The Life of the Plant

The cabbage, for instance, is distinguished by remark- able variety in the development of its organs ; in some kinds of cabbage the head is composed of thickened leaves, in others the stem presents a turnip-like swelling, in others the inflorescences turn into fleshy organs well known to everybody. In a fourth kind the stem grows tall, and hardens, so that sticks may be made out of it ; in a fifth the leaves acquire a bright colour, and so on. Obviously no one would ever suppose that the progenitor of all these forms, and hence also the transitional form, which serves as the link between them, possessed all these characters. As a matter of fact the wild original form of cabbage does not present a single one of these extreme characters. It is therefore very possible that in some cases the real connecting form should escape our attention. But it is beyond doubt, nevertheless, that in the majority of cases transitional forms between species no longer exist, and, as has just been observed, according to Darwin’s theory this very absence of them is one of the results of natural selection. For the sake of clearness we shall again refer to a comparison with artificial selection. When different varieties of cabbage arose, people who cultivated them evidently began to care for the most rare and extreme representatives of the several kinds. The specialist in the cauliflower did not worry about stems or leaves ; what he cared for was only the inflorescence, which he desired to be large and fleshy. A specialist in the decorative kind of cabbage cared only about the colouring and the form of the leaves ; one and the same plant could not produce at the same time both a head and a walking-stick, and so on. It is clear that plants which did not show an extreme development of some special character, but combined many of them less sharply defined, were no longer cultivated, were even destroyed, and hence were bound to disappear.

Consequently the appearance of more clearly defined representatives was inevitably followed by the extermination of the less clearly defined, as a result of which the connexion between extreme forms would break down and a series of variations disconnected with each other would come into existence. Something of this kind was bound to take place in the natural order of things, where any new form can prevail only if it is more perfect than others ; in such a case it is evidently bound to thrust out, to extinguish the others. Darwin also points out that it is to the advantage of every being to become as different as possible from its own kind, be- cause the less alike are the needs of two forms the less will be the struggle between them, the greater the possibility of their adaptation to the same environment, without any struggle whatever. Farmers have long known that the same plant cannot be grown for any length of time on the same field, that it is necessary to change it from year to year to another place ; partly upon this fact the rotation of crops is based. But what is true in time appears to be true also in space : farmers also know that from a certain area of land more hay can be gathered when mixed grass is sown than when the grass seed is uniform. Therefore it can scarcely be doubted that new forms are bound to expel, to extirpate their less perfect predecessors, and that of forms which arise simultaneously the more strongly differentiated have more chance of surviving. Thus, in varying, every organic form tends to break up into subordinate groups, and during the process the links between these forms are lost ; so that the result is the appearance of a series of divergent groups without any trace of immediate transition, and yet bearing evident tokens of either very close, or more remote, resemblance to one another, which used to be vaguely denoted by the term affinity, and which nowadays we simply call relationship. In a word, the present order of the organic world, with its specific and generic as well as other still larger groups, is but a necessary result of their common origin by means of natural selection.

With this we close the long chain of argument which contemporary biology, as represented by the great Darwin, can offer in explanation of the perfection and harmony of the organic world. Let us cast a glance back over what we have said on the subject. If we are able to analyse the majority of vital phenomena into their simplest physico-chemical principles, and can explain them by causes now at work, we are nevertheless obliged to go back to historical causes in order to explain all that concerns adaptation. In order to explain the perfection of organisms by this method, we must begin by proving that they actually have a history, and then that this historical development tends towards perfection. The united testimony of all the branches of biological science, of classification, comparative morphology, embryology, palaeontology, goes to prove the common origin of organic forms. The only objection to this theory is the belief in the immutability of specific forms ; but the criticism of the very conception of species, and moreover certain facts with regard to domesticated animals, which have been established with- in the limits of human memory, remove this objection. Having proved that all the facts speak in favour of, and nothing against, the conclusion that the organic world has a history, we studied the very nature of this historical process. Starting from such obvious properties of all organisms as variability, heredity, and the rapid rate of multiplication, we came to the conclusion that this historical process inevitably leads an organism towards perfection, through what Darwin rightly called ‘ natural selection.’ His theory does not therefore give any ready explanation of the existence of this or that special form, of this or that particular case ; it indicates the general method by which this explanation may be arrived at in any given case.

