The solar paradox: How the sun's dual power defines plant life

“ We shall now endeavour to explain how it is that everything on the earth, and no doubt on all the other planets also, is derived from the sun, so entirely, that we may affirm that vegetables, animals, man, in short, all living beings, are but the productions, the children of the sun; that they are, so to speak, woven out of solar rays.
In the first place, the sun is the primary cause of all those movements which we observe, in the air, in the water, or in the ground under our feet, and which keep up life, feeling, and activity on the surface of our globe. ”

“ The sun marks out for the planets their path through the heavens, and distributes to them their day and night, their seasons and their climate.
The sun is, then, the hand which holds the stars above the unfathomable abysses of infinite space, the centre from which they obtain heat, the torch which gives them light, and the source whence they derive the principle of life. ”

“ Plant-life is all directly or indirectly dependant on the heat and light of the sun—directly dependant in the immense majority of plants, and indirectly dependant in plants which, as the fungi, flourish in the dark: since these, growing as they do at the expense of decaying organic matter, mediately draw their forces from the same original source. ”

“ Thirdly, from the fact that plant-life alone has the power of subsisting on inorganic matter, and that plants furnish all the nourishment of animals, we may fairly infer that the life of the plant preceded that of the animal. ”

“ Like us, they can subsist only on food which the plant has previously produced; for in this world, from the beginning of time, the plant has been the only organism which could use the solar light and heat as forces to enable it to turn the dead elements of matter into living, growing tissues, and into organic compounds capable of nourishing the animal. Like us, the Protozoa expend the food which they have assimilated in the production of animal force, and in doing so cause it to be oxidized, or burnt away, and resolved again into dead matter. ”

“ The next theme of study was the solar system, under the laws governing which our world exists, part of which it is, and upon the members of which, the sun and the moon, we are so dependent that without them life on our globe would be impossible. That magic power whereby the plant world grows and ripens its fruits abides in the sunlight. It is the sun’s heat which raises water from the earth in vapor, which returns to us in showers and dew. It is that same forceful heat, poured out on vast continents, that gives rise to the winds that cool our summers and speed the sails of our commerce. ”

“ Falling on the cells of the simplest green plants, the potent sunlight dissociates the carbon from the oxygen in the carbonic acid floating in the atmosphere, and builds it up with the hydrogen of water in the tissues of the organism into starches and other organic products, which differ from the inert substances around them, mainly by the possession of locked-up solar energy. ”

“ If a potted plant is not convenient a slip or a seedling bean or pea placed in a tumbler of water will serve the purpose. Prepare several and place some in a sunny window and leave others in the room where it is darker, and observe them from time to time. In the case of those plants that were set in the sunny window moisture will be seen collecting on the inner surface of the tumbler. Where does this come from? It is absorbed from the soil by the roots and is sent with its load of dissolved plant food up through the stems to the leaves. ”

“ I now pass on to a different subject, the all-important process on which the life of green plants depends, an act therefore by which our own existence and that of all other animals is conditioned. I mean the process known as assimilation. This is the truly miraculous feat of using as a source of food the carbonic acid gas (CO2) which exists in minute quantities in the atmosphere. The plant is in fact a carbon-catching machine, and the machine is driven by the energy of the sun, and can therefore only work in light. ”

“ The heat of the sun’s rays, which is intended by nature to warm the soil, will be expended in evaporating the water from its surface; and thus the plants will never receive that genial warmth about their roots which so much favours their rapid growth. Where too much water is present in the soil also, that food of the plant which the soil supplies is so much diluted, that either a much greater quantity of fluid must be taken in by the roots,—much more work done,—or the plant will be scantily nourished. ”

“ SOME INTER-RELATIONS OF PLANTS AND ANIMALS The most important and fundamental difference between the animal and plant worlds is this: plants possess the power of manufacturing their food out of the inorganic materials of which it is composed, while animals cannot do this. Give an ordinary plant access to water with a pinch of mineral salts in it, to the air, and to sunlight, and by the agency of chlorophyll—the green colouring-matter of the leaves—the miracle will be accomplished, and dead materials transformed into living substance. ”

“ Under the light of the sun, acting upon green parts or foliage, that is, upon the chlorophyll, or upon what answers to chlorophyll, which these parts contain. The sun in some way supplies a power which enables the living plant to originate these peculiar chemical combinations,—to organize matter into forms which are alone capable of being endowed with life. ”

“ Some further consideration of the probable agency of light will occupy us, when we come to examine the phenomena of vital forces.
Light is essentially necessary to vegetable life; and to it science refers the powers which the plant possesses of separating carbon from the air breathed by the leaves, and secreting it within its tissues for the purpose of adding to its woody structure. ”

