The Life of the Plant

Albuminoids form the second predominant grolip of vegetable substances after the carbohydrates. They ard called albu- minoids from their likeness to the albumen of an egg. Wheat flour, taken as an example of vegetable food, contains something- like 17 per cent, of albuminoid matter, called gluten. There- fore, if starch and albuminoids are subtracted from cereal seeds, there will be only a small percentage left for all other substances. In addition to carbon, hydrogen, and oxygen, nitrogen enters into the composition of albuminoid sub- stances.

Though starch, as we have seen, cannot be formed otherwise than with the co-operation of light, the formation of albuminoids in a plant does not require light, or any other external source of energy. It depends instead upon the presence of ready carbo- hydrates. If some plants are only provided with a carbohydrate of some kind, say sugar, and some source of nitrogen, say ammonia, they are sure to form albuminoids even in total darkness. Accord- ing to exact experiment, without touching the problem unsolved as yet by chemists, as to the relation which exists between carbohydrates and albuminoids, we may say that plants are able to form albuminoids from a carbohydrate and ammonia. A physiologist can say to a chemist : give me sugar, ammonia, and a cell, and I will give you in return as much of an albuminoid as you wish. Its manufacture may certainly not always be

very profitable, but in the present case its very possibility, if even only in theory, is very important. Without entering into details as to the origin of other vege- table substances, less important to man as compared with albuminoids, we may nevertheless apply to them what has been said about the albuminoids, and thus arrive at the conclusion that the agency of sunlight is necessary only for the forma- tion of starch or, speaking more generally, of carbohydrates from carbonic acid and water ; no other substances require sunlight for their formation.

We can only now appreciate fully the significance of the pro- cesses taking place in the chlorophyll granule under the action of sunlight. In the first place, from the chemical point of view, it is here that inorganic matter, carbonic acid and water, is transformed into organic matter— here lies the source and origin of all the heterogeneous substances out of which the organic world is built up. On the other hand, from the physical point of view, the chlorophyll granule represents an apparatus for capturing the sun's rays, which then are laid up in store for future use.

Plants form organic matter out of the air, and stores of energy out of sunlight. They represent in every respect the machine invented by Moucliot and Ericsson — the machine set in motion by the energy of the sun, free of cost. This explains the result of the farmer's labour : by expending but a comparatively small amount of substance in the form of manure, he obtains great masses of organic matter ; by expending a certain quantity of energy he acquires great stores of it in the form of fuel and food. He burns down a forest, feeds sheep on the grass of his meadows, sells the corn of his fields, and yet everything returns to him again in the form of air, which, under the influence of sunlight, again acquires the form of forests, fields, and corn. With the assistance of plants he transforms air and light which have no market value into marketable quantities. He trades in air and sunlight.

These considerations dispose of the theories occasionally heard concerning the fate awaiting humanity when chemists shall have discovered the secret of synthesising complicated organic bodies, and have found means of preparing artificial food. On the strength of what has already been achieved by synthetic chemistry, we can scarcely doubt that in the future, it may be at no very distant date, science will realise these expectations. At all events there is no such fundamental difference between what has been already achieved and what yet remains to be done as to make such a hope improbable. And then if food actually becomes artificial, will not agriculture be a thing of the past ? Will not land lose its value ? Will not the economic order of things entirely change ? Let us see how far these conjectures are true. We saw that to form an organic body energy is required. On burning down a pound of bread eight hundred and ninety units of heat are produced. Therefore, to form it either naturally or artificially, a similar amount of heat must be used, or speaking more generally a similar amount of energy. Whence is this energy to be obtained ? The only source of energy free of cost is the sun. Therefore, in order to produce artificial food our remote posterity will have to imitate plants by covering the surface of the earth with artificial absorbers of sunlight. Such imitation cannot be said to be an easy matter, because from this point of view the plant presents a very perfect apparatus. A glance at the thickness of the grass in any meadow is enough to convince us of the fact that every bit of soil is at present utilised. Calculations furnish us with data still more eloquent. Thus, for instance, the total surface of the leaves of a clover plant exceeds twenty-six times the area of the land occupied by the plant, so that an acre of clover is equal to twenty-six acres of green surface absorbing the rays of the sun. Other plants occupy even larger surfaces still. The sainfoin has a leaf-surface thirty-eight and lucerne eighty-five times larger than the areas they occupy.