If we are able to discover the original cause of variation, and farther to indicate the continuous series of transitional forms (as we tried to do in the case of the sage) , the origin of the most complicated form, provided it be useful to the actual organism, will no more be puzzling ; it will be a question of time and selection. This explains why naturalists hail Darwin’s theory as the crowning of the stately structure of modern physiology. It really offers the key to the solution of the problem of the origin of organisms, gives a reason for their perfection, and solves the question we raised at the beginning of this chapter.

Having undertaken the study of the life of the plant, we tried in our first chapter to analyse this complicated phenomenon into its elements, by showing that a plant consists of organs, that these organs consist of simpler organs still — of cells — which in their turn present an aggregate of certain chemical bodies. On the ground of this analysis we then studied in the opposite, ascending, synthetic order, first the properties of these substances, then the life of the cell, the life of organs, the life of the whole plant, and, lastly, in this concluding chapter the life of the whole organic world. This apparently exhausts our problem, brings us to the end of the course through which I undertook to be your guide ; a long and toilsome way, wearying at times, but not, as I hope, utterly fruit- less. If for some of you ladies and gentlemen here present, the plant ceases to be a lifeless object marked by a Latin label, or an object exclusively of aesthetic enjoy- ment, and becomes also a source of fuller intellectual enjoyment : if by the discoveries of the microscope it assumes enormous dimensions, and becomes sufficiently transparent for you to look into the depths of the numberless cells where you will perceive protoplasm — the origin of all life — in ceaseless motion like the tide of the sea : if the same mental glance shows you the root buried deep in the ground, imbibing its liquid food and corroding the particles of the soil all along its course of many miles : if the green leaf revives in your imagination

the idea of that insignificant chlorophyll granule, wherein takes place the wondrous process of the transformation of the sun’s rays into chemical energy, source of all the manifestations of life on our planet: if you see in a flower surrounded by busy insects something more than a curious form, and involuntarily recall the wonderful ties which bind together the two kingdoms of Nature : if, finally, a dense mass of forest thicket or the luxuriant vegetation of a meadow where wild herbs crowd and intertwine, spreading the broad surface of their toothed leaves in the sun, stretching up their narrow blades, coiling round some chosen victim, stretching themselves from its summit to another, everywhere and always manifesting but one and the same tendency to take possession of the largest possible stretch of land, of the greatest amount of air and light — if this familiar everyday picture involuntarily recalls to your mind a whole range of new ideas, of the laws which inevitably and inexorably govern the organic world, leading it on to perfection and harmony- in a word, if a glance at a plant raises an endless file of questions in your mind, questions that demand answers, or better still if the desire comes to you to put these questions to Nature herself and to extort answers from her — I consider that our time has not been wasted, and I feel I may comfort myself with the thought that by affording you in the future some moments of conscious delight in Nature I have the opportunity of repaying, were it only in small measure, the debt of gratitude I owe to your long and indulgent attention.

‘ Can we really admit that all the sunbeams falling on this our earth are lost without further use, or should we not rather assume that they are but transformed when absorbed by the emerald green of the leaves ? ' We all know that if a man is deprived of food he grows lean. This fact is obvious and is understood by everybody, because from the logical point of view it is not difficult to connect these two phenomena, leanness and absence of food. A man's body, like everything else under the sun, becomes wasted ; it uses itself up. This waste is replaced by food. It is not difficult to imagine the food substance transforming itself into the sub- stance of the body, though it will be long ere science arrives at the explanation of all the details of the process.

Less obvious, though not less known, is the other aspect of the effect of food, another side of its influence upon our organism. Want or insufficiency of food causes loss of energy. A man or an animal when hungry grows weak. Food restores strength. The more work is done by an organism, the more food is required. Everybody knows that when a horse is expected to do hard work it receives an extra portion of oats to help it to do that work. This fact is universally known, and yet reflection alone is not sufficient to explain it.

