The Life of the Plant

kinds of cells : in the upper part of the leaf the cells are cylindrical in shape and arranged like stakes in a palisade, vertical to the surface of the leaf. In the rest of the leaf the cells are of varied and irregular form, with considerable spaces between them. All the cells of the ground tissue, especially those of the palisade layer, contain minute green granules ; we shall return to them presently, only noting here by the way that the leaf, like all green parts, is in itself colourless, and owes its colour to those granules. The epidermis which we see here in surface view and in section consists of flat, oblong, almost tabular cells. Between them on the lower surface of the leaf there are scattered organs of a special form — one of them happens to be cut in half at the edge of the section. We notice that it consists of two cells, each curved into an arch, forming between them an oblong slit. These are apertures, like ventilators, in the lower

epidermis, leading into the leaf from the outside air ; they are called stomata. Their number is enormous. One lime leaf contained more than a million of them ; and this number need excite no dismay, because the method of reckoning the stomata is very simple and precise. The restriction of these organs mainly to the lower surface of leaves explains why it is that, in experi- ments such as the one described above, gas is observed to be given off in most cases on the lower surface of the leaves. We shall return later on to these stomata.

Let us see what kind of gas is given off by a leaf when sunlight acts upon it. For this purpose we must wait until a sufficient quantity of gas is accumulated under the glass bell (fig. 34) . Then we take out the cork and introduce a slightly smouldering splinter into the neck of the bell. It instantly glows and bursts into flame, scattering sparks in all directions. This is the regular test for oxygen. Therefore the air given off by leaves is oxygen, or a gas very rich in oxygen. We have already seen that this experiment is successful only when carbonic acid is dissolved in the water. Is there not some connection then between the presence of carbonic acid and the formation of oxygen ? Evidently our experiment does not give a sufficient answer to the question. In order to find out what happens to the carbonic acid, we make another experiment. Let us take an apparatus consisting of a tube in the shape of a horse-shoe (fig. 36), with one side closed and the other with a stopper. 1 We pour some water into the tube and introduce carbonic acid in such a way as to fill the space in the left-hand closed end of the tube up to the movable pointer on the stand (as is shown on the figure) . We lower the long leaf of a cereal into the right-hand open end of the tube, and having filled the tube up to the very top with water close it with the

1 Known as Hofmann's apparatus, very much used in the practical teaching of chemistry. stopper, taking care not to leave air bubbles below it. Then we place the apparatus in the light. As before, the leaf becomes covered with minute bubbles, and these, after reaching a certain size, rise to the upper part of the tube ; here there accumulates a per- ceptible and ever-increasing quantity of gas. While the volume of gas constantly increases in the right-hand side of the tube the volume of car- bonic acid diminishes in the left. At the moment when the level of the water in the right-hand side reaches a' the level in the left-hand side will also be at a. Evidently the gas in the right-hand side is oxygen ; but to be quite sure, we can withdraw the stopper and test the gas with a splinter. Once having proved that it is indeed oxygen, we refill the tube with water and repeat the experiment. Again we get a certain quantity of oxygen while a corresponding quantity of carbonic acid disappears from the other side of the tube. We know it is carbonic acid because we introduced it our- selves, but for greater certainty, after several similar experiments, we fill up the right-hand side of the tube with water, replace the stopper and, turning the whole tube upside down, transfer the remaining gas from the left into the right-hand side. If we test this gas we find not only that a smouldering splinter does not glow in it but that even a burning one will be extinguished. This means that the gas was and remains carbonic acid. What happens in this experiment is easily understood ; carbonic acid continually dissolves in water in the left- hand side of the tube ; but the solution in the right arm

by the leaf from one part of the tube into the other, carbonic acid changes into oxygen. During this process of decomposition the volume of carbonic acid which disappears at one end of the tube and the volume of oxygen which appears at the other end are approxi- is decomposed under the influence of the leaf and oxygen is given off. Consequently a fresh quantity of carbonic acid dissolves and so on. This experiment proves very clearly, though not very exactly, that in passing

