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The Wonder Book of Chemistry by  Jean Henri Fabre


 

 

PLANTS AT WORK

[323]

"I
SHALL never forget," Uncle Paul resumed, "how rudely one of my friends was treated by a noted cook. One gala day he found the kitchen artist meditating on the triumphs of his profession, as he stood watching the various processes going on in his steaming pots and pans. Broad of face, with a wealth of chins one under another, an opulent nose richly adorned with pimples, a majestic stomach, napkin tucked under his waistband, cap of snowy linen—such was the man.

"The saucepans were simmering on the stove, and from under the lids came whiffs of odors so delicious that one could almost have dined by smelling them. A fat capon stuffed with truffles and a young turkey decorated with slices of bacon were roasting on the spit. At one side a fat thrush redolent of juniper was bestowing a part of its succulence on a slice of buttered toast.

"'Well,' said my friend, after the customary greetings, pointing to one of the saucepans, 'what masterpiece of your art have we here?' "'Ragout of hare with plovers' replied the artificer of tempting dishes, beaming with satisfaction and licking his fingers as he lifted the lid. Immediately there was wafted through the room a [324] smell fit to awaken the demon of sensuality in the most abstemious.

"My friend uttered words of high praise, and then continued:

"'You are clever, as every one admits; but zounds! It's not so very difficult a matter to cook good dishes when you have good materials to start with, to make the mouth water when you have a fat capon at your disposal, or to create a most appetizing odor with a brace of plovers. The ideal achievement would be to produce a roast or a ragout with no capon, no hare, no bird or beast of any kind. The old recipe, to make a hare pie, first catch your hare, is too exacting. Hares are not running about for every one to catch. It would be more convenient if we could take something else, something common and easy to get, and make our roast or ragout out of that.'

"The cook was at a loss how to reply, so evidently serious were my friend's words.

" 'What!' he exclaimed, 'a real hare ragout without any hare, a real roast capon without any capon? And you mean to say that you could do that?'

" 'No, not I; I am far from clever enough. But I know some one who is clever enough, and compared with whom you and your fellow artists are nothing but clumsy bunglers.'

"The cook's eyes flashed; the artist's self-esteem was wounded to the quick.

" 'And what, if you please, does this master of masters use? For I suppose he can hardly produce [325] his delectable dishes with nothing at all to work with.'

" 'He uses rather poor materials. Would you like to see them? Here they are, all complete.'

"My friend drew three small vials from his pocket. The cook took one. It contained a black powder, which the culinary artist felt of, tasted, and held to his nose.

" 'It is charcoal,' he declared. 'You are fooling me. Your charcoal capons must be fine eating! Let me see another of your vials. Ha, this is water, or I am much mistaken.'

" 'You are right; it is water.'

" 'Now for your third vial. Why, there's nothing in it!'

" 'Not so fast; there is something in it— air.'

" 'Air! Your air capons ought not to lie very heavy on the stomach. Are you in earnest?'

" 'Very much in earnest.'

" 'No joking?'

" 'Not the slightest.'

" 'Your artist makes his capons out of charcoal, water, air, and nothing else?'

" 'Yes.'

"The cook's nose turned blue.

" 'With water, charcoal, and air you say he could make this skewer of thrushes?'

" 'Yes, yes.'

"The cook's nose passed from blue to violet.

" 'And with charcoal, air, and water he could make this pasty of fat goose-livers, and this pigeon stew?'

[326] " 'Yes, a hundred thousand times yes!'

"The nose, now attaining its last phase, turned crimson. The bomb burst. The cook decided that the man was a maniac who was making fun of him. Accordingly he took my friend by the shoulders, spun him round, and propelled him through the doorway, throwing his three vials after him. Then the irascible nose gradually returned from crimson to violet, from violet to blue, and from that to its normal tint; but the demonstration to prove that a capon can be made out of charcoal, air, and water was never carried through."

"Of course your friend was only in fun, wasn't he, with those three vials?" asked Jules.

"Not at all. His three vials really contained the wherewithal to make the dishes the cook was preparing. Have I not already shown you that charcoal, or carbon, goes to the making of bread, meat, milk, and countless other things we use as food? Remember the slice of bread left toasting too long, and the mutton-chop forgotten on the broiler."

"I see now. Your friend was speaking of the chemical elements. Carbon is one, and that was in the first of the three vials. How about the other two?"

