The Life of the Plant

as it appears to ns now ; hence we are the more amazed at the brilliant deduction, at the ingenious conception to which the world owes one of its greatest discoveries in biology. Priestley proved by a series of experiments that continual combustion, or continual respiration in a limited volume of air, makes that air unfit for further combustion, for further respiration : in it a lighted candle goes out, an animal dies. Therefore, argued Priestley, all the atmosphere should become unfit for combus- tion, or for life ; yet the many centuries of the world’s existence testify to the contrary. Apparently there exists a process in Nature which restores this bad air into good air. Is this not due to plants ? On the 18th of August 1772 Priestley made the following experiment. He introduced under a glass bell over water, where a candle had previously gone out, or a mouse had died, a plant (mint), and kept it there for a time. The plant did not perish ; it even continued to develop, and when after a few days a mouse, or a burning candle, was again introduced under the glass bell, it appeared that the air had actually been renewed, that it had acquired once more the proper- ties of maintaining combustion and respiration. Hardly ever in any province of knowledge has a single experiment been followed by greater results. The same stroke demonstrated the most charac- teristic sides of the life of plants and animals, and the mutual relationship which exists between the two kingdoms of Nature. Priestley’s contemporaries appreciated the importance of this dis- covery.

The Royal Society conferred on him the coveted Copley medal ; and the President of the Society, Sir John Pringle, ex- pressed the importance of Priestley’s achievement in the following eloquent, though somewhat rhetorical, words: 'From this dis- covery,’ says he, ‘ we are assured that no vegetable grows in vain, but that, from the oak of the forest to the grass in the field, every individual plant is serviceable to mankind ; if not always distin- guished by some private virtue, yet making a part of the whole, which cleanses and purifies our atmosphere. In this the fragrant rose and deadly nightshade co-operate: nor is the herbage, nor the woods that flourish in the most remote and unpeopled regions, unprofitable to us, nor we to them ; considering how constantly the winds convey to them our vitiated air for our relief and for their nourishment.’ Priestley’s inference was that the plant restored air vitiated by respiration, and made it again

able to maintain respiration. His discovery of oxygen which soon followed, and the determination of the composition of carbonic acid, provided the explanation of the nature of this connexion between the two organic kingdoms. The animal inhales oxygen and exhales carbonic acid ; the plant inhales carbonic acid and exhales oxygen, retaining (precipitating) carbon. Plants and animals present a chemical antithesis. A series of further investigations showed that this process, which decomposes carbonic acid and restores good air, has yet another more important significance, which is that it provides the plant with food. Carbon remains inside the plant, forms its organic matter, and serves to build up its body. It follows that the carbonic acid of the atmosphere must be considered the main food of a plant. Although this function was long attributed to the black particles of the soil, i.e. to humus, the inadequacy of this view was established by exact experiments.

Priestley, however, had to experience one of the greatest dis- appointments that ever befell a scientist. He failed some time after, when he tried to repeat the experiment which made him so famous : he could not obtain his former results ; the plants persisted in refusing to decompose carbonic acid and set free oxygen. Although these disappointments did not shake his confidence in his earlier experiments, it became evident, never- theless, that some condition important for the experiment had been overlooked, owing to which fact the experiment could not be repeated. This condition neglected by Priestley was soon after discovered by Ingenhousz. In order to fully appre- ciate this discovery we shall dwell a little longer on the nature of the phenomenon itself.

Let us for the last time turn to our balls. We have been comparing chemical combination or combustion with the con- cussion of two balls against each other ; heat and light liberated during the process serve as a measure of the affinity or tension, i.e. of the mutual attraction of these bodies (represented in our illustration by the tension of the coils). In order to separate them again, to break up the connexion between them, in order to bring the balls into the former free position, we must on the contrary expend energy, and expend as much of it as is liberated at the moment of collision. Thus it becomes evident that a phenomenon contrary to that of combustion has to be accom-