1 Public lecture delivered before the Technical Society in St. Petersburg in 1875. 2 These lines of our talented Russian poet may j ustly excite our wonder if we consider that at the time they were written all the botanists, hav- ing lost sight of Senebier's remarkable intuitions, did not even suspect the existence of Robert Mayer's great generalisations. Thus we see that food not only serves to build up the vital mechanism of our body, it also sets this mechanism in motion* Feeding a man or an animal not only sustains his body— which is obvious; it also maintains his strength — which demands explanation. We ask: what energy can be concealed in a sack of oats, a loaf of bread, a piece of meat ? The answer is far from being a simple one, and does not occur to every mind.

In order to get a satisfactory answer to this question it will be necessary to study the properties of vegetable substance, and also the conditions of its production. We say f vegetable substance/ because we know that animal food proceeds in- directly from vegetable food. Meat is only grass or grain transformed by an animal organism. But before we tackle this problem we must agree as to the exact meaning of certain terms we are going to use. First, what do we mean by energy ? We will try to explain it by some examples. Such a method of explaining scientific truths is certainly not very exact, but on the other hand it is one of the easiest and quickest, and therefore the most convenient in the present case.

Man gets a conception of energy from his own experience, from his own muscular sensations. Energy is defined in mechanics as the ( capacity of producing work / and ‘'work con- sists in motion against resistance 1 (Ranldne). Let us study first of all some instances of the manifestation of energy, commencing with that most familiar to us, the energy of our muscles. Let us imagine two leaden balls, kept in the position C and O 2 by means of two steel springs to which they are attached. By overcoming the resistance of the springs, and moving the balls to €/ O 2 ', I do work, as it is generally called in mechanics.

I do a similar work when, by lifting a weight, I overcome its tendency to fall upon the earth when, so to speak, I tear it away from the earth. The raising of weights is the very simplest illustration of work ; such is the work of a porter, for instance. We know that in that kind of work the greater the weight, and the greater the height through which it has to be raised, the greater the amount of energy which has to be expended. If we raise a unit of weight, i.e. a pound or a kilogramme through a unit of height, i.e. a foot or a metre, we do a unit

of work : a foot-pound, a kilogramme-metre will therefore be units of mechanical work, with which we compare and by which we measure all work. Thus I did work when I separated the two balls. I used a certain amount of energy, which can be measured in terms of work done. Let us see what happened to our balls. Their relative position has changed, and they have acquired the power to move, without the application of any external force. I just remove my hands, thereby ceasing to exert any in- fluence, and the balls are set in motion : they return to their former position and knock against each other. Evidently they possess energy in the new position into which I brought them, and they had none in that from which I removed them.

This energy is latent, but ready to manifest itself at any moment in motion. This simple example clearly demonstrates the two different states, the two types of energy, so to speak : the active, apparent energy, manifested in the motion (of my hand, of the balls), and the latent energy, accumulated or stored, like the energy of a bent bow. We see familiar examples all around us : the energy manifested by a falling weight and the energy accumulated in a weight which may fall ; the energy manifested by an unbending bow, and that which is accumulated in a bent one. All these are cases of active and latent energy respectively. I must add that the weight once fallen, or the bow unbent, does not possess any energy whatever, either active, or latent.

It is evident, however, that these two types of energy are essentially different. In the first case it is manifest, in the second there exists only the possibility of its manifestation. We have just seen that bodies possess energy either as a consequence of motion, or by virtue of their position (e.g. a weight raised, or our separated balls). The first kind of energy is called actual or kinetic energy, whereas energy depend- ing on position has been called potential energy, i.e. energy existing as tension. Therefore, energy becomes manifested in motion, and is concealed in a state of tension. Both kinds are mutually transformable; our balls present an obvious illustration of this fact. Actual energy exerted in separating these balls has not disappeared, but has transformed itself into potential energy, into the tension of the springs. In that state it can be stored and kept, and then used again whenever wanted, retransformed into actual energy, into motion, and moreover at pleasure either directly or gradually. Every day of our lives we make such a storage of energy in winding our watches : the kinetic energy of the hand which does the winding transforms itself into the potential energy of the watch-spring, which in the course of twenty-four hours gradually passes into the state of actual energy, shown by the moving hands. Something analogous to this happens when a man saves money for his old age : he transforms the superfluity of his actual energy, mechanical or intellectual, into potential energy, so that he may use it when his actual energy comes to an end. On all sides in Nature we see similar transformations of motion into tension and vice versa. Keeping this transformation in view we soon arrive at the conclusion that energy as a rule does not arise anew nor disappear, that it is eternal ; in other words we become convinced that all the work done, or which can be done in the universe by the forces of Nature at any given moment, does not increase, nor decrease, but remains the same.