"The second is just as easy to account for. Hold a pane of glass over the smoke rising from a slice of bread when it is just beginning to burn on the stove, and you will soon see the glass covered with a fine film of moisture, exactly as if you had breathed upon it. This moisture comes from the rising vapor, and the latter from the bread. Therefore [327] bread contains water, considerable water in fact, however dry it may appear to be. If we could extract all its moisture from a mouthful of bread, you would be surprised at its quantity. It would astonish you to learn how much water we eat at every meal."

"But we don't eat water, we drink it," objected Emile.

"I say we eat it, for as it is found in bread it does not run, does not wet anything, does not quench thirst. It is solid rather than liquid, dry rather than wet, something to be chewed rather than drunk. Or, better, it is no longer water, but something else that unites with air and carbon to make bread."

"Well," assented the boy, "I'll admit there must be water in bread because it shows on a piece of glass held over a slice of toast that is beginning to burn. But there is still another vial to account for, the one with air."

"Here it will be impossible, with the simple means at our disposal, to furnish proof. Of the three substances named as being present in our food I have accounted for two, carbon and water; the presence of the third you must accept on faith."

"Agreed unanimously: there is air in bread. What other wonderful things are you going to tell us about?"

"We will see. This much, first, is agree upon: bread is composed of carbon, air, and water, which combine in a certain way, merge into one another, and cease to be merely carbon, air, and water, by becoming something else quite different from any [328] one of the three. White has come out of black, savor out of insipidity, nutritive qualities out of innutritious things.

"Meat subjected to the action of fire teaches the same lesson: it turns to carbon and emits fumes containing the constituents of air and water. We will go no farther, for our inquiries would always meet with the same answer. All that we eat or drink, all that goes to nourish us, is reducible to water, carbon, and air. Everything found in an animal's body, everything in a plant, is, with very few exceptions, made of nothing that is not found in water, carbon, or air. Let us be still more explicit. Carbon, being a simple substance, an element, is always carbon and nothing else; but water is composed of hydrogen and oxygen, and air of oxygen and nitrogen. Hence the four elements, carbon, oxygen, hydrogen, and nitrogen, are the materials of which all things in the plant and animal world are almost entirely made.

"So my friend with his three vials was speaking the truth, for all the savory dishes prepared by the cook were reducible to carbon, air, and water. In those little bottles were actually the elements, the prime ingredients, contained in roast capons, pigeon stews, pasties of fat goose-livers, cream tarts, and so on; but to put them together and make them into food, which chemistry in its brutal operations knows only how to destroy, the artist was lacking, the great artist of whom my friend spoke."

"And who is this artist?"

"Vegetation, my young friends, or, more par- [329] ticularly, grass. At the grand banquet spread for all the world, three dishes only are served, though they take an infinite variety of forms. From the epicure who dines on the choicest of delicacies contributed by every quarter of the globe, to the oyster that fills its belly with slime washed up by the waves; from the oak whose roots suck in nourishment from an acre of ground, to the mold on a piece of cheese—all draw upon the same source of supplies, all feed on carbon, air, and water. The only difference lies in the way these ingredients are prepared. Both wolf and man—the latter not unlike a wolf in his kind of food and in other respects also—eat their carbon as it is served tot hem in sheep, while the sheep finds its carbon in grass, and grass— Ah! Here we come to the point that shows vegetation to be the feeder of the world, with wolf, sheep, and man wholly at its mercy.

"In animal flesh both man and wolf find carbon, air, and water served up in compact form as a savory dish, while in grass the sheep finds them just as skilfully prepared, though less savory and less compact in form. But vegetation itself, so nourishing to the sheep, so well fitted to be made into sheep's flesh for the sustenance of man and for the building of his body—with what sauce does it eat its carbon and air and water?

It eats them with no sauce whatever, but in their natural state, or very nearly so. Blessed with a stomach truly marvelous in its capabilities, the plant digests carbon and takes in air and water, and out of these three substances, which no other [330] form of life will deign to touch as nutriment, makes the forage that hands on to the sheep its needed supply of carbon, oxygen, hydrogen, and nitrogen, all thenceforth united in nutritious form. The sheep takes up the preparation of these elements as found in the blade of grass and carries it further, improving it a little and turning it into flesh that finally, by the slightest possible change, becomes man's or wolf's flesh, according to the consumer."

"I begin to see how it all comes about," said Jules: "man makes his flesh from sheep's flesh and from the various other things he eats, the sheep makes its flesh from the grass it browses, and grass is made of carbon and the elements in air and water. So it is the plant that, in the beginning, prepares all our food for us."