panied not by liberation, or development of energy, but on the contrary by its absorption and expenditure. While combina- tion, i.e. combustion, takes place spontaneously, decomposi- tion requires the participation of an extraneous force* In order to burn down a piece of coal we must set fire to it, after which, it burns without any external assistance. We have noticed that in some cases coal can also burn spontaneously, when it comes into contact with the oxygen of the air. On the other hand, in order to decompose carbonic acid and water, to * un- burn * them, we must expose them to a very high temperature. Formerly it was supposed that the decomposition of such stable compounds was impossible without the co-operation of a third body, endowed with a still greater affinity for oxygen, and so able to sever that oxygen from hydrogen and carbon. But not so very long ago the attention of chemists was drawn to pheno- mena of decomposition, or dissociation, as they were called, apart from the action of any third body. In order to be dissociated, carbonic acid or water must be passed through red- hot tubes. Under the influence of the motion thus communicated to their particles, i.e. of heat, the connexion between them becomes loosened, so to speak; the compounds break down into their component parts, which must be immediately re- moved, lest on cooling they should recombine and prevent us from obtaining a complete separation. The amount of heat liberated at the moment of combination and absorbed at the moment of dissociation is strictly definite. There are exactly as many units of heat liberated during the oxidation of a pound of carbon into carbonic acid, as there are units of heat absorbed during the liberation of this pound of carbon from carbonic acid.

Thus we come to the conclusion that the dissociation of carbonic acid, which takes place in the plant, must be accompanied by the absorption of heat, of energy speaking generally, and also that the amount of carbon precipitated in this way in the plant may serve as a measure for this absorption. But whence will the plant obtain this energy so essential to it ? It cannot create it, because energy cannot be created. Apparently it must acquire it from without. The dissociation of carbonic acid in a plant can take place only on condition that there is a con- tinual supply of energy from an outside source. This was the condition which escaped Priestley's attention, and the discovery

of which made Ingenhousz famous . 1 Ingenhousz showed that the dissociation of carbonic acid inside the plant takes place exclusively in the sunlight. In Priestley's later experiments the plants probably did not . get enough sunlight, and therefore did not dissociate carbonic acid. Sunshine, the rays of the sun, are the very energy which loosens and separates the particles of carbon from those of oxygen during the decomposition of carbonic acid. Such an expression as f sunshine is a source of energy’ may sound strange at first. We know from daily experience how pleasant it is to warm oneself in the sun, and yet a long chain of arguments and calculations is necessary to persuade us that this is not only a source of energy, but even a very considerable source of energy — moreover, that this is almost the only source of energy used by man. In fact, apart from the energy of the tide, used in some parts of Europe, and which depends on the attraction of the moon (and also of the sun), all the other motive forces, all other sources of energy directly or indirectly depend upon the energy of sunlight. The flow of water in rivers and the circulation of air in the atmosphere, which set in motion our mills, are due to the sun. The latent energy of fuel, as we have already seen and shall presently see in greater detail, proceeds from the sun. Even phenomena so remote as those of electricity, which we use for practical purposes, can be connected with the activity of the sun. It is the sun that shines in a Voltaic arc obtained by means of a galvanic battery, which is easy to prove. The electric current which makes the carbon white-hot is the result of the oxidation in the batteries of a certain amount of metallic zinc. This zinc, however, is not found in Nature as a metal ; it is generally found in combination with oxygen, i.e, entirely burnt down. In order to deoxidise it, to restore its capacity for combustion, oxygen must be removed from it. This is done- by means of coal, which combines with the oxygen of the zinc ore, and burns down to carbonic acid.

But this coal, be it pit coal or wood charcoal, has been derived in the plant from carbonic acid, dissociated by sunlight. Thus it is that the rays of the sun 1 Such is the current opinion, but lately, after a careful study of the question, I have come to the conclusion that Ingenhousz’ priority is more than doubtful, and that the fact of the dependence of this process on sunlight was discovered by Priestley. are connected with those of the electric light. The visible, kinetic energy of the sunlight, expended in the decomposition of carbonic acid in plants, takes the form of latent potential energy, con- cealed in carbon after it has been liberated from the carbonic acid. This potential energy of carbon then passes over to the zinc during the process of deoxidation of the zinc ore ; carbon burns down and metallic zinc is obtained capable of combus- tion. Zinc oxidises in the galvanic battery, burns down, and its potential energy transforms itself into the actual energy of the electric current, appearing as light in the white-hot carbons. Such is the complicated chain of transformations of energy, which connect phenomena that take place on our planet with the activity of the sun. We can, however, form a more definite idea as to the significance of the radiation of energy from the sun, by making an approximate calculation of the amount of energy afforded by the sun. We can determine the number of units of heat cast by the sun upon a given area of our planet ; and then, knowing the mechanical equivalent of heat, we can express the energy of sunlight in units of mechanical work. According to the calculations of Mouchot the sunlight that falls during eight or ten hours on a bright day upon the surface of a square metre in Paris can do work approxi- mately equal to one horse-power. Ericsson calculated that if all the heat of the sun that falls upon the roofs of Philadelphia were to be used, it would amount to the force of 5000 steam engines, each of 20 horse -power.