mately equal. We learn from chemistry that when carbon burns in oxygen and carbonic acid is formed a given volume of oxygen forms an equal volume of car- bonic acid. In our experiment, therefore, the carbonic acid is entirely decomposed ; all its oxygen is given off, while all its carbon remains in the plant. We can perform this experiment still more satisfac- torily by using some large floating leaves of the water- lily. Water plants have the peculiarity that their stomata are scattered over the upper surface of the lamina which is in contact with the air ; its air cavities are continuous with similar cavities in the petiole. We sink the lamina in water under a piece of glass, which is shown in the figure (fig. 37) behind the vessel, and introduce the stalk into the tube filled with water. Let us commence by using boiled water, i.e. water without any trace of carbonic acid. We do not see anything taking place. We add some mineral water containing carbonic acid, and cover the vessel with a piece of cardboard ; and again we do not observe anything. But the moment we remove the cardboard and expose the apparatus to sunlight, a current of bubbles rushes from the cut end of the petiole. We collect the gas and measure its volume : from what we already know we decide that this gas, which is given off exclusively in presence of carbonic acid and in sunlight, contains oxygen. Let us gently draw the petiole out of the tube, close the latter with a finger, reverse it, and introduce into it a glowing splinter. It burns with a bright flame. We remove the splinter, extinguish the flame, and repeat the experiment ten times over with the same result, and conclude that this is actually a gas rich in oxygen. A leaf exposed to sunlight transforms carbonic acid into oxygen.

We have so far been experimenting in the presence of sunlight, but this phenomenon of the breaking up of carbonic acid by the plant can be demonstrated any time to a large audience by means of an improved magic lantern much in vogue nowadays— which throws on the screen a magnified image of the plant and the tube in which the gas given off by the plant is investigated. Here is one of the most convenient forms of this ex- periment (fig. 38) . A small glass cell made of a glass tube bent into the shape of a horse-shoe (e) and two glass plates (d) forms a kind of aqua- rium in which common water plants are grown. If we arrange for a sufficiently strong source of light, either sunlight, electric, or lime- light, we can throw upon the

seven or more feet across of this minute ac^uarium (diminished by one half in fig. 38), and we observe at all places where the stems or the petioles of the leaves are cut across that a curious phenomenon is exhibited by the plant: it gives off bubbles of oxygen in place of carbonic, acid it has decomposed . 1 In order that this may happen we must have a strong light, and the water must contain carbonic acid ; in the absence of either of these two conditions the giving off of bubbles will not be observed ; but, on the other hand, if the sunlight or the electric light is strong enough the bubbles rise in a continuous stream, like a string of beads. We have now to prove that this gas is oxygen, or rather that it is rich in oxygen, since it generally contains an admixture of other gases that are present in solu- tion in the water. For this purpose we pass the ends of several branches under the widened end of the graduated tube (a), which is full of water like that in the basin, and collect the gas which comes off. This tube is tightly closed at its narrow end with a stopper in the form of a glass rod ( c ) which passes right through its wider part ( b ). When a sufficient quantity of gas is thus collected we proceed to test it . 2 It might be oxygen given off by the plant, or atmospheric air, or carbonic acid dissolved in the water and therefore capable of penetrating into the cavities of the plant. We pour some solution of caustic alkali into the wide funnel- like upper part of the tube and gently raise the glass rod so that the alkali may pass into the lower part which is marked with divisions. Alkali, as we already know, absorbs carbonic acid . 3 At the beginning the tube showed, say, fifty divisions of gas ; there will be only forty-eight left after the absorption of the carbonic acid. Then we pour another substance into the funnel, a solution of so-called pyrogallic acid, which has the

1 Water plants have no stomata on their submerged parts but are provided instead with internal air cavities, into which oxygen diffuses, coming off later in bubbles through any chance apertures. 2 The circles on Fig. 37 represent the circles of light thrown on the screen by the magic lantern ; in order to get a larger image we look first at one part of the apparatus and then at the other. property of absorbing oxygen, becoming dark brown in colour during the process. We lift up the rod, and as soon as the first drops of this liquid penetrate into the tube and come into contact with the gas contained in it, the liquid becomes coloured and the volume of the gas rapidly decreases. In the end we shall have something like fifteen divisions of gas instead of forty-eight. This remaining gas is nitrogen, and this means that we had thirty- three parts of oxygen. Not more than five parts of oxygen will have penetrated with the fifteen parts of nitrogen in the form of atmospheric air, so that twenty- eight out of thirty-three divisions represent oxygen given off by the plant owing to the decomposition of carbonic acid.

