THE nutrition of plants has to be treated from two points of view; we have to see how the plant makes its food and how it utilises it afterwards. Since the making of the food depends upon chlorophyll we may well begin with this mixture, a mixture which is perhaps the most important group of substances on the face of the earth. All living creatures depend on the products of its activity for food, and modern industrial civilisation, based on the combustion of organic remains of past ages, could not exist had not green plants produced the material that the furnaces burn. If nettleare chopped up and treated with acetone
and water, a deep-green solution is obtained; this solution contains a mixture of the four pigments known to be present in chlorophyll. When a drop of this solution is placed on white blotting-paper, the liquid spreads, and there is a thin yellow edge surrounding a green centre. This is a rough demonstration that the solution contains a mixture of substances. There are in fact four pigments in crude chlorophyll, two green and two yellow; of the latter, one, carotene, is familiar, as it gives the colour to carrots. All four pigments are extremely complicated substances; the green ones, which are much more so, are made up of carbon, hydrogen, oxygen, nitrogen and a little magnesium, the yellow ones of carbon and hydrogen only (and, in some, oxygen as well). Land plants contain rather under one per cent of their weight of the mixture.
PLANTS THAT NEED AN IRON TONIC
MUCH is known of the chemistry of chlorophyll, but we are still ignorant of the way in which it works in the plant. It contains no iron, but iron is used by the plant as it builds up its chlorophyll. Some plants are hindered from taking in iron from soils that contain much lime. Incautious liming of the soil in gardens may be shown subsequently by the development of yellow streaks and spots in theof the plants, and, in calcareous districts, such yellow markings are often seen in the leaves of the wild plants. They indicate a deficiency of chlorophyll in the leaves, no doubt because the abundant lime in the soil has upset the intake of iron by the plant.
Most plants are unable to develop chlorophyll except in good light, and ifare allowed to germinate in unbroken darkness, they give rise to sickly yellow plants with little or no chlorophyll; these plants seldom survive for long, for even if they are put in the light and then form chlorophyll, they are usually so weak that they fall easy victims to enemies which are unable to attack fully healthy plants.
Chlorophyll may disappear from plants if they are darkened; the yellowing of grass which has been accidentally covered up is a familiar example of this. Recovery to a normal green condition depends on the length of time that the plant has been deprived of light. If recovery is possible, it seems that weak light is more favourable than strong light. When a plant is in the light, it is forming chlorophyll, and at the same time it is losing chlorophyll, since light breaks the
pigments up. The stronger the light, the more rapid is the break-up, and, when a plant is in a weakened condition it may not be able to form chlorophyll quickly enough to compensate effectively for losses in strong light.
During those parts of the year when growth is in active progress there is some kind of regulation of the events going on in the cells, so that, with normal conditions, the green cells remain green and always contain about the same amount of chlorophyll. In autumn, however, changes occur in plants which lose their leaves at that season. The chlorophyll is broken up, giving brown substances which are in part responsible for the colour changes seen in autumn; they are not, however, entirely responsible, since other brightly coloured substances owing their origin to disorganisation in the cells of the leaves, also make their appearance.
THE UNEXPLAINED MYSTERY OF A PLANT’S FOOD CARBON dioxide enters leaves by means of the stomata, and finds its way into the spaces between the cells of the leaves. The walls of these cells are wet, and the carbon dioxide dissolves in the moisture, passes in solution through the walls and enters the cells. Once there, in some fashion which still awaits complete explanation, the cell brings about union between water and carbon dioxide, with the liberation of oxygen. The oxygen passes into solution, diffuses as gas into the spaces within the, and escapes from these through the stomata, into the open air. As a result of the union between the carbon dioxide and the water, sugars are formed in the cells of the leaf, and these sugars form the starting-point for further food-making operations.
There is no doubt that some of the early stages in these operations are carried out by means of energy from the sun which is arrested by the chlorophyll and used for bringing about chemical changes. Many suggestions have been made to explain the changes which go on, but the experimental investigation is very difficult, and the results are not clear enough to allow of dogmatic conclusions. It is well demonstrated that green plants can only utilise water and carbon dioxide for food manufacture if they are supplied with light —not necessarily, however, with bright sunlight—and it follows that the energy from the sun lies at the bottom of all the changes which lead to the building up of food from simple beginnings.