"Yes, the plant, and only the plant, has that important task. Man gets the materials of his body either from the plant itself or from the sheep and other animals that contain those materials already prepared; the sheep or other grazing animal gets them from the plant, where they are found in an advanced stage of preparation; and the plant alone gets them from the original source, eating the uneatable, carbon and the elements in air and water, and, by a marvelous process of which it alone knows the secret, converting them into nutriment fit for the sustenance of animal life. So, then, it is vegetation, finally, that spreads a table for all the earth's inhabitants. Were its work to cease, all forms of animal life, absolutely all, unable to derive nourishment from carbon in its natural state or from air or [331] water, would perish of hunger, the sheep for want of grass, the wolf for want of sheep, and man for want of any and every kind of food.

"I see now," said Emile, "why you called the plant the great artist that knows how to make everything with what was in your friend's vials. It does it all with carbon, air, and water."

"The plant does not eat as we do: it soaks in its food; that is, carbon, for example, is not consumed by it in the natural state known to you as fine black powder, but after it has first been changed from its solid form and dissolved. Now, the solvent of carbon is oxygen, which converts it into carbonic-acid gas, and that is the plant's chief food."

"You say plants live on carbonic-acid gas, that fatal gas that kills us if we breathe a few breaths of it?"

"Yes, my lad, it lives on what would kill us, prepares our food by using what would be death to us if there were enough of it around us. Remember that whatever breathes, whatever burns, whatever ferments, whatever decays, sends out carbonic-acid gas into the atmosphere. The atmosphere, receiving these fatal emanations, would therefore in the course of centuries become unbreathable and suffocating to all animal life on the earth if other agencies did not forestall this accumulation of the deadly gas. Let us now see, to begin with, what statistics have to say as to the amount of carbonic-acid gas that is continually being produced.

"The carbonic-acid gas exhaled by a man in twenty-four hours is estimated at about four hundred [332] and fifty liters, which would represent two hundred and forty grams of burnt carbon and four hundred and fifty liters of oxygen taken from the air to effect this combustion. At this rate the carbonic-acid gas produced in a year by the whole human family would amount to about one hundred and sixty billion cubic meters, which represents eighty-six billion, two hundred and seventy million kilograms of burnt carbon. Piled in one heap, this carbon would make a mountain a league around at its base and from four hundred to five hundred meters high. Such is the quantity of fuel required for the maintenance of man's natural heat. All of us together eat carbon to this extent, and in the course of a year we breathe it out, a breath at a time, in the form of carbonic-acid gas. Then we start on the consumption of another pile of the same size. How many mountains of carbon, then, since the world began, must mankind have breathed out into the atmosphere!

"We have also to take into account the vast variety and extent of animal life on land and in the sea, and its requirements in the way of carbon, requirements that would in the course of a year represent a mountain of perhaps the size of Mont Blanc. Animals far outnumber us, abounding as they do all over the earth, including oceans as well as continents. What a quantity of carbon to keep the flame of life burning! And to think that it all sooner or later goes into the air as a fatal gas, a few breaths of which would quickly kill us!

"Nor is that the whole story. Fermenting sub- [333] stances, like the juice of the grape and like the dough that is to be baked into bread, and all decaying matter,—such as, for instance, is found in the form of manure spread over a cultivated field,—these, too, are rich sources of carbonic-acid gas. Even if the manure is of no great strength it will send out one hundred cubic meters or more of carbonic-acid gas in a single day for every acre of land on which it is spread.

"Coal, wood, charcoal, and other fuel used for heating our houses, and the great quantities of coal needed for running our factories-do not these also contribute to the atmosphere their share of the harmful gas we are speaking of? Just think of the quantity of carbonic-acid gas vomited into the air from the smoke-stack of a great factory that consumes coal by the car-load! Think, too, of the volcanoes, those giant natural chimneys that in a single eruption throw up enough of this gas to make the belchings of a factory furnace seem like a mere whiff on the breeze.

"It is evident that carbonic-acid gas is constantly being poured into the atmosphere in torrents that defy computation; and yet animal life has no reason to fear suffocation, either new or in the future. The atmosphere, continually being tainted, is as continually being purified: as fast as it is laden with carbon it is purged of it. Now, the health-office charged with the safeguarding of the general physical welfare is the plant, my little friends, the plant that lives on carbonic-acid gas to prevent our dying by breathing it, and with it prepares the food that [334] is to sustain us. This fatal gas in which is taken up so much of the putrefaction of all things is the plant's chief sustenance. To the plant's wonderful stomach, putrefaction is satisfaction. What death has cut down, the blade of grass builds up again.