The apparatus we have just described, together with electric light, makes it possible for us to-day to demon- strate before an audience on a dark winter evening a phenomenon that generally takes place in nature only in the daytime during the warm season of the year ; we can throw it upon the screen as easily as an ordinary lantern slide. Of course the apparatus can also be used without the lantern as a simple and convenient method of investigation.

So far we have been studying the decomposition of carbonic acid by plants submerged in water. This form of experiment is the most convenient for a pre- liminary study of this phenomenon, because it clearly shows the giving off of gases by the plant. We must now ascertain whether the same kind of decomposition also takes place as the result of contact of the leaf with air which contains carbonic acid. Here is a very rough and simple form of experiment, by which the phenomenon was demonstrated for the first time a hundred years ago by the great chemist Priestley. We take a glass jar (as in fig. 34), pour into it a small quantity of water and fix at the bottom of the jar a lighted bit of candle, of course large enough to stick up

out of the water. We cover this bit of candle with a glass bell (similar to the one in fig. 34) in such a way that its edges dip into the water at the bottom of the jar. The air in the jar will be thus enclosed and isolated by a layer of water from the external atmosphere. The bit of candle will go on burning under the glass bell for a while and then die out. This means that the oxygen necessary for the process of combustion has already been exhausted from the air under the glass bell, and has been replaced by carbonic acid, as the result of the com- bustion. If at this moment we were to introduce (through the neck of the bell) a burning splinter, it would certainly die out in the same way as the candle. But if we carefully introduce some leaves or a green branch into the glass bell through the water, and place the whole apparatus for a long time out in the light, we shall find eventually that the splinter will continue to bum under the bell — which will mean that oxygen has reappeared. - Apparently the plant has transformed into oxygen the carbonic acid formed by the burning candle. We might have given another form to the experiment : we might have put a mouse under the bell instead of the candle and taken its death as a proof of the fact that the air under the bell no longer con- tained a sufficient quantity of oxygen for respiration. By then introducing a green branch under it and exposing it to the sun, we could have restored to the air its power of supporting respiration by giving back to it oxygen.

Hitherto in all our experiments we have studied only qualitatively the transformation of carbonic acid into oxygen under the influence of the plant, or rather we have estimated the relation between the disappearance of carbonic acid and the appearance of oxygen only approxi- mately. For the quantitative study of this phenomenon science is provided with other methods incomparably more precise, but the description of them would be out of place here on account of the technical details involved. I will only say that in those methods we make use of the property of carbonic acid, already familiar to us, of being absorbed by caustic alkali. A definite amount of carbonic acid is supplied to a plant, or to a single leaf, placed in a graduated glass tube closed at its upper end (as in fig. 37), which is then exposed to the light. The experiment being over, we determine, by means of an alkali, the quantity of carbonic acid remaining in the tube. Knowing how much gas was introduced, and how much is left, we can tell how much has disappeared.

This method has helped to solve many interest- ing problems ; for instance, that of determining the percentage of carbonic acid in the air most favourable for the plant. The experiment proves it to be some- thing like eight per cent. ; a greater percentage is evidently noxious to the plant. Another question dealing with the same point deserves our attention. We have proved experimentally that plants decompose carbonic acid supplied to them in our apparatus ; but it is questionable whether we are entitled to infer from these experiments that a plant in its natural condition is also able to decom- pose the carbonic acid of the air. You remember that we supplied in our experiments comparatively large quantities of carbonic acid to the plant, as a rule in the proportion of several parts in a hundred, whereas the atmosphere contains only a few ten-thousandth parts of it. It may seem unlikely that a plant should be able to discover and assimilate the particles of car- bonic acid so sparingly diffused in the air. In order to settle this question, the renowned French chemist, Boussingault, made the experiment which we will now demonstrate. We take a large glass globe with three apertures (fig. 39). A vine branch with leaves is introduced through the lower opening into

the globe, while still connected with the vine, and therefore under perfectly natural conditions. With the help of an aspirator a continual stream of fresh external air, as is shown by the arrows on the figure, is slowly passed, first through the glass bell and then through the apparatus A connected with it. We mark on the aspirator the volume of air passed through the globe during the experiment ; we also analyse the air of the place where the experiment is made, and

