A pure culture of bacteria. Notice that the bacteria are all the same size and shape.
Size and Form.—In size, bacteria are the most minute plants known. A bacterium of average size is about 1/10000 of an inch in length, and perhaps 1/50000 of an inch in diameter. Some species are much larger, others smaller. A common spherical form is 1/50000 of an inch in diameter. They are so small that several million are often found in a single drop of impure water or sour milk. Three well-defined forms of bacteria are recognized: a spherical form called a coccus, a rod-shaped bacterium, the bacillus, and a spiral form, the spirillum. Some bacteria are capable of movement when living in a fluid. Such movement is caused by tiny lashlike threads of protoplasm called flagella. The flagella project from the body, and by a rapid movement cause locomotion to take place. Bacteria reproduce with almost incredible rapidity. It is estimated that a single bacterium, by a process of division called fission, will give rise to over 16,700,000 others in twenty-four hours. Under unfavorable conditions they stop dividing and form rounded bodies called spores. This spore is usually protected by a wall and may withstand very unfavorable conditions of dryness or heat; even boiling for several minutes will not kill some forms.
A figure to show the relative size and shape of (1) a green mold, (2) yeast cells, and (3) different forms of bacteria; B, bacillus; C, coccus; S, spirillum forms. The yeast and bacteria are drawn to scale, they are much enlarged in proportion to the green mold, being actually much smaller than the mold spores seen at the top of the picture.
Where Bacteria are most Numerous.—As the result of our experiments, we can make some generalizations concerning the presence of bacteria in our own environment. They are evidently present in the air, and in greater quantity in air that is moving than quiet air. Why? That they stick to particles of dust can be proven by placing a little dust from the schoolroom in a culture dish. Bacteria are present in greater numbers where crowds of people live and move, the air from dusty streets of a populous city contains many more bacteria than does the air of a village street. The air of a city park contains relatively few bacteria as compared with the near-by street. The air of the woods or high mountains fewer still. Why? Our previous experiment has shown that dirt on our hands, the mouth and teeth, decayed meat and vegetables, dirty money, the very hairs of our head are all carriers of bacteria.
Fluids the Favorite Home of Bacteria.—Tap water, standing water, milk, vinegar, wine, cider all can be proven to contain bacteria by experiments similar to those quoted above. Spring or artesian well water would have very few, if any, bacteria, while the same quantity of river water, if it held any sewage, might contain untold millions of these little organisms.
Foods preferred by Bacteria.—If bacteria are living and contain no chlorophyll, we should expect them to obtain protein food in order to grow. Such is not always the case, for some bacteria seem to be able to build up protein out of simple inorganic nitrogenous substances. If, however, we take several food substances, some containing much protein and others not so much, we will find that the bacteria cause decay in the proteins almost at once, while other food substances are not always attacked by them.
Growth of bacteria in a drop of impure water allowed to run down a sterilized culture in a dish.
What Bacteria do to Foods.—When bacteria feed upon a protein they use part of the materials in the food so that it falls to pieces and eventually rots. The material left behind after the bacteria have finished their meal is quite different from its original form. It is broken down by the action of the bacteria into gases, fluids, and some solids. It has a characteristic "rotten" odor and it has in it poisons which come as a result of the work of the bacteria. These poisonous wastes, called ptomaines, we shall learn more about later.
Conditions Favorable and Unfavorable to the Growth of Bacteria.—Moisture and Dryness.—Experiment.—Take two beans, remove the skins, crush one, soak the second bean overnight and then crush it. Place in test tubes, one dry, the second with water. Leave in a warm place two or three days, then smell each tube. In which is decay taking place? In which tube are bacteria at work? How do you know?
Moisture.—Moisture is an absolute need for bacterial growth, consequently keeping material dry will prevent the growth of germs upon its surface. Foods, in order to decay, must contain enough water to make them moist. Bacteria grow most freely in fluids.