This broadest physical generalisation, called the law of the * conservation of energy/ is { the highest law in physical science which our faculties permit us to perceive * (Faraday). There are, however, cases in which this law seems not to hold ; energy seems sometimes to use itself up, and motion instead of changing into tension seems to vanish altogether. We have exactly such a case in our balls. I separate them and let go. The balls knock against each other, and seem to lose

in this very knock all the energy acquired from the movement of my hand. They do not move any longer, neither have they any capacity for motion, i.e. tension. It seems evident that energy has disappeared. But this is only apparent. The moment the balls knocked against each other, the moment their motion disappeared, there appeared another kind of energy — heat. In knocking, the balls became heated. It would be rather difficult to prove this in the present case, because the rise in temperature is only slight, but the fact cannot be doubted by any one who has ever struck fire. Illustrations of this transformation of energy are to be met with at every step. When metal is bored the borings become very hot ; a piece of wood can be set on fire by rubbing it against another piece of wood ; sparks fly from under the brake of a train when it is suddenly stopped ; a leaden bullet partly melts when it hits against a solid obstacle. These phenomena of the transformation of mechanical energy into heat long ago attracted attention ; they led the famous Boyle, more than two centuries ago, to express an idea, which has been scientifically developed only within the present genera- tion : ‘ When we drive a big nail into a wooden plank/ writes Boyle, ' we notice that it requires a great many blows before it becomes heated ; but when we drive it down to its head, so that it cannot move any longer, a few blows are sufficient to make it hot.

While every blow of the hammer drives the nail deeper and deeper into the wood, a progressive movement of its mass is provoked ; but the moment this movement is checked the shock produced by the blow, unable any longer either to drive the nail any further or split it, is necessarily bound to spend itself on the inner oscillation of particles ; and heat as we know consists of such motion/ Modem physics actually teaches that heat is a rapid invisible but palpable oscillation of par- ticles in a body. Thus the visible motion of balls produced by the movement of my hand has passed into the invisible motion of the particles of the balls. This motion, i.e. heat, was communicated first to bodies in the nearest neighbourhood of the balls ; then, spreading more and more, it is dispersed in space. It is dispersed, but has not ceased to exist. The energy, used by me to separate the balls, has not vanished entirely. In doing this work I ultimately raised the temperature of the universe though to an infinitely slight degree. Innumerable investiga-

tions have shown that during this transformation of mechanical work into heat or, vice versa , of heat into mechanical work, a constant, strictly quantitative ratio is observed. A certain amount of mechanical work on transformation gives rise to a definite amount of heat and vice versa. The quantity which expresses this constant ratio is called the mechanical equivalent of heat. It can be determined in various ways : here is the simplest and most obvious, although not a very accurate method of determination, which was devised by the French scientist Hirn. It consists roughly in the following : a heavy iron hammer is made to fall from a certain height upon an anvil on which a piece of lead is laid. This piece of lead becomes hot from the blow of the hammer. We take a kilogramme- metre for the unit of work, as explained before, and for the unit of heat the rise in temperature of one kilogramme of water by one degree Centigrade. Knowing the weight of the hammer as well as the height from which it falls, knowing also the weight of lead and the amount of heat it has acquired, possessing moreover some other data, which we will not mention here, we can find out how many units of work have been expended as well as into how many units of heat they have been trans- formed. Exact determinations give the value of 426 for the mechanical equivalent of heat. This number indicates the constant ratio by which heat transforms itself into mechanical work or vice versa. This means that a unit of heat on trans- formation into work gives 426 units of mechanical work, i.e. can do work equal to raising 426 kilogrammes to the height of one metre, or of one kilogramme to the height of 426 metres. On the other hand, by expending 426 units of mechanical work to heat water, we can raise the temperature of one kilogramme of it by one degree.

