Popular Science Monthly/Volume 30/December 1886/Energy in Plant-Cells
FEW people have any true conception either of the kind or amount of actual energy displayed in the life and growth of a simple plant. In ordinary experience the manifestations of vital energy are always associated with the activity of some animal. Life in the animal seems at its best; its forces are more concentrated, hence more vivid in display, and in every way appeal more certainly to our attention. An animal can move, can exhibit strength, can do work, hence has force, exhibits energy—vital energy, if you please. But in the plant-world these forces are less noted, although going on in much the same way to the accomplishment of life's purposes; and, if less obtrusive in their action and simpler in behavior, are also less difficult to study and easier to understand. To see where some of these forces are exerted, how they are manifested and how controlled, is, in so far as circumstances may allow, the purpose of this article.
The most patent display of energy on the part of a plant is in connection with the growth. Every one knows how a growing seed will send a shoot to the surface through a hard covering of overlying earth. And above-ground the tip of the growing plantlet persistently defies gravitation. Roots find their way through the interstices of clay, and crowd into the enlarging crevices of rocks. The bark of a tree, under tension, in equilibrium of pressure and resistance as long as the tree lives, evinces an energy very appreciable in amount. The amount of force concerned in this case we are not left to imagine, but may at least approximately estimate.
Along the highway that passes one of our Iowa farms were planted many years ago a row of soft-maple trees, designed to serve at length as posts for carrying the wires of the fence. When the trees attained suitable size they were put to the use intended by nailing to each tree a piece of pine lumber four feet long and two by four inches in section for the better attachment of the wires. Since the erection of the fence in the way indicated, the growth of the trees has produced some very striking results. The blocks were attached to the trees by heavy iron spikes (Fig. 1). These seem to have rusted into the tree, and by their points to have held firmly, while by the continual deposition of new layers the tree has crowded off the block, drawing the head of the spike directly through the pine wood;
that is, new material has been thrust in between the wood of the tree on the one hand and the block on the other, until the block has been fairly wedged from its place. Now, it is asserted on excellent authority that the force necessary to accomplish this result amounts to a pressure of about thirty pounds to the square inch; i. e., the forces of growth in a soft maple are capable of exerting in all directions a force of thirty pounds to the square inch. Now, these results may seem somewhat surprising, but our surprise is in no degree lessened when we begin to study the machinery by which this energy is exerted. If we could make a cross-section of one of the trees in question, we should find by far the greater part of the tree in a condition of nearly absolute fixity, incapable of enlargement in any direction. Outside is the bark, likewise largely incapable of exerting force, most of the cells haying long since yielded up their living matter. Only on the line of division separating bark from wood do we find a structure whose cells are capable of life, growth, and multiplication. This structure is so thin that only the finest line would be needed for its delineation, were we to draw the whole section, natural size (Fig. 2).
|Fig. 2. — Cross-Section of an Exogenous Stem.|
Furthermore, this layer is made of cells whose walls are exceedingly delicate and thin. So much more feeble, in this regard, are the cells here than on either side, that this layer, the cambium, is the line of separation when, in the growing season, you easily strip the green bark from the wood. The energy, then, which we have estimated, must finally rest upon these thin-walled, delicate cells. Not only is this true, but we may also easily conclude that all the pressure by which the cleat is wedged from the tree must come from the growth and multiplication of the same diminutive organisms. It is plain here that the force concerned is not capillary, for that is certainly as active in the woody parts of the tree as in the cambium, there producing no expansion whatever. Neither does it seem that the energy expended must be attributed to osmosis, although the cell may be by construction a simple osmotic apparatus. Osmosis there undoubtedly is, but it is exactly similar here to osmosis everywhere, and, while accounting for certain things as capillarity accounts for certain other things, still does not mean growth. Let us see what osmosis can do. If two liquids of unequal density be separated by a membrane pervious to either or both, an interchange between the two fluids occurs until equilibrium of density is established, the greater quantity of the commingled fluids being found at last on that side of the membrame at first occupied by the denser fluid. Suppose now, for illustration, a chain of cells extending from some leaf on the maple-tree down to some rootlet in contact with a drop of water, each cell-content of less density than that above it, and we should have a current setting toward the leaf, and likewise, though less in energy and amount, a current in the opposite direction. Certainly, something of this kind actually happens, not in a single row of cells, but involving all active cells of the tree, so that water from the soil is carried to the leaf, and the products of the latter are diffused throughout the organism. We may even conceive the cells beneath our block of wood to be distended to repletion by the process just described, yet all this is not growth. Given this machinery at the beginning of our experiment, and we can see that the connection of the block would be strained as when wooden wedges, by absorbing water, burst the rock. But the cells once distended, the limit of pressure is reached, and everything would remain in statu quo. And now appears the energy of life's forces. After osmosis and diffusion have done their best, the living matter of the cell is able, notwithstanding the pressure, to enlarge the cells, increase their number, and thicken their walls, and this it is that at length produces the phenomenon we have seen, and brings the spikes, heads and all, through the yielding wood.