Further, on calculating the enormous amount of heat that falls upon the earth, he exclaims : * Archimedes undertook to lift the world by a lever, whereas I maintain that by concentrating the heat of the sun we could obtain a force capable of arresting the motion of the earth/ Both Mouchot and Ericsson, however, did not confine themselves to calculations : they made experiments, which clearly demonstrated the stores of energy presented by the rays of the sun. Mouchot made several very simple kinds of apparatus, in which it was possible to boil water, soup and vegetables as well as to bake bread exclusively by means of the heat of the sun. In the end he also made some steam and hot- air engines set in motion by the sun. Of all the applications of sunlight suggested by Mouchot, his pumps for irrigating fields are perhaps the most curious. They not only act by

means of energy obtained free of cost, but also work most effec- tually. They are regulated according to the very need for water : the amount of water is regulated by the amount of the radiant energy of the sun, hence also according to the severity of the drought . 1 We can prove easily enough from what has been said that sunlight is a powerful source of energy, and that this very energy decomposes carbonic acid in plants. The plant is unable to provide itself with the energy necessary for the purpose ; it only plays the part of a mechanism, if I may so say, of a connector for transmitting the energy of the sun.

The plant therefore presents a regular contrast to the animal from the physical as well as the chemical point of view. The life of the plant consists in a continual transformation of the energy of sunlight into latent chemical energy ; while the life of the animal, on the contrary, manifests the transformation of chemical latent energy into heat and motion. The spring is wound up in the one to unwind in the other. We should be wrong, however, if we imagined that this function of the sunlight became intelligible the moment Ingenhousz dis- covered its participation in the process of the dissociation of carbonic acid. More than half a century had to pass before the actual mechanical details of the process were worked out. This achievement science owes to Mayer and Helmholtz. While light in former days was talked of only as an incomprehensible though beneficial influence, Mayer was the first to state that sun- light is actually used up in the literal sense of the word, and is absorbed by the plant ; that the energy of the ray transforms itself into chemical tension ; that in burning fuel, and in the vital processes of our organism, we use the stored up energy of the sun. It would be better to listen to his own eloquent way of putting it :

‘ Nature, says he, seems to have set itself to capture the light that falls upon our planet, to transform the most mobile of all forces into an immobile form, and to conserve it as such. With this end in view it has covered the crust of the earth with organisms, which, during their life-time, absorb the sunlight and form at the expense of this energy the stores of latent chemical 1 See my article on tlie * Struggle of Plants with Drought' in my Russian book, Agriculture and the Physiology of Plants. Moscow, 1906.

energy incessantly accumulated. These organisms are plants . The vegetable world is a kind of storehouse, where the sun’s rays become arrested and stored for further use. The physical existence of mankind depends on this economical solicitude of Nature, and a single glance at our luxuriant vegetation involun- tarily provokes the sensation of prosperity/ Thus it is that in the dissociation of carbonic acid and the formation of organic plant substance we have essential con- ditions for any technical process. We possess in the sun- light a motive power ; in the plant — a machine to which the motive power is applied ; in the carbonic acid — a raw material ; in the organic matter of the plant — the manufactured product.

Let us investigate more closely the inner mechanism of this process. Let us study first of all the source of energy, the sunbeam. We know that the sunlight, like any other white light, is not homogeneous ; we know that it consists of many heterogeneous rays, differing, among other things, in their colour. Rays of seven different colours are generally discriminated ; they are the colours of the rainbow- — red, orange, yellow, green, blue, indigo, violet. This decomposing of a colourless ray into its seven component colours is best performed by means of a glass prism. If a small aperture is made in a shutter facing the sun, the sunlight in passing through this aperture will produce on the floor an image of the sun in the form of a round patch. Now if we place a prism with its edge downwards in front of the aperture, the image will move on to the wall ; but instead of a round patch we shall obtain a band showing the seven colours of the rainbow just mentioned : the band will be red at one edge and violet at the other. This rainbow band is called a spectrum . Whenever a ray of white light falls upon the surface of a body of any kind, it becomes partly or entirety absorbed. If all the rays become absorbed by the body, its surface appears black ; if all the rays are reflected in equal measure its surface appears white. If some of the rays are absorbed and others reflected, the body acquires the colour of those rays which are reflected from the body and strike our eye. The same holds true with regard to transparent bodies. If the body absorbs all the rays it is not transparent, it is opaque ; if it lets all the rays pass through it, it is entirely transparent, and as colour-

