MANY attempts have been made to solve some of the problems of the way in which the plant works, by making chemical analyses of the plant body. It is comparatively easy to analyse the plant and to find what chemical elements are present, but such analyses tell us little of the substances which are actually present in the plant. However, such work shows that the number of chemical elements of universal occurrence in plants is small; it is made up as follows : carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, calcium, potassium, magnesium, iron. In addition, many plants contain appreciable amounts of sodium, chlorine and silicon. The elements do not occur as elements inside the plant; they are always united with other elements, often in very complicated fashion.
Wheat grains and potato tubers are resting structures containing much starch, a carbohydrate plants often store in their resting structures. The figures suggest very clearly how it is that wheat is a better food for animals than are potatoes. The high ash content of the, calculated in relation to
their dry weight, is of interest; leaves commonly contain much mineral material.
The ash contains all the elements present in the plant except carbon, hydrogen, oxygen and nitrogen. The elements found in the ash enter the plant from the soil, dissolved in water taken up by the. Much of the solution passes into the leaves of the plant, where the water escapes into the air, leaving the mineral substances behind in the leaves. Some of the accumulation is got rid of by the plant when the leaves fall. At first sight such a circumstance may seem of little moment, yet so complex is the inter-relationship between plants growing under natural conditions that even so trivial a matter as the disposal of excess mineral substances may have its significance. The ash of leaves often contains a fair amount of compounds of calcium, and rotting leaves may sometimes add sufficient calcium to the surface soil to prevent it from becoming acid, even though there may be an acid soil beneath it. The lesser celandine cannot grow well on an acid soil, but it is often found in abundance on the shallow layer of soil kept suitable for its growth by the addition of compounds of calcium from fallen leaves.
PLANTS WITHOUT STEM, ROOT OR LEAVES ANALYSES such as those which have just been given apply to the whole plant; they tell us little about the composition of its protoplasm. Up to the present it has not been found possible to obtain sufficient quantities of the protoplasm of any of thefor purposes of analysis. There is, however, a group of lowly organised plants, the slime moulds, whose bodies consist of a naked mass of protoplasm without any cell walls; these plants have no , no and no leaves. The protoplasm is not pure, for it always contains a certain amount of rubbish, but it is the best material available. Analysis of this material shows that about 55 per cent of the dry weight is protein, indicating that protoplasm is specially rich in nitrogen, sulphur and phosphorus. Probably most of the nitrates, sulphates and phosphates taken from the soil by plants are used up in making protoplasm.
A MIXED DIET AND DOSES KEEP THE PLANT WELL THE chemical analysis of plants has led to a number of developments : for example, the modern industry in artificial manures, such as superphosphates and sulphate of am-
monia, and to the utilisation of deposits of potash and phosphates in various parts of the world. After it had been shown that plants contained certain elements it was an obvious step to investigate what plants took from the soil, and this work has stimulated much study of the soil and of means to increase its productiveness. Experiments have shown that plants do not flourish unless they can obtain supplies of nitrogen, sulphur, phosphorus, potassium, magnesium and iron, and that some plants need small supplies of other elements as well. It is noteworthy that although plants contain much carbon, they do not obtain this from the soil. Sufficiency, however, is by no means all that is necessary for a plant to flourish. The water in a fertile soil holds, in solution, supplies of all the nitrogenous and mineral substances that the plants need, but if these substances are supplied to the plant singly, and not mixed with the others, they may beinstead of beneficial; in the soil water, the substances are so balanced that they cancel out one another’s poisonous properties and all is well. It has been shown, too, that many plants need, for their best growth, tiny amounts of special substances. For example, broad beans need very small doses of borax or of zinc which appear to produce a stimulating effect on growth out of all proportion to the amount of the dose.
All growing plants contain a great deal of water, so much indeed that it has been suggested that the plant consists of water held in shape by small amounts of a few other substances. When it is recalled that a fresh lettucecontains about 95 per cent of water, the idea does not seem so odd as it does at first sight. Water is of the greatest importance to plants, and this in a variety of ways.
There are some properties of water which fit in well with the special needs of the plant. It dissolves a very large number of substances and so provides a means of bringing material into the plant and of moving it inside the plant. It is transparent to light, and so opposes no serious hindrance to the entry of this important source of energy into the plant. The heat relations of water are of special significance; it heats up slowly and cools down slowly, and consequently the large amount of water in the plant affords protection against sudden and violent changes of temperature. Further, when liquid water is changed into water vapour, it takes up much heat from the substances with which it is in contact—the porous earthenware butter coolers are kept cool by the water evaporat-
ing from them. Plants, too, lose much water vapour and are cooled in the process, without doubt to their advantage in hot weather, when water loss is particularly great.