determine the proportion of carbonic acid contained in it. When once we know the amount of air which has been passed through the globe containing the plant, and the proportion of carbonic acid contained in the air, we can easily determine the quantity of carbonic acid that has entered it. We have now only to determine the quantity of carbonic acid that has come out of the globe in order to find out how much of it has been decom- posed by the leaves. The apparatus A serves this purpose. I will briefly describe its significance, again leaving out all technical details, since my point is merely to explain the fundamental idea of the experiment and

not the way it is carried out. The main part of this apparatus consists of two bent glass tubes ( a ) through which the stream of aspirated air passes, and which are meant to absorb the carbonic acid. For this purpose one of them contains small pieces of caustic alkali. Caustic alkali will become heavier in absorbing carbonic acid ; and therefore what we have to do is to unfasten the part of the apparatus marked a and to weigh it before and after the experiment. The increase in weight will indicate the quantity of carbonic acid remaining in the air after it passes out of the globe. It happened that under favourable conditions of illumina- tion the air came out of the ball almost deprived of carbonic acid. Consequently, in passing over the green surface of the illuminated plant, the air has left behind it almost all its carbonic acid ; in spite of the fact that its particles are so scantily diffused in the atmosphere, lost, so to speak, in the mass of its other components. This result will become more comprehensible if we recall the diffusion of carbonic acid into our artificial cell . 1 Then carbonic acid pressed into the cell onty because the cell contained none of this gas ; but in the leaf also, since it is continually decomposed, it dis- appears, without, so to speak, leaving any traces ; and therefore, according to the laws of diffusion, it must continually be replaced by fresh quantities from the atmosphere.

Boussingault’s classical experiment was made more than half a century ago, and has hardly ever been repeated since. Lately a distinguished English chemist, Horace Brown, undertook a whole series of similar ex- periments of an improved form, by which he succeeded in removing any remaining doubts upon the subject. The insignificance of the total area of the minute openings afforded by the stomata made it still seem incomprehensible, however, that a plant should succeed

in extracting from the atmosphere the meagre propor- tion of carbonic acid contained in it, until it was dis- covered that, owing to peculiarities in the diffusion of gases, demonstrated for the first time in Brown’s experiments, carbonic acid enters the leaf through these stomatal apertures almost in the same quantity as it would diffuse had the whole surface of the leaf taken part in the process of absorption. Horace Brown wittily remarked, in commenting upon this remarkable dis- covery, that a plant evidently knows more about physics than we are inclined to admit ! His experiments also showed that if the percentage of carbonic acid in the air were increased five times, from the usual proportion of rdnnr to roW, the quantity decomposed by the plant also increased to about five times as much ; a result which justifies us in concluding with greater certainty than before, how successfully the plant can use up the atmospheric carbonic acid, seemingly available in such minute quantities.

On coming into contact with a green plant in sun- light carbonic acid is decomposed : its oxygen is set free, while its carbon is deposited in the plant. Let us try to trace the further fate of this carbon inside the plant. Let us again turn to the microscope. Almost without exception every observation of this phenomenon, and every experiment, point to the conclusion that this process takes place only in the green parts of the plant. We can really say with certainty that if an organ is not green, it does not decompose carbonic acid ; while if it does decompose it, the green colouring matter when not directly visible is hidden by other colouring sub- stances. These green granules, which contain this green colouring matter, called chlorophyll , serve as organs in which the decomposition of carbonic acid takes place.

Apart from chlorophyll there is no assimilation of carbon in the plant. This chlorophyll is found in cells in bodies of varied form, in small granules, or in disc-shaped (fig. 40) 1 or band-shaped bodies (fig. 58). These bodies are called chloroplasts. If we keep a plant in the dark for a time and then examine these chloroplasts under a microscope their structure appears quite uniform (fig. 40 a on the left) ; but if we then set the plant in the light, we find after a occasionally after even a few minutes, that tiny granules have appeared in them (fig.

40 a on the right) . In some plants these granules increase in size, in time protrude them- selves and continue to grow on the side in contact with the chloroplast (fig. 40 b ) . They then have a characteristic stratified appearance, and we recognise them as grains of starch. We need not, how- ever, wait until they develop ; we can detect starch in a granule as small as a pin point by colouring it blue with iodine, a reaction with which we are already familiar.