less as water or glass ; but if the body arrests some rays and lets others pass through it, it will acquire the colour of the rays it lets through. If we analyse by means of a prism the light reflected by a coloured body, or the light which has passed through a coloured body, it is obvious that we shall no longer obtain an entire spectrum of seven colours, but one from which the absorbed rays will be absent. Vegetation presents us with a similar phenomenon. In bright sunlight forests and meadows appear green. It is clear that if the leaf reflects the green colour it must absorb part of the white light which it received.

Before drawing any inference, however, from this fact, let us investigate more closely the cause of the green colour of the leaves. Whatever green part of a plant we may choose for investigation under the microscope, we very soon arrive at the conclusion that in itself it is colourless; it consists of bubbles called cells, the walls of which are as transparent as glass, and the liquid which fills them as colourless as water. But this liquid contains bodies or grains emerald green in colour. They are generally called chlorophyll granules or chloroplasts. It is to these granules containing chlorophyll that the plant owes its green colour, in much the same way as the blood owes its colour to the red corpuscles which flow in the colourless lymph. Now let us observe what happens to the sunlight when it falls upon the surface of the green leaf ; which rays are going to pass through the leaf and which will be arrested by it ? For this purpose we must let a ray of light pass through the leaf, and then analyse it by means of a prism. When we do so we notice the difference which takes place in the spectrum. The rays absent in the spectrum, those in place of which black spaces are observed, have obviously been arrested by the leaf, have

been absorbed by its substance. We can perform this experi- ment more exactly still. Since the colour of the plant depends on the chlorophyll, we can study the absorption of light by the chlorophyll itself. Chlorophyll can be extracted from leaves by means of spirit. We all know that any kind of infusion of leaves acquires a splendid green colour, which is the colour of the chlorophyll. Therefore instead of nearly opaque leaves we can use for our experiment a nearly transparent chlorophyll solution. We fill a glass with this solution and place it in the path of the sunlight, and then proceed to analyse with a prism the light we thus obtain. This is the kind of spectrum we get. The extreme red rays (from A to B, tig. 82) will have passed through unabsorbed ; whereas in place of the brightest red, the orange and part of the yellow rays, the spectrum will have a black band (tig. 82 from B to D) ; 1 the green rays (between D and a little to the right of b) will not be absorbed, and will give a green band in the spectrum ; the blue and violet rays will be likewise absorbed. Hence, instead of all the seven colours the spectrum of chlorophyll will show only two coloured bands : a dark red and a bright green, with a black space between them. Hence we conclude that the green colour of a plant is not pure, but a mixture of green and red. This can be easily proved by a curious experiment. The commonest blue glass absorbs green rays and lets through some red rays. It follows that if we look at green vegetation through a piece of this glass it will arrest green rays on their way to our eyes, and let through only red rays. German opticians have taken advantage of this fact, and offered the public a rather amusing instrument called the erithro-phytoscope, which is simply a kind of blue spectacles, but the moment you put them on the whole world changes its aspect.

A fantastic landscape with coral woods and meadows unrolls itself under a deep blue sky. It might be useful to draw the attention of some artists to this fact who are in the habit of colouring their landscapes with that malachite green colour, 1 Fig. 82 represents a photograph of the absorption spectrum of chloro- phyll. The blackest part lies within the red part of the spectrum. The process of photographing the spectra has presented great difficulties, even down to our own day. In the summer of 1893 I succeeded in obtaining satisfactory photographs, which were demonstrated at a con- gress of naturalists and physicians at Moscow in January 1894. The letters mark the so-called Fraunhofer lines of the spectrum of the sun.

never to be observed in Nature! In their unsuccessful en- deavours to represent' Nature, these artists probably tend to portray her in the clearest green possible, whereas the colour of our vegetation is in fact a mixture of green and red. 1 We must, however, return to the main object in view. We wanted to know which rays are absorbed by the plant, and we found that chlorophyll absorbs certain red, orange, and yellow rays, as a result of which its spectrum presents a black band in place of them. This fact can be tested even for a single chlorophyll granule under a microscope. This time instead of throwing the spectrum upon a wall, we can obtain it under the microscope by means of a lens, and in this spectrum of the size of a pin’s head we investigate our chloroplast. We notice then that it appears transparent green in the green part of the spectrum, transparent red in the extreme red, and entirely opaque, as black as soot, in the red rays (marked by RC in fig. 82) absorbed by the solu- tion. This means that the living grains of chlorophyll also absorb these rays.