HOW THE PLANT DRINKS
WE have already noted that water provides the means for the entry of many substances into the plant. In order to understand how entry occurs it is necessary to devote some attention to a matter which, at first sight, has little to do with the plant. Somewhere about 1750, Nollet, a Dutchman, found that when a pig’s bladder was filled with alcohol, tied up firmly and placed in water, the bladder swelled, and sometimes burst; conversely, a bladder of water shrank when it was immersed in alcohol. It was evident that the two fluids were not passing through the bladder at the same rate, and that the water was going through faster than the alcohol. These experiments were the first of many made to discover the power possessed by some membranes to influence the passage of substances through them, and we now know of many sorts of membranes which let some substances pass through them easily, but limit or prevent the movement of others. Such membranes are semi-permeable membranes and they are of great significance in the economy of living creatures.
It is not very difficult to fasten a long glass tube to a thin bag of collodion in such a way that the joint is watertight. If we then fill the bag with a strong solution of sugar and hang it in water, we may make the following observations. First, we note that there is a steady rise in the level of the water in the glass tube and this may continue until there is a column of water several feet high in the upright tube. The rise does not continue indefinitely. It ceases after a time, the level stays constant and then slowly falls, until finally the liquid stands at the same level inside and outside the tube.
At the beginning of the experiment there was water outside, and water and sugar inside the bag of collodion. If samples are taken from the water outside while the experiment is going on it is found that every sample contains more sugar than did the preceding one; it is understood of course that equal samples are taken. At the end of the experiment, sampling shows that the sugar is now evenly shared in all the water present, both inside and outside the bag.
All the time that the experiment was going on, water was
passing easily through the walls of the bag in either direction, but sugar was passing much more slowly. Consequently, for a time there was a strong solution of sugar inside and a weak solution outside. Under these circumstances, water tended to pass from the weak to the strong solution, and this was shown by the rise of fluid in the upright tube above the bag. As the column rose in the tube it exerted an increasing downward pressure. This acted against the accumulation of water in the bag, since it tended to force water out.
Presently a point was reached where the passage of the water into the bag was just balanced by the amount of water being pressed out, partly by the column in the upright tube, and partly by the resistance to stretching offered by the somewhat elastic collodion bag. Sugar continued to pass slowly out of the bag, so that the two solutions, inside and outside, were approaching a point where they were of equal strength. As this went on, the pressures expelling water from the bag became more and more effective, and the loss of water was indicated by the slow fall of the column. Finally, when the sugar was evenly distributed, levels were the same inside and outside, since the apparatus no longer contained two solutions of different strength, separated by a semi-permeable membrane.
The experiment may be made in another way. A solution of sugar is placed in a bag of collodion which is then sealed up and sunk in water. At first, water passes readily into the strong solution, the bag swells and its wall is stretched. A point is slowly reached where the bag will stretch no more; then, provided the bag does not burst, nothing appears to happen for a time. As sugar is slowly lost from the bag, internal pressure falls and the bag continues to contract until it is no longer stretched; when this point is reached, samples will show that the sugar is evenly distributed throughout all the water present. It is evident that in experiments of this kind we are dealing with a balance of pressures.
WATCHING THE ROOT HAIRS GROW
E return to the plant. Let us suppose that we line a glass pot with blotting paper, fill the pot with clean sand, and push some mustardbetween the pot and the paper; we then damp the sand and put the whole thing in a warm place. After a few days the germinate and produce downwardly directed roots. These show a smooth,
tapering tip, and a little behind this is a length ofcovered by long, delicate white hairs. If we watch the preparation for a few days we see that as the elongates, the hairs towards the base shrivel and are replaced by fresh hairs formed nearer to the tip. The general effect is that as the tip of the root grows downwards, the zone of hairs also moves downwards, always keeping at about the same distance behind the tip.
The microscope shows that each hair is an outgrowth from one of the surface cells of the root. The hair is part of the cell, and not a separate structure. One wall covers the whole and the lining of cytoplasm and the vacuole run out into the hair. The superficial cells of the fairly young parts of the root, with the hairs that they bear, form the apparatus used by the plant to collect soil water. Incidentally, care must be taken to distinguish clearly between root hairs and very fine rootlets; the latter consist of many cells and may themselves bear root hairs.
Mustard seeds, treated in the manner indicated, give very clear specimens of root hairs, but when plants are grown in soil, root hairs are not always easy to find, for they are often short, and are always closely applied to the particles of the soil. Since the root hairs have sticky walls it is difficult to wash away the soil without destroying the hairs.