Starch grains, then, are formed in the chloroplasts, and continue to grow where they are in contact with chlorophyll. We can easily prove that the formation of starch is connected with the decomposition of carbonic acid. To begin with, no starch is formed in the chloro- plasts when the plant is not supplied with carbonic acid ; nor, in the second place, is there any formation of starch 1 Fig 40 a. On the left, chloroplasts without starch ; on the right — with starch grains inside them, b Single large starch grains.

in the dark. Thus starch is formed and carbonic acid decomposed only in presence of chlorophyll ; both processes are conditioned by the presence of light, and only when carbonic acid is present and being decomposed does any starch appear. It therefore seems more and more evident that starch is the very substance we are looking for, which is formed out of the carbon in carbonic acid. Its com- position supports the conclusion : like other carbo- hydrates it can be regarded, as the name suggests, as made up of carbon and water. Cells always contain water ; so we may explain the origin of a carbohydrate by supposing water and carbonic acid to combine, and at the same time all the oxygen of the latter to be withdrawn. Such is the course of events as far as they are known to us ; but we must remember that our in- formation on this subject is as yet far from being complete. We know that a cell receives carbonic acid and water, gives off oxygen and forms a carbohydrate ; we know that these processes must have a causal connection, that they take place in the same chloro- plast and follow upon each other with striking rapidity. As to how it all happens, where the oxygen comes from, whether it is produced entirely from carbonic acid or also partly from water (which is more probable), and whether other simpler or perhaps more complicated combinations precede the formation of starch — up to the present we do not know anything about these questions, and it would certainly be out of place to enter here into speculative comment upon phenomena as yet unexplained by science.

It is far more important to remember that, in observ- ing these processes of carbonic acid decomposition and starch formation, we are witnessing one of the most important phenomena of life, one on which depends not only the life of the leaf and of the plant, but the life of the whole organic world. This transformation of the simple inorganic substances carbonic acid and water into the organic substance starch represents the sole natural process by which organic matter is formed upon our planet. All organic substances, however diverse they may be, and wherever they are found, whether in plant, animal, or man, had their origin in the leaf, have been formed from substances manufactured by the leaf. Nature does not possess any other laboratory for the formation of organic matter, except the leaf, or, more strictly, the chloroplast. In every other organ and organism, organic matter is merely transformed ; only here does it arise anew from inorganic matter.

From starch, for instance, is formed soluble sugar which reaches the furthest parts of the plant by passing from one cell to another. From this sugar the hard skeleton of the plant called cellulose is formed. And, lastly, from this same sugar and the inorganic substance, ammonia, the most complicated organic substances, such as proteids, can be formed. Thus the leaf assimilates carbon, and within itself forms an organic compound with which not only the plant itself but the whole animal kingdom is also supplied.

We have at last arrived at the source of carbon in the plant, and have explained how it penetrates into it. We have thus explained the first stage of the phenomenon of nutrition ; we now know whence and by what means all the elements that enter into the composition of the plant are obtained, carbon being the last of the series. So far we have been examining the activity of the leaf and of the plant in general, exclusively from the chemical point of view — from the point of view of the transformation of matter. Starting from the funda-

mental law of chemistry, that matter is neither created nor destroyed, we tried to discover the sources of the matter composing the plant, the way in which it penetrates into the plant, and the changes it undergoes during the process. But the vegetable body is a storehouse, so to speak, of energy as well as matter — heat-energy for in- stance. Burning a single seed of a birch-tree will not warm our frozen hands for a moment, whereas a birch- tree a hundred years old will serve to heat our stove for many a day. A birch-tree therefore accumulates heat during its life-time, which we use as such, or else as a source of mechanical force.

Where does this heat, this energy, come from ? We raised a similar question with regard to matter. Just as we admit that matter neither disappears nor is created, so do we also assume that neither does energy disappear, nor is it created. In fact just as the chemists of last century came to the conclusion that matter is indestructible, so also the physicists of the present day have come to the conclusion that energy is never destroyed. The different sources of energy may suffer endless change, passing from one form into another, or become concealed in a state of tension ; but they are never destroyed, never created anew.

What is this latent energy, this heat concealed in our fuel, and whence does it come, since it could not arise spontaneously ? In order to explain this we must glance again at the chemical phenomena already familiar to us that take place in the leaf, but this time from the purely physical point of view of the trans- formation of energy involved. All chemical phenomena can be divided into two categories : those in which heat, light, electricity — in a word energy — appears, is given off ; and those in which energy disappears, is absorbed. Phenomena of the former category take place spontaneously, or require but an