We have mentioned many examples of the transformation of mechanical energy into heat * examples of the opposite are also often met with. The steam engine may be taken as a striking illustration ; heat developed by burning fuel becomes transformed through the medium of steam into mechanical work. The heat of the sun evaporates water from the surface of the earth, causes it to rise to a certain height whence it falls down again upon the earth, runs from the mountains into the valleys and thence into the ocean, producing all the way mechanical work.

e.g , setting in motion our mills. The same energy of the sun heats the atmosphere at certain spots, so as to produce those terrible manifestations of mechanical energy known as whirl- winds, hurricanes, etc. So, then, heat transforms itself into mechanical work and vice versa , and during these processes a strict quantitative ratio is maintained. The same is true with regard to other forces of nature, such as light, electricity, chemical affinity. They are all capable of mutual transformation either immedi- ately or by acquiring the latent form of tension, which subse- quently manifests itself in one way or another. It is only as we continually keep in mind this possibility of the mutual transformation of different forces that we come to realise how true is the law of the conservation of energy.

Let us dwell for a little upon the correlation that exists between heat and chemical affinity, for it will naturally bring us back to the question raised at the beginning of this Lecture. Chemistry teaches that the atoms of the elements are endowed with mutual affinity in various degrees. The atoms of hetero- geneous bodies tend towards each other in much the same way as bodies tend towards the earth, or as these balls tend towards each other by reason of their springs. Our model is meant to illustrate in an obvious way this very fact of chemical affinity. The ball marked by the letter C represents carbon, the ball O 2 oxygen. The atoms of carbon and oxygen tend to combine and to form carbonic acid, in which two atoms of oxygen are combined to one of carbon (C0 2 ). In the same way the atoms of hydrogen (H) tend to combine with the atoms of oxygen and form water, H 2 0, in which two atoms of hydrogen are combined with one of oxygen. On the other hand, the atoms of carbon and hydrogen are endowed with a comparatively much weaker affinity, and therefore, even though combined with each other, tend at the first opportunity to recombine each in its turn with oxygen, thus forming carbonic acid and water.

At the moment of combination the atoms knock against each other in the same way as these balls do. But whenever bodies knock against each other heat is generated. The same takes place in the case of the blows of atoms. These blows, this collision of the particles of carbon and hydrogen with those of oxygen, is exactly what we mean by combustion. Just as heat and light are generated the moment steel is knocked against a flint, so the blow of the particles of oxygen of the air against the particles of carbon and hydrogen in coal-gas generates the heat and light that we observe in the gas flame. The only difference between the two processes consists in that in the first case we see the motion, the blow, as well as the accompanying phenomena, i.e. light and heat ; whereas in the latter case we see only these phenomena, and get an idea of the collision of particles only from the results. Before combustion we have a hydrocarbon {i.e. a compound of carbon and hydrogen), the coal-gas, and oxygen, and after combustion carbonic acid and water.

Therefore every atom of carbon and hydrogen stands to oxygen in the same relation as the ball C stands to the ball O 2 . Like the balls they are in a state of tension, and possess a store of latent potential energy which we call chemical affinity or chemical attraction. In the separated atoms of carbon and hydrogen we have a fresh illustration of the potential energy of position , which at the moment of the collision of atoms in combustion passes into the energy of motion , i.e. into heat and light.