“ Let humble plants, then, be introduced in the waters, and they would at once begin to use the solar light for the purpose of decomposing carbonic acid, and forming carbon compounds which had not before existed, and which independently of vegetable life would never have existed. ”

“ The locking up of potential energy in the protoplasmic cell of the plant requires the expenditure of a vast amount of energy, but the solar ray, aided by the subtile alchemy of the leaf, is competent for the task; and, while the chlorophyl of the leaf assists in weaving organic tissues from the air, this outward dissipation of energy is delayed for a while, giving us food for our bodies and fuel for our fires. ”

“ It is, then, a portion of solar radiation, stored up at first by the plant, which the animal makes disposable and converts to use, whether for resisting cold or for securing the regular play of his motive functions. Thus we may say, with exact truth, the sun is the inexhaustible source, as it is the perpetual spring of life. From this point of view, science confirms the intuitions of oldest date, and man's poetic dreams in the childhood of the race. ”

“ Take a young oak-tree with its first tender leaves; if deprived of the sun's light and heat, its growth would be stayed, and its life die out. But with the aid of the sun's rays, it absorbs carbon from the gases in the air, each particle of carbon absorbed being absorbed by the power of the sun, through the agency of the plant; and with each particle of carbon stored up is also, as it were, stored up the labor of the sun by which that particle was set free from its former fetters. ”

“ At the head of the assemblers of organic matter stands the plant, the animal’s senior. Directly or indirectly, it is to-day, as it was in the geological period, the chief purveyor to beings more generously endowed with life. ”

“ But 272we know that when, under fit conditions, plants are exposed to the Sun’s rays, they give off oxygen and accumulate carbon and hydrogen. In darkness this process ceases. It ceases too when the quantities of light and heat received are greatly reduced, as in winter. Conversely, it is active when the light and heat are great, as in summer. And the like relation is seen in the fact that while plant-life is luxuriant in the tropics, it diminishes in temperate regions, and disappears as we approach the poles. ”

“ We all know that a plant becomes pale and sickly if it has not sunlight, and the reason is, that without these light-waves it cannot get food out of the air, nor make the sap and juices which it needs. When you look at plants and trees growing in the beautiful meadows; at the fields of corn, and at the lovely landscape, you are looking on the work of the tiny waves of light, which never rest all through the day in helping to give life to every green thing that grows. ”

“ So absolute is the dependence of plants for their respiration on the action of the sun's light, that if it be intercepted by clouds, the escape of oxygen from them suffers a marked diminution. If the light of the sun be suddenly stopped, which occurs during a total solar eclipse, the escape of oxygen ceases, and the plants transpire carbonic acid only, as they always do during the night. ”

“ Plants are the accumulators of the power which animals distribute and disperse.
But it will be observed, that the existence of the matter of life depends on the preëxistence of certain compounds, namely, carbonic acid, water, and ammonia. Withdraw any one of these three from the world and all vital phenomena come to an end. They are related to the protoplasm of the plant, as the protoplasm of the plant is to that of the animal. ”

“ The cells themselves are of microscopic size. The chlorophyll itself occupies only quite a small portion of the corpuscle in which it is immersed; yet on its activity as spread in this infinitesimal quantity through the leaves the whole organic{133} world, animal as well as vegetable, depends. [9] Utilizing the energy which comes through space from the sun, it builds up organic compounds; from the energy thus stored comes all the varied life and vital movement which fill our world—the opening of flowers, the hum of insects, the march of armies, and our own restless thought ”

“ A short analysis will disclose to us that, though minerals have various activities, they are not “living.” The plants have a very definite and well known function—the transformation of solar energy into organic chemical energy. They are a class of life which appropriates one kind of energy, converts it into another kind and stores it up; in that sense they are a kind of storage battery for the solar energy; and so I define the plants as the chemistry-binding class of life. ”

“ What Plants Are.—What is the use of the earth's being warmed by the great sun, and watered by the rain and dew? Let us see. Everything that grows out of the earth is called a plant, and all plants need water to drink, and sunshine to keep them warm. Some need a great deal of water and warmth; others want only a little.
Plants cannot grow without heat and moisture. Different plants belong to different zones.
In the Frigid zones the winters are long and very cold with little light. Hardly anything grows here except mosses and a few low-growing plants that flourish during the short summer. ”

“ Some plants yield a large amount of water under a pressure sufficient to raise a column several feet; others force out little, but under considerable pressure (less easily demonstrated) . The vital processes (i.e., the life processes) . 100. The pupil having studied roots, stems, and leaves, should now be able to describe the main vital functions of plants: what is the root function? stem function? leaf function? 101. What is meant by the “sap”? 102. Where and how does the plant secure its water? oxygen? carbon? hydrogen? nitrogen? ”