As sugar accumulates in the cells, at times when that substance is being formed, it necessarily follows that the concentration of dissolved material in the cell sap must rise. Continued increase of this kind might well upset the general relationships of the cells, since, the more concentrated the cell sap, the more vigorously does water enter from without. We find that, as sugar is made in the green cells, much of it is converted into starch; this, being insoluble in water, appears as small solid granules within the cells, and so long as it remains in this form it plays no part in the activities of the cell. Some of the sugar is used by the cell which makes it, and some of it diffuses out of the cell and passes to other parts of the plant, supplying cells which possess no chlorophyll and cannot manufacture for themselves.
During times when the green cells are exposed to light, the rate at which sugar is being made greatly exceeds the rate at which it is being used up, and it is easy to show during the daytime that starch accumulates rapidly in the green cells of the plant. As the light becomes weaker, the manufacture of sugar, and therefore of starch, falls off, and presently a point is reached when the cell is using sugar, or losing sugar by diffusion, faster than it is making it. Then the starch is converted back into sugar, and, as this is used or passes elsewhere, more starch is converted. In some plants all the starch accumulated during the daytime may be turned back into sugar and used or transported to some other part of the plant during the night; in others two or three days of complete darkness are necessary to ensure that all the starch in the leaves has undergone the change.
There is some doubt about the part played by chlorophyll in relation to the formation by the plant of food materials containing nitrogen, and other of the more complicated substances of the plant. It seems well established that few of these substances are formed at the expense of light energy, and it may be that none are. It is quite possible that the green plant uses light energy only for the formation of sugar, and that all the other changes which occur in the cells derive their energy from the subsequent breakdown of the sugar.
TINY SUGAR FACTORIES IN THE LEAVES
THE sugar-starch relation just mentioned plays an important part in the opening and closing of the stomata. A simple form of stoma consists of a couple of sausage-shaped
cells—guard cells—lying side by side in the epidermis of the leaf, and firmly united with the neighbouring epidermal cells; the two guard cells are not, however, joined to one another except at the ends, so that there is an opening between them. They contain chlorophyll and so are able to manufacture sugar. In the light, they do so actively, and as the concentration of sugar within them rises, they take up water actively from neighbouring cells. As a result the guard cells swell, and owing to the manner in which they are joined to neighbouring cells they curve as they swell and the pore between them opens. In darkness manufacture ceases, sugar is used or lost from the guard cells, their water content falls, and, as they shrink, they straighten out and the pore is closed.
It has been mentioned several times that the plant makes use of energy from the sun in its manufacturing processes, and it has been implied that solar energy is also used up in the liberation of water from the plant. We may try to construct a kind of balance-sheet as an indication of the way in which the plant uses this energy. The figures given below were obtained from experiments with the commonsunflower, and they do not suggest that, as a machine for using energy, the plant is specially efficient.
Arbitrary value of solar energy . . . . . . Proportion used in food-making . . . . . . o- Proportion used during loss of water vapour .. 48- Proportion transmitted, reflected or otherwise lost 50- That is, the plant uses for constructive purposes less than I per cent of the solar energy falling on it. The large amount used in connection with the elimination of water suggests the efficiency of this process in cooling the plant. As a rough measure of the rate at which the plant builds up material we may note that it has been calculated that, in a long summer’s day, a leaf forms about as much starch as would be equal in weight to a piece of tissue paper of the same size as the leaf.
HOW FOOD IS USED TO GIVE THE PLANT ENERGY THE many chemical changes which go on in the plant need a supply of energy for their accomplishment. A great deal of this energy is obtained by the breakdown of the food materials already formed by the plant. It appears
probable for instance that many of the more complicated food substances are formed with the aid of energy liberated when the sugar present in the plant is broken down by means of oxygen, with the formation of water and carbon dioxide. Everyone knows that sugar burns readily and gives out a good deal of heat as it does so. In the same way the plant is able to break down sugar and liberate energy from it, but the process is not exactly comparable with burning, since it goes on more slowly and more gradually; yet the results are the same in the end.