"It is plain enough that there is never any lack of carbonic-acid gas in the air we breathe, and also that it does not accumulate to the point of becoming a menace to life, as one might at first infer from the abundance in which it is produced. In this dish is some lime-water that I poured out yesterday, when it was perfectly clear. Look at the surface and you will see a delicate transparent crust, which cracks and breaks if you touch it with the point of a pin. It might be taken for a thin sheet of ice. What can it be? The answer is plain: the air, coming into contact with lime-water, has yielded to the latter some of its carbonic-acid gas, and carbonate of lime has consequently formed, not in this instance as a white chalky powder, but as a transparent crystalline sheet."

"I've often noticed," said Jules, "the same sort of crust on water in which lime has been slaked for making mortar. I should have taken it for ice, but as it didn't melt in the summer sun I concluded it must be something else."

"It was carbonate of lime, formed exactly like that in this dish by the union of carbonic-acid gas from the atmosphere with lime dissolved in the water. Now that our talk has led to this subject, let us say a little more about mason's mortar. You know how it is prepared. The lime-burner first [335] heats a mass of broken limestone very hot in a lime-kiln, and the heat drives out the carbonic-acid gas, leaving the lime by itself. This lime the mason slakes with water and mixes with sand, thus producing mortar, which is laid by the trowel between the stones that go to make a building. At first a soft paste that readily fills in all empty spaces, it gradually becomes impregnated with carbonic acid from the atmosphere, this process being aided by the loosely compacted grains of sand in the mortar, and finally, in consequence of the union of the carbonic acid with the lime, turns to stone as hard and firm as that from which the lime was originally produced. In course of time, therefore, the hardening of the mortar being accomplished, the entire structure of stone and mortar becomes welded into one solid mass, so that often the stones themselves, if one tries to separate them, will break before the mortar will relax its hold. It is, then, carbonic-acid gas from the atmosphere that hardens the mason's mortar by turning the lime it contains back into limestone.

"There is always, as I have said, carbonic-acid gas in the air around us: the hardening of mortar and the forming of a brittle crust on lime-water when exposed to the air prove this sufficiently. But there is not much of this gas thus floating about us; for if the chemist subjects the air to delicate tests such as are beyond the resources of our modest laboratory, he finds that never and nowhere is there more than one liter of carbonic-acid gas in two thousand liters of air. What becomes, then, of the [336] enormous volumes of this gas continually being added to the atmosphere? Vegetation feeds upon it and thus causes it to disappear, as we shall soon see.

"The surface of a leaf is riddled with numerous excessively small holes called stomata, or mouths. On a single leaf more than a million can be counted; but, being so thickly assembled in so small a space, they are of course too small to be seen without a magnifying-glass, a microscope. I cannot show them to you in their natural state, but only as represented in this picture. Well, through these tiny mouths the plant breathes, not pure air as we do, but poisoned air, fatal to the animal, but life-giving to the plant. It inhales, through these millions of myriads of stomata, the carbonic-acid gas dispersed throughout the atmosphere, receiving it into the substance of the leaves, where under the action of the sun's rays an incomprehensible process is accomplished. Stimulated by the light of the sun, the leaves work upon the fatal gas and rid it completely of its carbon. In other words, they decompose the carbonic-acid gas, undo what combustion has done, separate the carbon from the oxygen combined with it.


[Illustration]

STOMATA, HIGHLY MAGNIFIED.

"You must not think it is any easy task to restore to their original condition two substances combined by burning, by oxidizing. The chemist will have to exercise his utmost ingenuity and use his most po- [337] tent drugs if he wishes to break the hold that carbon and oxygen have on each other in carbonic-acid gas. Well, it is just this task, which would tax the resources of a chemical laboratory, that the leaves accomplish quietly, without effort, instantaneously, but only on the express condition that the sun gives its aid.

"If the sun does not shine, the plant can do nothing with carbonic-acid gas, which is its chief food. Then it languishes in a half-starved condition, reaching out as if seeking the much-needed light and losing the green color, the hue of health, from its leaves and stems, until finally it perishes. This sickly state caused by lack of light is called etiolation, or bleaching. In market-gardening it is resorted to for the purpose of making certain vegetables more tender and their flavor less strong. Thus it is customary to tie closely together the stems or stalks of a plant used for salad, in this way keeping the sunlight from the central portion and causing it to turn white and tender. So, too, it is usual to heap up the earth around the stalks of artichokes and celery, as their flavor would be unbearable without this treatment of darkness. Lay a tile flat on the grass, or invert a flower-pot over a plant, and after a few days you will find the matted grass of the shaded plant yellow and sickly.