Mixed grasses would probably give still higher numbers. Here another curious theoretical problem occurs to us: Can we indefinitely increase by means of plants the amount of organic matter obtainable from a certain area of land ? Can we expect that by means of improvements we shall indefinitely increase the productiveness of our soil, or has it a limit ? This is the problem of the future fate of humanity. The data we already possess permit us to decide this question in the affirma- tive. There is a limit, and we are even able to determine it approximately. We have already said many times that the

formation of organic matter in a plant is accompanied by the absorption of as much heat as is liberated in burning it down. Thus, for instance, if a plant liberates 1*000 units of heat when it burns, we may conclude that at least a similar amount of the heat of the sun has been used up in its growth ; and however we may manure our soil and cultivate our land, if the sun does not provide it with these 1.000 units of heat we shall not get our plant.

Thus it is that, knowing on the one hand the amount of combustible matter (to be ascertained by analysis) contained in a crop obtained from a certain area of land, and knowing on the other hand the amount of heat cast by the sun upon this area, we have all the necessary data for calculating the profit and loss of the energy of the sun in our field, and hence to decide what proportion of it we use and what there is yet to be used. Such calculations for the crops which yield the largest amount of organic matter, the richest harvest, bring us to the following conclusions. The largest annual increase of matter in woods represents something like T J<yth of all the amount of heat received by the area of land which they occupy during the period of growth. The increase in bulk of the roots is not taken into this calculation. The Jerusalem artichoke, one of the plants which undergoes the most intensive cultivation, uses in tins way r J ir th of all the energy it receives from the sun. The organic matter in the richest crop of hay (rye grass), including the root remains, stores up x fo th of the energy of the sunlight. Lastly, the best crops of oats and rye (grain, straw, and root remains) represent -^ T th of all the energy they receive from the sun. Thus by means of a plant we are able to avail ourselves approximately of a quantity varying from -nnrffth to - f -J- (T tli of the total amount of sunlight which falls upon the surface of our forests and fields during the growing period . 1 Are we therefore entitled to say that by improving our methods of culture we shall be able to increase the crops by one hundred if not by one thousand times before we reach the limit of productiveness ? Is the plant able to store up all the energy it receives from the sun ? Certainly not. We know that no mechanism or organism makes exception to

1 It is clear that these numbers are not Very strict. The number quoted for the amount of heat that falls upon a given area, taken from PouilleCs data, is only approximately true. this rule, or transforms into useful work all the energy obtained ; and this consideration alone is sufficient to prove that the physiological limit of vegetable productiveness cannot coincide with the physical. Contrary to the figures just mentioned, which have been taken from the results of various cultures, an objection of the following kind can be brought forward : although field vegetation, as we have seen, presents a highly developed surface for absorption, we cannot, however, say that it absorbs all the sunlight that falls upon it. The following experiment will give us from this point of view more trustworthy statistics. By exposing to sunlight green leaves with a surface of accurately measured area, determining by means of analysis the amount of carbonic acid decomposed by this leaf in the best light, say during the space of an hour, and determining also the amount of heat that falls upon the selected surface of the leaf during that hour, we shall obtain all the data necessary for calculating the correlation between the absorption of energy and its utilisa- tion in decomposing carbonic acid. Direct experiments of this kind gave on the average T ^th of all the energy received, •Aj-th at the best. Some recent calculations show that this quantity can be increased to ^ 0 -th. This last figure may probably be considered as approaching the limit of physiological pro- ductiveness, because the plants in these experiments were placed under the most favourable conditions possible. Thus we see how closely all our most intensive cultures approach what we have called the physiological limit, ix. the largest amount of organic matter which can be obtained by means of a plant from a given area of land.

Even at this limit, however, only T J-^-th and in the best instance -g^th of the energy received is retained. This will cease to puzzle us if we consider the fact that apart from this uniquely pro- ductive work from man’s point of view, other work takes place in the plant, entirely unproductive for man . 1 In the first place the plant evaporates during the whole of its life-time such great 1 It is still more important to take into account the fact that a leaf can-

not even absorb all the light of the sun : otherwise it would be black instead of green. Modern investigations prove that the leaf absorbs on the average 25 per cent, of all the radiant energy received from the sun — this is the physical limit; 3*3 per cent, are utilised in physiological experiments, and x per cent, in the fields. quantities of water that in amazement we are inclined to dis- credit the figures. The evaporation of this quantity of water apparently requires much more heat than is used in the decom- position of carbonic acid. Therefore, together with the pro- ductive work in the formation of organic matter, the plant uses still more energy in work useless to man — in evaporation. But this, although the most important, is not the only other expenditure of energy in a plant. The plant absorbs that water from the soil, and therefore has to raise it to a certain height. This work may be expressed in foot-kilogrammes. It may be neglected in the case of our field-plants, but is considerable in our trees . 1 We can imagine what a large amount of work underlies the raising of the masses of water evaporated in forests by giant trees like the eucalyptus of Australia, the tops of which, according to one botanist, might have cast their shadow even on the summit of the pyramid of Cheops. However, evaporation and the raising of water are not achieved solely by the energy derived immediately from the heat of the sun, though a considerable amount is certainly supplied in this way. To these causes of unprofitable waste of the sun's energy must also be added another. We cannot avail ourselves of all the organic matter stored up by the plant during its life-time, because it uses, burns down, part of that matter itself. We can say that it uses in this way as much as -/ (J th of all the matter, so that as regards the accumulation of organic matter the plant makes twenty steps forward and one backward.

