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Quantities of starch formed, and their significance for the plant. The absorption of energy—the conversion of energy in the plant. The plant is a complex machine for concentrating and storing energy and material from without.

Sachs measured the increase in dry weight (due to the carbohydrates formed in the chlorophyll-corpuscles) per square meter of leaf-surface, exposed for a definite period, by drying rapidly at 100° C. equal areas of the leaves concerned, and comparing the weights.

Of course the results are not to be pushed too far, in view of the fact that some of the starch is continually passing away to be utilised, and of the difficulties of comparing the weather, the intensity of light, currents of air, hygroscopic conditions of atmosphere, and other variable factors which influence the matter. For instance, the stomata open and close to different extents according to the conditions of light and moisture, and this affects the whole mechanism of transpiration especially, and therefore the supplies of water and mineral salts. Nevertheless, some interesting and valuable results have been obtained in connection with this important subject.

It was found, for instance, that the foliage of a sun-flower or of a vegetable-marrow may be forming starch at a rate of considerably over a gram per hour in every square meter of leaf-surface exposed on a fine day; while in particularly clear and warm sunny weather Sachs obtained as much as 24 to 25 grams per square meter per diem.

When one reflects that 200 square meters is not an extravagant estimate for the area of leaf-surface exposed on a tree, for a period which even in our latitudes may be considerably over 100 days of, say, ten hours' light, we need no longer wonder at the rapidity with which wood is produced in the stems, and similar estimates (which I have purposely kept lower than the estimates for continental and tropical climates) may suffice to show how quickly potatoes or the ears of corn, etc., may fill up with the starch or other carbohydrates which render them valuable as crops. We want more measurements in these connections, moreover, for there are several ways in which they are of scientific value and practical importance.

It is evident from what has been said that every grain of starch formed represents so much energy, packed away for the moment in the storehouses of the plant; and we know that—quite apart, however, from intermediate transformations of the energy thus stored—this energy reappears in the kinetic state eventually, when the starch is burned off, in presence of oxygen, and transformed into carbon-dioxide and water. It matters not how quickly or how gradually this combustion occurs, or whether it is accomplished by burning in a fire, or by slow and complex stages in respiration or metabolism: the point is that the unit of weight of starch yields so many units of heat when its structure tumbles down to the original components, carbon-dioxide and water.

Clearly, if we know how many units of heat are yielded by the combustion of one gram of starch, we can obtain an estimate of the amount of energy, measured in terms of heat, which the foliage gains and stores up—an estimate which will approach the truth in proportion as our estimate of the total assimilative activity is correct.

A word of warning is necessary here, however, for those best acquainted with physiology recognise that however useful such calculations as the above may be, and undoubtedly are, to give a general idea of the fact that the energy represented is large, it would be a mistake to suppose that such estimates give even an approximate measure of the energy of potential which may be got from the carbohydrate, and still less of the amount of work that may be got from its employment, according to the way it is employed or presented in the plant. To take a single instance only. If the carbohydrate is rapidly burned off to carbon-dioxide and water, very little is got out of it in the way of work—most, if not all, of the energy set free escapes as heat: whereas if the carbohydrate is slowly and gradually oxydised, passing through various stages and giving rise to powerfully osmotic bodies in the process, or if it is built up into protoplasm, or into the structure of a cell-wall, relatively enormous quantities of work may be got out of its surface-energy, and heat may be absorbed. Whence it follows that we cannot measure the power for physiological work of a body by merely obtaining its heat of combustion, any more than we can infer its significance in metabolism from its chemical properties.

The general conclusion that the plant stores large quantities of energy may of course be arrived at by simply estimating the enormous quantities of food-material which we obtain annually from agricultural plants.

Modern physiologists have attempted to proceed further than this, however, in their essays to form an estimate of the relations between the available energy in the solar rays and that used and stored in the plant.