"But when, on the other hand, a plant receives the sun's rays in full force, carbonic-acid gas is decomposed in no time, the carbon and the oxygen separating and each resuming its original attributes. Freed of its load of carbon, the oxygen becomes [338] again what it was before the union took place; it is once more a breathable gas, if diluted with nitrogen, a gas that will support life and make a fire burn. In this pure state it is given out by the stomata to take its part again in combustion and respiration. It entered the leaves a fatal gas, it departs a vivifying gas. It will find its way back some day with a fresh load of carbon, deposit its burden in the plant's storehouse, and, once more purified, recommence its atmospheric round. Bees go and come, one after another, from the hive to the fields and from the fields to the hive, by turn, in the one instance lightened of their heavy load and eager for a fresh burden, and in the other laden with honey and winging their way heavily back to the hive. Oxygen is the swarm of the plant hive: it reaches the stomata with a load of carbon taken from the veins of animals, from burning fuel, or, it may be, from substances undergoing decay, and it gives this carbon to the plant and then departs on its untiring round.

"As to the carbon that the leaves separate from the oxygen, it remains in the plans and enters into the composition of the sap, turning finally into sugar, gum, oil, starch, wood, or some other vegetable substance. Sooner or later these substances are decomposed by the slow combustion of decay, or by the less gradual combustion that takes place in animal nutrition, and the carbon once more enters into the formation of carbonic-acid gas and returns to the atmosphere to feed another generation of plant life, which will hand on to animal life the food products [339] it has with the help of this carbon manufactured.

"Now I will ask Emile if he remembers that old tree stump we were talking about that was some day, it might be, to contribute its carbon to the making of a slice of bread and butter to eat. Does he remember the oak that was to give of its carbon to produce a loaf of white bread or some other article of food? Was I not right in saying it is quite possible that to-day we eat, in the form of a buttered roll, what we some time before burned on the hearth as a stick of wood?"

"I haven't in the least forgotten," replied Emile, "how puzzled I was when you told us those things might some day give us our bread and butter. Now I begin to see how it might come about. A stick of wood burns in the fireplace, and its carbon goes out of it combined with oxygen, making carbonic-acid gas, which is scattered in the air. Plants take in this gas as food and, the next thing we know, the carbon has turned to flour in the grains of wheat, or it may have become grass and been eaten by cows to make butter; and so we have the slice of bread and butter all complete. And then there's no reason why this carbon from the stick of wood, after it has traveled through the air, should not become a stick of wood again and be burned once more in a fireplace, so that it might go the same round again and again, no one knows how many times."

"Yes, my boy, it can keep going back and forth in that way indefinitely; for the same carbon is forever passing from atmosphere to plant, from plant to animal, and from animal to atmosphere, [340] this last being the common storehouse whence all forms of life derive the principal material of which they are made. Oxygen is the common carrier of this material. The animal gets its carbon from plants or from other animals, in the form of food, and in the end makes carbonic-acid gas out of it, with the help of oxygen, and out it goes into the air. Plants take this unbreathable gas from the air and give back pure oxygen in return, using the carbon in making food for men and animals. Thus the two kingdoms, animal and vegetable, help each other, the former making carbonic-acid gas to feed the latter, and this in turn making breathable air and nutritious substances out of the fatal gas."

"Those are the most wonderful things you have told us yet," declared Jules, deeply impressed by these marvelous transformations. "When you began to tell us about the vials that made the cook's nose turn blue, I thought it was going to be just a funny story, and never dreamed it would turn out to be so interesting and so serious."

"Yes, my child, what I have just told you is indeed interesting and serious, perhaps too serious for one so young as you; but I could not resist the temptation to acquaint you with the beautiful concord existing between the plant that feeds the animal and the animal that feeds the plant.

"Let us now descend from these heights and go on to another experiment. We wish to prove that plants do really decompose carbonic-acid gas. The simplest way to do this is to let the operation go on under water, a method that enables us to observe [341] the release of the gas,—that is the oxygen,—and to collect it. Ordinary water always contains a little carbonic-acid gas in solution, obtained either from the soil or from the atmosphere; therefore we shall not have to supply this to the immersed plant.

