and less frequent dose is better.
Disaster also follows over-watering. Less than half of a good porous compost is solid matter and the rest is water and air. The air there is as vital to the plant as the water because roots need oxygen. If the compost is kept sodden all the time, the air is excluded, the roots cannot function, so the plant goes short of water even though the roots are drowning in it, and it wilts and dies. That is the simple reason for the constant warnings about the dangers of over-watering. There are plants which can extract the oxygen they need from water - plants like water lilies and pond weed - and some, like arum lilies, can use both methods and can survive in relatively dry soil or in a pond.
The stems form the vital link between the roots and the leaves - multi-lane highways servicing the plant. They are complicated structures, as you can see if you look at a cross-section under a magnifying glass, but two of their functions are especially important. One is the conduction of water and mineral salts from the roots. The other is the supply line for the synthesised food. Both operate throughout the plant; in the leaves their routes follow the easily recognisable mid-rib and the network of veins. Stems may need supporting but they do not demand as much constant looking after as leaves and roots. Only when we are pruning or propagating them do they take over the star role.
I know this has all been a bit technical, but it does help one to understand a plant's needs and do the right thing by your plants. It does seem strange that one can treat two plants in exactly the same way, and one of them will die and the other thrive. But then, that's life.
Happy gardening,
Bill Hutchings
Return to the October 1999 Features page
return to Home page and main index
page last updated 2 OCTOBER 1999