This state of tension in the atoms of carbon, this tendency of theirs to unite with the atoms of oxygen, does not attract , our attention in daily life, because an impulse is necessary to produce their combination. In order to burn a piece of coal we must set fire to it, i.e. the process of combustion must be initiated from without. This tendency of carbon to combine with oxygen is, however, manifested more obviously in phenomena of spontaneous combustion. It has long been known, for in- stance, that rotting hay in stacks is capable of taking fire spon- taneously, but a case of it has only recently been investigated in Germany. When some large hay-stacks were opened in con- sequence of spontaneous combustion being indicated by the smoke coming out of them, it appeared that the hay inside was already charred, and that the soft, shiny, graphite-like carbon caught fire directly it came into contact with the air. It appeared later on that such spontaneously combustible carbon could be prepared artificially if hay was charred in the absence of air; in a sealed glass-tube, for instance. Carbon prepared in this way catches fire the moment it comes into contact with the air. This and similar examples obviously prove that combustion,

i.e. combination with oxygen, can take place spontaneously, i.e. without any previous setting on fire. Both carbon and hydrogen have the property of combining separately with oxygen, and of developing heat and light during the process ; hence they each possess energy stored in the form of chemical tension. But the same is true with regard to com- pounds of carbon and hydrogen as well as of any substance capable of combining with oxygen, i.e, capable of combustion. Substances of which plants and animals are built — all organic bodies, are combustible, and therefore are stores of latent energy.

We use these stores when we burn wood or coal in our engines. The potential chemical energy transforms itself into actual energy during the process, into motion of particles, i.e. into heat, which in its turn transforms itself into external mechanical work, and so into the visible motion of bodies such as the motion of our locomotives. This collision between the atoms of carbon and hydrogen and those of oxygen can, however, take place without any such ostensible liberation of energy as occurs in combustion ; they can combine without any visible manifestation of light, without the production of high temperature. This happens when com- bination does not take place suddenly, but gradually. In both cases the quantity of heat liberated by the combustion of a certain quantity of carbon will be the same, but its liberation in the first case covers a longer period of time, and hence is less obvious. Respiration is a good illustration of this slow com- bustion. Everything that breathes, whether man or animal, slowly burns away. This is easily proved by placing a burning candle, or a living bird or mouse, under a glass bell. We shall soon see that the results will be identically the same : the candle will cease to burn, the animal will die ; while the air, in which before the experiment oxygen was present and no carbonic acid, will now contain carbonic acid, and the oxygen will have correspondingly decreased in quantity. Thus the carbon of every living organism continually combines with the oxygen of the air, burning down into carbonic acid.

In order to restore this continuous waste of his body, man is obliged to take in fresh quantities of carbonaceous matter in the form of food. Food in the organism plays the same part as fuel in an engine, i.e. it burns down, though of course not directly. being first transformed into the substance of our body. What is lost to the organism as matter is acquired as energy. But we can accept the following statement as an axiom, says the famous physiologist, Claude Bernard : * every manifestation of activity in a living organism is necessarily connected, with the destruction of a fart of Us matter.’ In the organism as well as in an engine a certain proportion of matter burns down, and this is accom- panied either by the manifestation of heat, or by the mechanical work into which this heat is transformed, such as the work of our muscles. According to Frankland, a pound of wheat bread stores up something like 75,000 foot-pounds of potential energy. There is no doubt that an organism, just like an engine, cannot transform into useful work all the energy stored up within it in the form of fuel, i.e. all the potential energy of the oxidisable parts of its food. Physiological experiments prove, however, that in this respect the living organism far outstrips any steam engine.

We have proceeded so far towards the solution of the question raised above that we already know the kind of energy con- tained in our food : it is the latent energy of its carbon and hydrogen which are always ready to combine with the oxygen of the air. A fresh problem arises at this point in the course of our investigation. Wood burns, animals burn, man burns, everything burns, and yet nothing burns right away. Forests are burnt down, and yet vegetation is not exterminated. Genera- tions pass away, but mankind is always alive. If everything were only to burn away, the surface of the earth would contain neither plants nor animals any longer; there would soon be only carbonic acid and water.

Evidently another process must also be going on in Nature, a process contrary to combustion, during which the substances entirely burnt down are 4 unburnt, * transformed into substances once more capable of burning. The formation of carbonic acid must be accompanied by a reverse process, the decomposition of the carbonic acid produced by universal combustion. The first man whose attention was drawn to the logical neces- sity for such a process in Nature was the great chemist Priestley. As a matter of course this idea could not present itself to his mind in the same form, or with the same precision and clearness.