If we reflect on such phenomena as the cool shade of a tree, and the deep gloom of a forest, and on experiments which show that an ordinary leaf certainly lets very little of the radiant energy of the spectrum pass through it, it becomes evident that many of the rays which fall on the leaf are absorbed in some form, and it becomes very probable that much of the solar energy, other than that we term light, is retained in the leaf for other purposes than assimilation—or, at least, no other conclusion seems possible in view of all the facts. Engelmann's researches with purple bacteria are almost conclusive on this point, and we may regard it as extremely probable that the plant makes other uses of rays, perceived by us as heat-rays, as sources of energy. Researches on the influences of temperature on assimilation and other functions point to the same conclusion; and Pfeffer and Rodemann definitely state that heat is converted into work in the osmotic cells. And the study of the absorption bands in the spectrum of the living leaf becomes more intelligible in the light of these conclusions. Moreover, the fact that a plant still carries on processes of metabolism when active transpiration has lowered its temperature below that of the surrounding air—and the plant therefore receives heat from the environment—points to similar conclusions.

The importance of the conclusion is immense, for even if the plant had no other sources of energy than the darker heat rays of the solar spectrum, it is clear that it ought to be able to do work.

The above may suffice for the general establishment of the conclusion that the plant absorbs more radiant energy than it employs solely for assimilation, and emphasises our deduction that it is a machine for storing energy.

The question now arises, how is this relatively enormous gain in energy employed by the plant? Our answer to the question is not complete, but modern discoveries in various directions have supplied clues here and there which enable us to sketch in some degree the kinds of changes that must go on.

Not the least startling result is that, important as carbon-assimilation is as the chief mode of supplying energy, it is not the only means that the plant has of obtaining such from the environment, and it is even possible—not to say probable—that energy from the external universe may be conveyed into the body of the plant in forms quite different from those perceptible to our eyes as light.

In the most recent survey of this domain, it is pointed out that we may distinguish between radiant energy, as not necessarily or obviously connected with ponderable matter, and mechanical energy, which is always connected in some way with material substance. All mechanical performances in the plants depend on transformation of some form of these, evident either as actual energy doing mechanical work, or as energy of potential ready to do work.

In so far as molecular movements are concerned, we have the special form of chemical energy. The evolution of heat, light and electricity by plants are instances of radiant energy, and so on.

Many transformations of energy in the plants are due to non-vital processes—e.g. transpiration, warping actions, etc., but we cannot always draw sharp lines between the various cases. Nor can we directly measure the work done in the living machinery; but from the effects of pressures and strains, the lifting of heavy weights, driving of root-tips into soil, osmotic phenomena, etc., it is certain that the values may be very high.

The following classes of processes in living protoplasm and cells may be taken as indicators. First we have transformation of chemical energy, without which continued life is impossible: in many cases—e.g. the processes connected with oxygen respiration—these result in the development of heat. Secondly, we have those remarkable manifestations of energy known as osmotic processes, which depend on surface actions, and with which may be associated other surface effects, such as imbibition, secretion, etc., and in connection with which heat may be evolved or absorbed. It is true the substances which exhibit the properties here referred to may be produced, or placed in position, by chemical energy, or they may be absorbed by roots, etc.; but the proximate energy exhibited by them is not derived from chemical energy, and may be out of all proportion to the chemical energy of the substance or substances concerned. Moreover it is significant to note that a highly oxydised body may develop much osmotic energy, as well as a highly combustible one.

It is of the greatest importance to realise the truth that much work can be, and is done in the living plant, by conversions of energy of potential independent of and out of proportion to the chemical energy available by decomposing the substances concerned; even the heat of respiration may be superfluous here, for the plant may absorb heat from without, and convert it into work.

Tensions often arise in the plant, and do work expressed as movements—e.g. the springing of elastic Balsam fruits, stamens of Parietaria, etc.

Osmotic energy not only results in enormous pressures and tensions, but causes movements by diffusion and diosmosis, and any given osmotic substance which carries this energy with it is not necessarily formed always in the same way in the cell—e.g. glucose may arise from starch, or from carbon-dioxide, or from oil.

Surface-energy is also expressed in the powerful attractions for water exhibited in imbibition, swelling, capillarity, absorption, surface tensions, etc.

Transpiration induces relatively enormous disturbances of equilibrium, and does work in moving water quite independent of chemical energy.