Thus when they fall upon a plant or rather upon the chloroplasts enclosed in its cells, certain of the sun’s rays become absorbed, cease to be light any longer. But there is no loss of energy : it has only changed, passing into a state of tension. What kind of work is done by these rays in a plant ? Let us recall the conclusion we have just arrived at, that sunlight decomposes carbonic acid in plants. May not this work take place at the expense of just those rays absorbed by the chlorophyll granules? This suggestion gains in probability when we learn that the chloroplast is the very organ, the very apparatus in which the decomposition of carbonic acid takes place. Priestley noticed that the de- composition of carbonic acid and the giving off of oxygen take place exclusively in the green parts of the plant, i.e. in leaves or green stems. He was even able to prove that this activity is due to the green substance. If a vessel of

1 It is difficult to give any definite advice on the subject in the absence of necessary technical information. The spectrum of chrome green is nearest of a!! mineral green colours to the spectrum of chlorophyll, its green colour being a mixture of red and green rays. At all events we cannot obtain the green colour of foliage by mixing together yellow and blue (the blue of the spectrum). water or some kind of extract is allowed to stand in the light, a green deposit soon appears on the walls of the vessel Now- adays we know that this deposit is composed of microscopic plants, of algae ; but in Priestley’s time this fact was not known, and the deposit was even known as *' Priestley’s matter/ Priestley was able to prove that this matter gave off oxygen. This experiment already showed that the green substance decomposed carbonic acid, even outside the leaf or stem, and that it was precisely to this green substance that these functions were due. Other doubts, however, rose later on. There are plants which are not green in Nature, and yet they also decompose carbonic acid. Such are the numerous plants with red, black, and other coloured leaves, which are more and more gaining a footing in our gardens and greenhouses ; such are also the brown and red weeds growing at the bottom of the sea. In the former the matter has been easily explained. The variegated colour in such plants depends on bright solutions which exist in their cell-sap, and which conceal the green chloroplasts. These are easily seen under a microscope, and can be also revealed in the following way. We have only to dip a red or almost black leaf of Coleus , or some other plant with similarly coloured leaves, into dilute sulphurous acid, and it immediately turns green. This depends on the fact that in decolourising the red pigment sulphurous acid does not affect the chlorophyll. It was rather more difficult to prove the presence of chlorophyll in seaweeds. It was impossible to find the green grains in them even under a microscope ; they were all brown or red. Chemistry, however, showed that the green chlorophyll is concealed behind another substance. It is easy enough to observe this fact simply by walking by the seaside.

Weeds cast on the shore very often manifest as they decompose all shades of colour from their natural colour to green. This is because in dead plants the more brightly coloured substances are washed away by water, while chlorophyll remains insoluble. Thus, even here the decomposition of carbonic acid takes place only in parts which contain chlorophyll granules. This rule has no exception. As has been already said, we must see in a chloroplast the apparatus, the mechanism to which the energy of the sun is applied. It was very interesting to test by means of an experiment the truth of this hypothesis, and to see whether the decomposition of carbonic acid actually did take place at

the expense of rays absorbed by the chlorophyll. In order to do so we had only to perform Priestley’s experiment simul- taneously in different parts of the spectrum. The experiment was performed in the following way : a series of glass tubes (fig. S3 II, i, 2, 3, 4, 5) were filled with a mix- ture of air together with a certain per- centage of carbonic acid. Green leaves from the very same plant and of similar size were introduced into each of them. Then the vessels were exposed to the spec- trum of the sun ob- tained in a perfectly dark room. After a few hours it was de- termined, by analys- ing the gas, in which tubes the carbonic acid

had decomposed and in which not — those in which it had decom- posed more, and those in which less. This experiment en- tirely proved the hy- pothesis. It turned out that the decom- position of carbonic corresponded to the black band in the spectrum of chlorophyll (fig. 83, 1 Red.— and fig. 82 between R and D), which means that rays which do not get absorbed by the chlorophyll do not decompose carbonic acid; whereas rays which become absorbed decompose it the more, the more they themselves