“ The important point is that it is the sunshine which is used by plants to make all these refractory bodies, such as water, carbonic acid gas, and others, unite together to form sugar, starch, and wood.
As the earth revolves upon its axis, sunlight falls successively on every acre of land. Almost everywhere it is intercepted by green foliage. Each leaf of every plant receives and absorbs as much as it can, and, for so long as the light lasts, its living particles are hard at work: water or sap is hurrying up the stem and streaming out of the leaves as water vapour. ”

“ In deserts the saltness gains the upper hand, as we have already shown, and rarities resembling one another are left between the particles of sand; the sun has therefore no power to produce or perpetuate any continuous plant life; and so in deserts separate species of plants will not occur, but species similar to one another.
4Plants which grow on the surface of the water will only do so when there is density in the water ”

“ The probability of the absorption of water vapor from the air by plants of the desert is one of perennial interest, especially to those who take a sentimental view of desert life. The spines of cacti, especially the large curved and hooked ones of the Echinocacti, will take up water vapor, as has been demonstrated more than once in my work, but the very small amount of moisture thus acquired is not available to the living cells and is quickly lost when the plants are exposed to direct sunlight. ”

“ Arctic. On our loftier hills, indeed, the heat is on occasions oppressive.
Again, the mountain climate, with its heavy rainfall and long cold period, tends to the formation of peat; and the acids thus engendered in the soil, as well as the low temperature prevailing during most of the year, render difficult the absorption of water by the roots of plants. The conditions under which alpine plants, then, live may be summed up as follows: a long cold winter, a short summer; great exposure; scarcity of food-supply. ”

“ As long as practical, surface cultivation should be given, especially after rains, but usually at this time the plants make such rapid and vigorous growth that there will be little time for the work. Their tops and branches soon fill the entire space and quickly shade the ground so densely as to keep down weed growth. Of course, whatever tall weeds may spring up here and there have to be pulled out by hand. ”

“ We found at the beginning of this book that ability to get about is one of the main differences between animals and plants. Like so many first thoughts, this is, however, only a half truth, for while most plants, seemingly by a kind of fatality, are anchored forever to the place of their birth, many of them do move certain parts of themselves and that quite regularly. While some of these movements have already been hinted at as a possible response to transpiration or too intense light, there are others where the advantage to the plant, if any, has yet to be demonstrated. ”

“ But let the plant have a chance for its life—a fair field, but no favour—let the ground be free from the shade of tree, walls, or anything else. Let the plant have light and air, and let the Peronospora do its worst. Then every plant which is not immaculate when ripe, which does not resist the shock absolutely, destroy it. No matter how beautiful; no matter how protific, destroy it. It is not to be depended on for a moment. It is not a "staff of life," and why cultivate it; when any number of millions as prolific and as palatable as it, can be obtained with the utmost facility? ”

“ Every morning the plants will require spraying, but this must never be done at night or damping off may follow. All through their time in the greenhouse it is important to keep the boxes near the glass. Towards the end of March remove to cold frames, keeping the lights rather close for a few days, but gradually giving more air until the lights can be taken off for a short time daily. ”

“ This home economy of plants, or how they meet the environment and each other, is called Plant Ecology, a phase of botany now much studied, for it tells us more directly than most other plant research what the actual response to various factors of the environment may be. Just as plant distribution is the reflection of many, usually widely operating forces, so ecology narrows down to individual plants or groups of them the impact of the immediately surrounding conditions upon vegetation. ”

“ We allude to the continual changes that are going on throughout the whole living fabric of the plant while in its active condition. Cell membrane, the protoplasm, the entire mass of liquid and solid constituents of which the plant consists, are, as we have seen, made up of molecules, each, as it were, with a life of its own, undergoing continual changes according to different circumstances, acting and reacting one upon another so long as any active life remains. ”

“ The extraordinary efforts that the plant world makes to keep all but a minute fraction of the earth clothed with some sort of vegetation we have seen in the pages just turned. No other phase of the study of plant life is so replete with interest as plant distribution. ”

“ How vegetable life originated, from what inorganic forms it was evolved, is a secret which science has not yet discovered; but since those dim first beginnings it has never been absent from the Earth, so far as we know, and has increased and multiplied, and passed through a thousand changes to higher and higher forms, till it has attained to the beautiful and bountiful and varied plant world which we know, covering with a green mantle most of the land surface of the globe and filling the shallower lakes and seas ”