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 role 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.
The student who desires to pursue this subject further should read Sachs' Lectures, XX. and XXV., and Pfefifer's Physiology, pp. 442 - 566, but he will hardly arrive at the best that has been done without consulting Pfeffer's "Studien zur Energetik der Pflanzen" in the Abhandl. der Math. - Phys. Classe der Kgl. Sachss. Gesellsch. der Wiss. (Leipzig, 1892), p. 151; and Kassowitz, Allgemeine Biologie (Vienna, 1899), Bk. I., pp. 1 - 127.