are absorbed. This is graphically illustrated in fig. III. From the line ah perpendiculars are drawn, the length of which corre- sponds to the amount of carbonic acid decomposed in the corresponding parts of the spectrum (I) in tubes 1, 2,3, 4, 5 (II). The broken line 1, 2, 3, 4, 3 (III) clearly shows tire part of the spectrum in the sphere of which carbonic acid is decom- posed most energetically. Thus spectroscopic investigation serves to prove on the one hand that certain rays of the sun, after having crossed without any modification immeasurable depths of space, on meeting on their way a chlorophyll granule cease to be light any longer, disappear in performing work of some sort. On the other hand, the experiment in the spectrum just described points to the fact that it is these very rays which cause the decomposition of carbonic acid into carbon and oxygen, and are used up in this chemical Work. We may conclude that a complete correlation is to be observed between the accumulation and expenditure of energy in the plant.

We have thus discovered the source of energy and also the apparatus to which this energy is applied, Le. the chloroplast. We have seen the work done during the process ; we have now to discover the product obtained, to follow the further fate of the carbon liberated from the carbonic acid and to realise what is produced from that carbon in the plant. Here our curiosity can be satisfied by the microscope. We take a green organ of some kind, say a leaf, and dissect it so as to investigate it under a microscope ; or, what is better still, we take a vegetable body, such as a green alga, commonly called green-slime, which can be directly observed under a microscope. Having made sure that the chloroplasts do not contain any foreign body to begin with, 1 we expose the green organ to sunlight either in the open air or in an artificial atmosphere containing carbonic acid, i.e. we place it under con- ditions favourable to the decomposition of carbonic acid. After some time we again investigate the chloroplasts under a microscope, and discover in them colourless grains which were not there before. It is easily proved that these grains consist of starch. Among the properties of starch is that of turning

1 Which is ensured by keeping the plant in the dark for some time. dark blue under the action of a solution of iodine. It is by this method that we detect starch in the chloroplast. The formation of starch, however, is not observed in the absence of light or carbonic acid hence we are justified in concluding that the formation of starch is the result of the decomposition of carbonic acid* This is confirmed by the rapidity with which the one phenomenon is followed by the other. The decomposition of carbonic acid is manifested a few seconds after sunlight falls upon the surface of the leaf, and five minutes later starch is already found in the chloroplast. This correlation of the two processes becomes still more apparent if we take into considera- tion the chemical composition of starch. Starch can serve as a representative and type of the group of vegetable substances known as carbohydrates. The carbohydrates contain carbon, hydrogen, and oxygen. Their name is derived from the fact that in them hydrogen and oxygen are in the same ratio as in water, so that they seem to consist of carbon and water. In order to form a carbohydrate out of carbonic acid and water, we have only to remove all the oxygen from carbonic acid, i.e. we must perform exactly what takes place in a plant during the decomposition of carbonic acid. Therefore carbohydrates have precisely the same composition as would be expected of substances formed in the plant from carbonic acid and water.

In this way the microscope fully confirms the results obtained by means of analysis. Whenever carbonic acid is broken down in a chlorophyll granule carbohydrates form inside it. The following example is a good proof of this correlation of the two processes, A bright spectrum of the sun is thrown in a dark room upon the leaf of a living plant, previously deprived of starch. In an hour’s, time the leaf is removed, decolourised with spirit, and treated with a solution of iodine. It appears that starch has formed in those parts of the spectrum alone which are absorbed by the chlorophyll, and tiie greater the absorption the more abundant the formation of starch; in other words, the leaf obtains an impression of the spectrum of chlorophyll in the form of starch coloured almost black by iodine (fig. 83 IV — compare with fig. 82 and 83, 1 ).

The group of carbohydrates forms the largest component of our vegetable food. Thus starch forms three-fourths of the weight of a wheat grain and four-fifths of the dry matter of the potato. There are many substances besides starch which belong to this group, e.g. sugar, and cellulose, the sub- stance which forms the solid skeleton of plants, from thin blades of grass to the trunks of trees. All these bodies have a similar composition, and differ only in their greater or less density and other physical properties. Sugar, for instance, dissolves in water ; starch does not, it only swells and forms a kind of semi-liquid paste ; cellulose scarcely swells at all In a sense we can say that starch is condensed sugar, and cellulose con- densed starch. Other carbohydrates can be easily derived from starch. As a matter of fact sugar is artificially obtained from starcli in the manufacture of potato molasses. Cellulose has not yet been prepared artificially, but certainly is derived from starch in the plant : thus, for example, the starch of the germinating seeds of cereals changes into the cellulose of which the rootlet is built up.