ROOT HAIRS AS COLLECTORS OF RAW MATERIAL THE root hairs, closely applied to the soil particles, are the chief absorptive organs of the plant. They occur only on young roots, generally on restricted areas just behind the tips. Older parts of the root, over which the impermeable corky layer is forming or has formed, have lost their root hairs.
The particles of a normal soil are surrounded by a thin film of water which holds many substances in solution. Root hairs make intimate contact with the soil particles, often becoming moulded to them. Each cell, with its root hair, is closely comparable with the collodion bag as a device for absorption, with two important differences. In the bag the collodion wall acts as the semi-permeable membrane; in the cell the wall affords support to the lining of cytoplasm inside, and contributes the elastic part of the combination, but it is not a semi-permeable membrane. The lining of cytoplasm is the semi-permeable membrane, and—the second
important difference—it is alive, and can only act as a semipermeable membrane as long as it is alive. If roots are killed by heat or by poisons, there may be no visible difference between their cells and those of living roots, but experiment shows that the root is no longer an efficient absorbing mechanism.
The soil water of fertile soils is not so strong a solution as is the fluid in the vacuoles of the cells of roots growing in the soil. Consequently, water and other substances pass from the weak solution outside to the stronger solution inside. Everything goes in in solution, for the roots cannot take in solid material. As the cells absorb water they swell and the cell wall is stretched. The power of the cells to continue to absorb is determined by the balance set up between the tendency of material to pass into the cell because of the stronger cell sap inside, and the resistance offered to stretching by the walls of the cell. If this balance is exactly reached, absorption will cease, but it is probable that this never happens. The process by which a plant takes in food through its roots is known as Osmosis.
The entry of water into the root hairs and superficial cells of the root is only the beginning of the story; we must still consider how the material is distributed to the rest of the plant. Inside the young root there are many thin-walled cells which, except that they bear no hairs, are very like the superficial cells, and have the same power of taking in watery solutions. Now, the cells on the outside of the root are closest to the source of supply, and it is therefore a fair assumption that, of all the cells of the root, they are the most likely to be nearest to the condition of balance where the pressure of the contracting walls opposes most resistance to the entry of more water. The cells next inside them will not be quite so close to this condition, and they will have a somewhat greater demand for water; this they will satisfy as far as they can by taking water from the superficial cells, and with the water, other substances also. Cells still further within the root will be still less satisfied, and so will take water from those just outside them, and, as we pass deeper and deeper into the root we find cells whose demands for water are less and less easily met. The effect of all this will be that water will be always passing inwards from the better supplied to the worse supplied cells; consequently, even the cells on the outside of the root will never quite reach a stage where they
retain their full water content, and will therefore continue to take it from the soil.
In this way we may picture the passage of material from the soil into and through the thin-walled living cells of the root, but we presently come to a point, deep in the root, where there is a transfer from the living cells to the dead vessels of the wood, by means of which the water passes into the general body of the plant; at present, we have no satisfactory explanation of the manner in which this is done.
HOW WATER RISES FROM ROOTS TO LEAVES w
E do know something, however, of the ascent of water from the roots to the stems and leaves. We have already seen that dyes travel upwards in the wood, showing this to be the path followed by watery solutions in the plant, but before we can deal with this matter we have once more to leave the plant for a time.
Paradoxical as it may appear, a thread of water, suitably supported, can take and can transmit a pull much in the same way as a wire can do so. This may be shown in the following way. A glass tube about five feet long is provided with an open, expanded end. This is loosely packed with wet plaster of Paris, which, as it sets, swells and fills the expansion. The tube is then filled with water containing no air in solution, and set vertically with its lower end dipping into mercury. Water evaporates from the plaster plug; it is replaced by water from the tube below, and mercury rises into the tube to occupy the space vacated by the evaporated water.
As evaporation continues the mercury rises higher and higher. At first, this rise has no special significance, for it could be due to the weight of the air pressing up the mercury, as it does in a mercury barometer. In time, however, if the apparatus has been well prepared, it is found that the column of mercury rises until it stands much higher than the column in a barometer placed by the side of the apparatus. Since the column in the barometer is supported by the weight of the air, and since at any given time the column in the barometer shows the maximum effect that the weight of the air can exert, it follows that the extra length of the mercury column in our apparatus is being supported by something other than the weight of the air.
It is a fair conclusion that the mercury is being pulled up
by the thread of water above it, and that this in its turn is supported by the numerous fine threads of water in the plaster plug, these ending at the upper surface of the plaster where evaporation is in progress. Calculation and experiment have shown that threads of water may be sufficiently strong to take pulls greater than those necessary to lift water from soil level to the tops of the highest trees. In a broad way, the plant may be directly compared with the simple apparatus we have just considered.