We thus have the curiousthat the plant takes in water and carbon dioxide and brings them together to form sugar, liberating oxygen in the process, and also that it takes in oxygen, and uses it to break down the sugars (and other substances) and liberate water and carbon dioxide. This second process, the breakdown of complicated substances to simple ones, with the liberation of energy, is common to all living creatures, whether plant or animal, and it is important to realise that the process goes on inside the cells of the creature. Moreover, it goes on all the time, so long as the cells are alive, and so contrasts with the more simple of the manufacturing processes of the green plant, which only proceed in the light.
QUICK-BREATHING PLANTS WHICH GROW WARM THIS utilisation of material, with the assistance of oxygen, is called respiration; it appears that the commonest substances used in the process are sugars, and, when this is so, the plant gives out approximately as much carbon dioxide as it takes in oxygen. Fats can also be used, and they are commonly used by young plants, since manycontain stores of fatty substances; these need more oxygen for their breakdown, so that a plant using fats in respiration takes in more oxygen and yields less carbon dioxide. For short periods green plants are able to carry on respiration without an external source of oxygen, since they can take oxygen from some of the material present and use it to attack further material, but they cannot live indefinitely without supplies of free oxygen.
So far as we know, the green plants utilise only complicated substances for proposes of respiration, but it is worth notice that some of the bacteria of the soil, bacteria which play an important part in the breakdown of organic material and its
reconversion into the simple forms utilisable by higher plants, are able to oxidise very simple substances such as ammonia, and to gain in this way sufficient energy for their needs.
In general, respiration goes on rather slowly in plants, and the energy liberated, probably mostly in the form of heat, is not set free fast enough to raise the body temperature of the plant to any appreciable extent; in this respect we have a sharp contrast with the warm-blooded animals, in which the rapid rate of respiration keeps up the body heat to the necessary level. Sometimes, however, plants may respire fast enough to produce temperature effects; germinating seeds are usually a trifle warmer than their surroundings, and in some plants which develop theirvery quickly, the rapid utilisation of food materials at that time may result in the liberation of considerable quantities of heat. Of course, any heat that can be detected by means of a thermometer placed in the neighbourhood of the plant is heat lost to the plant, but the fact that it can be shown that heat is escaping indicates that respiration is proceeding with special vigour.
We may obtain a good deal of heat energy from sugar by burning it, but we can only do this by first raising the temperature of the sugar to a point where combustion begins. In the plant, sugar is utilised by oxidation, but the plant does not start the process by the application of a high temperature. It seems that the plants possess a number of very curious substances—enzymes—which have a remarkable power of causing chemical change in other substances, at ordinary temperatures, and without much loss of the enzymes themselves, and there is little doubt that the utilisation of food materials in respiration, at the low temperatures alone possible in the plant body, is brought about by these enzymes. These powerful substances are .sometimes of industrial importance; for example, the conversion of sugar into alcohol by yeast, a process which is a modified kind of respiration, depends upon the enzymes of the yeast plant.
ENERGY BUILT UP BY RESPIRATION
IT appears then that respiration is a process by means of which plants and animals break down the complicated substances which serve as food, and that the process ends with the formation of carbon dioxide and water, and the liberation of energy. There can be no doubt that in the
living cells in which the process is carried out the breakdown of the food goes on in a number of successive stages, each yielding simpler products and some free energy. Respiration is carried out by all living things, and goes on at all times, furnishing the creature with a continuous supply of energy.
In green plants in the light, it is not easy to show that respiration is in progress, as it is masked by the more vigorous process of food manufacture going on at the same time. It has, however, been shown that green plants respire at the same time that they make up food, and the realisation that these opposing processes are going on in the same cell at the same time serves to emphasise the very complex nature of the proceedings of living creatures. Complete breakdown of the food materials to the simple substances from which they were made is only possible when the plants are abundantly supplied with oxygen; if the supply of oxygen is restricted the process may continue in a modified form, but this yields less energy, it may result in the formation of substances which ultimately poison the organism, and it can only be carried on successfully by a few specialised fungi and bacteria.