Again, what may be termed excretion-energy, as expressed in the separation of a solid body—e.g. a crystal—from a solution, may be for our purposes regarded separately. Any change in the condition of aggregation of a substance in the plant may result in movements and the overcoming of resistances.

It will be evident from this short digression—and this is the point I wish to emphasise—that in the interval between the securing of a grain of starch, representing so much energy won from the external universe, and the reconversion of this grain into its equivalent carbon-dioxide and water, by respiration, resulting in the loss of the above energy as heat, the starch referred to may have undergone numerous transformations in the living machinery of the plant, and have played at various times a rôle in connection with the most various evolutions of energy.

If we try to picture a possible case, we may take the following. A given starch-granule, after being built up in the chlorophyll-corpuscle, is decomposed, and yields part of itself as glucose, which passes down into other parts of the plant in solution. Part of it is merely re-converted into starch, and temporarily stored: another part passes into the arena of oxydation-processes, the sum of which constitute respiration, and may serve for a time in the molecules of an organic acid: yet another part may be converted into a constituent of the cellulose cell-walls; while part may be brought into play in the reconstruction of protoplasm.

In this last connection a discovery made by Schulze about 1878, and followed up later by Pfeffer, Palladin, and others is of importance. Seedlings growing in the dark, or in an atmosphere devoid of carbon-dioxide in the light, become surcharged with nitrogenous bodies known as amides, formed during the breaking down of the proteids in the destructive process preceding and accompanying respiration: if the seedlings are allowed free access to light and carbon-dioxide, however, the amides disappear. The explanation is that they are combined with some of the materials of the carbohydrates, and again built up into the material of the living protoplasm.

Returning to our hypothetical starch-grain—or, rather, its parts—we have some of it retained as starch, in excess, simply because it is not needed at the moment: another portion gives up its energy in respiration, and this does work on the spot, or is lost as heat; or in the body of an organic acid, or its salt, the part in question may do lifting or pressing work by osmosis, or cause diffusion-currents from one cell to another. In the constitution of the cell-wall we may have part of our starch-grain aiding in imbibition or in the establishment of elastic tensions in turgidity: and, finally, parts may be built up into the living protoplasmic machinery of the plant.

What is true for the starch-grain is also true for any particle of salt, or water, or gas which enters into the metabolism of the living plant, regard being paid to the particular case, and circumstances in each case.

Enough has been said to show that the plant cannot be properly studied merely as the subject of chemical analysis or of physical investigation; you might as well expect to understand a watch by assays of the gold, silver, steel and diamonds of which its parts are made up, or to learn what can be got out of the proper working of a lace machine by analysing the silk put into it, and the fabric which comes out, and by taking the specific gravity of its parts and testing the physical properties of its wheels and levers.

This is not the same thing as denying the value of such knowledge, in the case of either the dead machine or the living plant: it is merely emphasising the supreme importance of the study of the structure and working of the active machinery in both cases.

Nor is it pertinent to remark on the apparent hopelessness of physiology being at present able to explain the seemingly infinite complexity of the living machinery of protoplasm and its activities. The modern locomotive is also a complex affair in its way, but it is profitable to investigate it and to know all one can of its working and possibilities, for obvious reasons: a little reflection will convince us that it is also worth while to investigate that complex machine, the plant—the working organism which alone can really enrich a country. Moreover, we ought to be encouraged by the satisfactory progress now being made, and the splendid practical results which are accruing, rather than dismayed by the prospect of unflagging labour which will be required in the future.

Enough has perhaps been said to establish the general truth that the plant is a complex machine for storing energy and material from outside, and we have seen that modern research has at least gone a long way towards determining how the living machine works.

It is hardly necessary to point out that important practical consequences may result from these phenomena of the accumulation of surplus starch or other carbohydrates in the leaves during the day, and of their disappearance during the night into the lower parts of the plant. For instance, foliage cut for fodder in the morning is far poorer in starch than if cut in the evening, and it would be very instructive to have experiments made on a large scale to test the result of feeding caterpillars or rabbits, for instance, with mulberry, vine, or other leaves in the two conditions.

Again, we now see what complications may arise if a parasitic organism gains access to the stores of carbohydrates in process of accumulation, or attacks and injures the machinery which is building up such materials, etc.