THE PLANTS WAY OF MAKING UP LOSSES
IT is easy to show that plants are always losing water vapour from their exposed surfaces, and it is equally easy to show that water is always rising from below to replace losses. The leaves and young branches of the plant provide an evaporating surface comparable with, and no doubt much more efficient than, the surface of the plaster plug, and the system of conducting vessels inside the plant forms an elaborate counterpart to the upright glass tube. It seems highly probable that, in essentials, the water rises in the plant in the same kind of way that it rises in the glass tube.
It follows that the plant has a water supply system by which losses are automatically replaced, provided always that there is sufficient water in the soil to allow the stock in the plant to be maintained, and provided also that the plant can take in that water as fast as it loses water from its leaves. The mechanism is not perfect. Any garden, in the middle of a hot day, will show some plants that look limp; they are flagging because they are losing water faster than they are taking it in. The same plants may, however, be stiff and fresh looking in the late evening or the early morning, even if they have not been watered in the meantime.
In the heat of the day plants often lose water faster than they take it in, their soft cells shrink, and the leaves and young stems flag; expenditure exceeds income. With cooler conditions, expenditure decreases, income and expenditure balance, and the plant receives and retains enough water to keep it fresh and rigid. Sometimes, income exceeds expenditure by evaporation, and water which is in excess of requirements may then pass out of the plant in liquid form; this accounts for much of the dew on grass in the early morning; the water has not come from the air but has been exuded from the tips of the leaves.
The relation between income and expenditure of water is of vital importance to the plant. Growth is impossible unless the young growing parts are fully charged with water; a plant flagging from insufficiency of water is not growing. If a plant is wilted for several hours a day in hot weather the loss of growing time may be serious—so serious indeed that it has been found worth while to provide artificial shading for some crops in the tropics. The shaded plants gain many hours of growth and the improvement in the crop more than pays for the expense of providing the shade.
WHEN THE PLANT MUST CHOOSE BETWEEN FOOD AND WATER PLANTS lose water chiefly through their stomata. These may close when the water supply is scanty, and so exert an automatic check on loss, but they do not act very accurately, and since stomata tend to remain open in bright light they may cease to act as checks on the loss of water from the plant at the very time when a check is most needed. Further, the stomata are essential to two other important activities of the plant, and for these purposes they must remain open. The manufacture of food, at any rate in its preliminary stages, can only go on in the light, and the plant uses carbon dioxide taken in from the air through the stomata. If, therefore, the stomata are closed in the daytime the intake of carbon dioxide must be hindered, and this must limit the making of food.
On the whole, the average plant does not seem well equipped for controlling its water supplies. It must have plentiful supplies of water for all its many activities, and it must obtain carbon dioxide, and also oxygen from the air for the manufacture and utilisation of its food. The gases from the atmosphere enter the plant through the stomata which must be open if the gases are to pass in; but which, if they are open, may allow too much water to pass. Somehow, therefore, the plant has to strike a balance between these two processes. It is well known that many plants have structural devices which limit the loss of water, and the widespread occurrence of such devices indicates how acute the water problem must be. The loss of water from the plant is not a mere matter of evaporation. If oneon a twig is killed by chloroform, and then the whole twig is cut and put to dry in the air, the killed leaf loses water more quickly than the others. Evidently living material, even when it is dying from drought, can
impose some check on straightforward evaporation. There is good reason to conclude that the loss of water from the plant is chiefly responsible for the maintenance of the stream of water that passes through the plant. If plants lost water only by evaporation, there could hardly be a water current through a plant submerged in water, as a submerged plant could not evaporate water into the water around it. By means of dyes it can be shown that there is a stream of water passing through plants growing submerged in water, and this suggests that the plants are losing water by some means other than evaporation, and that probably land plants also have the same means at their disposal. Probably both excretion and evaporation are concerned, and indeed evaporation must be concerned, since dead plants left standing in soil continue to take water from the soil and give it off into the air. In those plants the usual mechanism for the collection of water has been destroyed, and it appears that the dead cell walls act as a sort of wick, lifting water from the soil to regions where evaporation is possible.
Plants do not lose water to the air so rapidly as water evaporates from an open surface. In one experiment, exposed water evaporated at the rate of about three pints from each square yard in twenty-four hours. At the same time, and under the same conditions, an equal area of beech leaves lost about a quarter of a pint of water. This does not seem much, but it appears that beech trees covering the ground fairly densely may transfer in a day nearly a quarter of a million gallons of water to the air from an acre of ground.