"Into a wide-mouthed glass jar filled with ordinary water we put a recently cut plant in full leaf. A water-plant is preferable, as with it quicker and longer-continued results are obtained. We next invert the jar with its mouth in a bowl of water, and then set the whole thing in the sunlight. Soon the leaves are covered with tiny bead-like bubbles, which rise to the upper end of the inverted jar and accumulate there, forming a layer of gas. This gas, as has been proved by trial, will rekindle a match that has just been extinguished, provided by trial, will rekindle a match that has just been extinguished, provided it has a spark left; and thus the gas is shown to be oxygen. Hence the carbonic-acid gas dissolved in the water must have been resolved by the leaves into its original elements, the oxygen being set free and the carbon remaining in the leaves.

"But let us put aside our laboratory outfit and resort to the simplest possible proof of the fact in question. Let us go to the nearest pond, where, in some stretch of still water, we shall find a population of tadpoles living and flourishing, either lying in the sun at the water's edge or swimming out into the deeper part, where they frisk and play in freedom. In this pond, also, are to be found various mollusks crawling slowly along, and small shell-fish propelling themselves by jerks as they lash the water with the end of their tail; larvae, too, encased [342] in little molds of fine sand, and black leeches lying in wait for their prey; and finally, sticklebacks, graceful little fish armed with spines on the back, hence the name.

"All these creatures, of whatever sort, breathe oxygen but it is oxygen held in solution by the water. If the vivifying gas were lacking in the pond, all this teeming population would infallibly perish. Another danger menaces it, also. The bed of the pond is black mud, being an accumulation of decaying matter such as rotting leaves, dead plants, ejections from the aquatic population, the lifeless bodies of various kinds of tiny creatures, and other refuse. This bed of decomposing matter is continually throwing off carbonic-acid gas, as fatal for the stickleback and the tadpole to breathe as for us. How, then, is the water kept clear of this unbreathable gas and enriched with life-giving oxygen for the maintainence of the pond's population?


[Illustration]

"Water-plants perform this office of sanitation by feeding on the dissolved carbonic-acid gas, decomposing it under the sun's rays, and giving out oxygen in its place. Decay supports the plant, and the plant supports the animal. Among the various kinds of plants entrusted with the sanitation of stagnant water I will mention the confervae, that is to say the delicate, green, thread-like growths that overlay the floor of the pond or other body of fresh water with a sort of thick velvet carpet, or float [343] about in jelly-like flakes. Put one of these plants into a bottle of water, and after a short exposure to the direct rays of the sun you will see the plant covered with innumerable tiny beads of gas. These are bubbles of oxygen from decomposed carbonic-acid gas. Caught in the sticky network of the water-plant, these bubbles increase until they finally buoy up the plant and so raise it, all covered with foam, to the surface of the water.

"This experiment calls for no special outfit. Break off a green fragment from some aquatic plant, put it into a glass of water, set the glass in the sun, and soon you will see your little oxygen factory in full operation. When the process is well under way, set the glass in the shade, and the release of gas will stop at once; but put the glass back in the sun, and the formation of gas-bubbles will promptly begin again, thus proving the need of the sun's rays in this wonderful process. I cannot imagine a more beautiful or lucid experiment. It is, too, so simple that I leave you to perform it yourselves.

"The bit of green water-plant making breathable gas in a glass of water set in the sun, will enable you to understand the work of sanitation accomplished in large bodies of water by aquatic vegetation just as a like purification of the atmosphere is effected by terrestrial vegetation. All the green plants of various kinds growing in a standing body of fresh water become covered with tiny beads of oxygen if they receive the sun's rays to help them in their work, and this oxygen dissolves in the water and gives it new life. It is thus by means of the lowest [344] forms of vegetation that standing water, instead of becoming pestilential, may be kept in a condition to support innumerable species of aquatic life.

"From all this you may learn a little lesson that will perhaps be of use to you. How many times you have tried to keep sticklebacks alive in a glass jar! But the attempt has always failed. In water not frequently renewed the little fish soon died, perishing as soon as the small amount of oxygen contained in the water was exhausted by their breathing. Hereafter, if you wish to succeed, put a good-sized clump of confervae into a jar. Plant and fish will help each other, the plant supplying the stickleback with oxygen, and the stickleback providing carbonic-acid gas for the plant; so both will prosper even in unrenewed water. In short, then, if you would keep your water animals alive, do not forget to give them their indispensable companions, waterplants."


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