All these causes of waste of the sun's energy which we have enumerated illustrate for us, so to speak, the expenses of the production of organic matter by the plant. We see, therefore, that although the plant is a very perfect apparatus for utilising the energy of the sun, it nevertheless leaves much to be desired, since at the best it transforms into work useful for man only 1 -J 1T th or ^th of all the energy it obtains from the sun. Man has to face the problem either of perfecting the plant in this respect, or of inventing in its place an artificial apparatus, which shall utilise a greater percentage of the energy acquired, and

1 The raising of the sap to a great height may be considered as unpro- ductive, only with regard to the production of matter, but on the other hand it is all-important as it furnishes us with timber. moreover work the whole year round* How far he will succeed is for the future to decide. One thing is certain, that when by means of his artificial apparatus man shall gather from all the free area of land about a hundred times more organic matter than is contained in the richest crop to-day, he will be able to say that he has reached the limit ; that he can go no further. Man will then make no further demand of the soil or his art, for more fuel or more food — he will not be able to get any more, because the sun will not be able to give any more. Then the law of Malthus will manifest itself in all its ominous cogency : mankind will have to keep a strict account of life and death ; it will have to take account of the death-rate before reproducing itself, as has been already anxiously suggested by perspicacious economists. No extra mouth, in the literal sense of the word, will then find room at the banquet of Nature. Will mankind ever attain this limit ? By what new processes of synthesis will Berthelots of the future benefit it ? What new sun-machines will be furnished by future Mouchots and Ericssons ? Who can tell ? One thing is certain, that our planet will acquire then a very dismal aspect. When man shall have arrived at the utilisation of all the energy of the sun instead of only part of it as we do at present, then, instead of the emerald green of our meadows and woods, our planet will be covered with the uniform mournful black surface of artificial light-absorbers. Lord Kelvin foretold that our planet will find its death from cold, that our world would be wrapped in its icy embrace; but I do not think this prophecy has alarmed more than a very few. It will come to pass long after our day, and we all know the proverb : apres moi le deluge.

Yet we cannot help shuddering at the idea of what life will be like when the earth is transformed into a universal factory, with no possible escape into the open even on a holiday, even for a single hour ! Let us turn from this gloomy and fantastic picture of what I am happy to say is a very remote future, and go back to the question raised at the beginning of this lecture, which we are now able to answer fully and categorically. We can do so best under the following figure. Once upon a time a ray of sunlight fell somewhere upon the earth. It did not fall, however, upon sterile soil, but upon a green blade of wheat,

or rather upon a chlorophyll granule. The ray was extin- guished when it struck the granule ; it ceased to be light any longer, but it did not cease to exist. It was used up in the work it did inside the granule : it broke the connexion between the atoms of carbon and oxygen which were combined as carbonic acid. The liberated carbon in some way or other combined with water and formed starch. This starch was transformed into sugar, and after many peregrinations inside the plant was precipitated again inside the grain as starch, or as gluten. In either case it entered into the composition of bread, which serves us as food. It was transformed into our muscles, into our nerves. And now in our organisms atoms of carbon strive to recombine with the oxygen which is carried by the blood to all the parts of our body. The ray of sunlight, concealed in, these atoms during this process in the form of chemical tension, reacquires the form of actual energy. It is this ray of sunlight which warms us, and by which we move. May be it sparkles in our brains at the present moment.

This illustration is the most detailed answer which science can give in reply to our question. We can express it shortly in three words. Food plays in our organism the part of a source of energy only because it is a preserve of sunshine. The scientific importance of this result is obvious. It will also be appreciated by people indifferent to scientific truths. A poetical dreamer who looks sadly upon the prosaic labour of a scientist will be pleased to learn from him that he — the poet himself — is much the same ethereal being, built of air and light, as the immaterial productions of' his fancy. The haughty noble who prides himself upon his ancestry, and looks down somewhat contemptuously upon the modest lot of the toilers on the field of science, will certainly treat with more respect this same science on hearing that she entitles him as well as the Emperor of China to call himself ‘ the Son of the Sun/ 1

1 Helmholtz: Ueber die Wechselwirkung der Naturhrafte, p. 127. Printed by T. and A. Constable, Printers to His Majesty . at the Edinburgh University Press