But let us look at another example illustrating this same thing. In the manufacture of beer, as every one knows, the alcohol of the beverage is produced by fermentation, a process induced through the activity of brewer's yeast. Now, brewer's yeast, as may be shown by any good microscope, consists essentially of minute single cells, each of which is capable of performing alone all vital functions; i. e., each cell can assimilate food, grow, and reproduce its kind—the two functions last named being here, as elsewhere, dependent on the first. The food of the yeast-cell in this instance is grape-sugar or glucose. From this comes as a sort of by-product of assimilation carbon dioxide in large quantities. The liberation of this gas in the wort produces the frothing which constitutes so noticeable a feature of fermentation. The glucose being the source of supply whence the gas is eliminated, it is plain, all questions of temperature aside, that gas will appear so long as glucose remains in sufficient quantity to nourish the yeast. The amount of glucose found in different grades varies, but certain it is that no beer is entirely free from this yeast-food, so that, when the brewer is ready to deliver his beer to the customer, he is painfully aware that his goods are in anything but stable condition. Hence beer for shipment is placed in oak quarter-barrels (kegs), bound with many broad iron hoops, and made by shape and in every way as stout and strong as possible. Prior to filling at the brewery, beer-kegs are subjected to water-pressure of thirty-five pounds to the square inch, and yet, notwithstanding care in construction and rigor in the test applied, beer-kegs will once in a while actually burst; i. e., the strain caused by internal pressure passes (probably far surpasses) the limit of the test. Here then, again, we have a gauge by which to estimate the energy of life's forces. The pressure is due to the evolution of gas; but the gas, as has been said, is disengaged only as a consequent of vitality, of growth, and, at the moment preceding the explosion, the cells are acting, the processes of growth accomplishing, under a pressure of not less than forty or fifty pounds to the inch. The yeast-cell grows, pushes forth bud after bud, liberates particle after particle of carbon dioxide, all under increasing pressure the further the process goes, until at last at the supreme moment oak and iron can endure no more—the barrel bursts! Here no one can quote osmosis, although as between the contents of the cell and the surrounding liquid osmosis doubtless there is, as there was in the case of the maple; but osmosis is certainly not responsible for the gas-pressure under which the cells are working.
The illustrations cited certainly establish the truth of the proposition with which we began, viz., that plant-cells may display actual appreciable energy. Indeed, it would seem that no one could look in upon the living streams of any transparent, active cell, as these sweep within narrow limits in tireless ebb and flow, and not be convinced that in some way at least life is the exponent of force. What that force may be, no one can positively as of knowledge say. If we affirm that the energy of the plant-cell is to be traced to the sun's rays, we state but a partial truth. The sunlight simply continues an energy already started, simply keeps the machinery wound up, or rather prevents its running down, and we can conceive of no cell whose primary energy is not derived directly from the nearest link of the infinite chain preceding.
Take now into consideration what we may call the directive energy of the cell (call it accumulated habit, hereditary endowment, or what you will), which determines the direction and the limits of the cell's growth, which locks within the compass of a single bit of protoplasm the destiny of millions of succeeding living atoms, combining to the accomplishment of most wonderful and varied functions, so that every germ-cell at the least has its own individuality, its own future, its own ideal, into which in the order of Nature it comes, and we begin to see that, even were the physical energies of the cell cleared up, we are yet as far off as ever from the solution of life's problem. As Emerson puts it, "Life is life which generates," and generation implies an energy to which all other energy in a living cell yields homage.
In thus measuring the energy which cells exert, it seems to me we lay our finger on the very pulse of the living world; we feel the push of its ceaseless stream, and in the impact of the latest wave catch the full force of that primal impulse in which life's history on earth began.