Light.—If we cover one half of a petri dish in which bacteria are growing with black paper and then place the dish in a light warm place for a few days, the growth of bacteria in the light part of the dish will be found to be checked, while growth continues in the covered part. It is a matter of common knowledge that disease germs thrive where dirt and darkness exist and are killed by any long exposure to sunlight. This shows us the need of light in our homes, especially in our bedrooms.
Air.—We have seen that plants need oxygen in order to perform the work that they do. This is equally true of all animals. But not all bacteria need air to live; in fact, some are killed by the presence of air. Just how these organisms get the oxygen necessary to oxidize their food is not well understood. The fact that some bacteria grow without air makes it necessary for us to use the one sure weapon we have for their extermination, and that is heat.
Heat.—Experiment.—Take four cultures containing bouillon, inoculate each tube with bacteria and plug each tube with absorbent cotton. Place one tube in the ice box, a second tube in a dark closet at a moderate temperature, a third in a warm place (about 100° Fahrenheit), and boil the contents of the fourth tube for ten minutes, then place it with tube number two. In which tubes does growth take place most rapidly? Why?
Bacteria grow very slowly if at all in the temperature of an ice box, very rapidly at the room temperature of from 70° to 90° and much less rapidly at a higher temperature. All bacteria except those which have formed spores can be instantly killed as soon as boiling point is reached, and most spores are killed by a few minutes boiling.
Sterilization.—The practical lessons drawn from sterilization are many. We know enough now to boil our drinking water if we are uncertain of its purity; we sterilize any foods that we believe might harbor bacteria, and thus keep them from spoiling. The industry of canning is built upon the principle of sterilization.
Canning.—Canning is simply a method by which first the bacteria in a substance are killed by heating and then the substance is put into vessels into which no more bacteria may gain entrance. This is usually done at home by boiling the fruit or vegetable to be canned either in salt and water or with sugar and water, either of which substances aids in preventing the growth of bacteria. The time of boiling will be long or short, depending upon the materials to be canned. Some vegetables, as peas, beans, and corn, are very difficult to can, probably because of spores of bacteria which may be attached to them. Fruits, on the other hand, are usually much easier to preserve. After boiling for the proper time, the food, now free from all bacteria, must be put into jars or cans that are themselves absolutely sterile or free from germs. This is done by first boiling the jars, then pouring the boiling hot material into the hot jars and sealing them so as to prevent the entrance of bacteria later.
Uses of Canning.—Canning as an industry is of immense importance to mankind. Not only does it provide him with fruits and vegetables at times when he could not otherwise get them, but it also cheapens the cost of such things. It prevents the waste of nature's products at a time when she is most lavish with them, enabling man to store them and utilize them later. Canning has completely changed the life of the sailor and the soldier, who in former times used to suffer from various diseases caused by lack of a proper balance of food.
Pasteurizing milk. Why should this be done?
Pasteurization.—Milk is one of the most important food supplies of a great city. It is also one of the most difficult supplies to get in good condition. This is in part due to the fact that milk is produced at long distances from the city and must be brought first from farms to the railroads, then shipped by train, again taken to the milk supply depot by wagon, there bottled, and again shipped by delivery wagons to the consumers. When we remember that much of the milk used in New York City is forty-eight hours old and when we realize that bacteria grow very rapidly in milk, we see the need of finding some way to protect the supply so as to make it safe, particularly for babies and young children.
This is done by pasteurization, a method named after the French bacteriologist Louis Pasteur. To pasteurize milk we heat it to a temperature of not over 170° Fahrenheit for from ten minutes to half an hour. By such a process all harmful germs will be killed and the keeping qualities of the milk greatly lengthened. Most large milk companies pasteurize their city supply by a rapid pasteurization at a much higher temperature, but this method slightly changes the flavor of the milk.
Cold Storage.—Man has also come to use cold to keep bacteria from growing in foods. The ice box at home and cold storage on a larger scale enables one to keep foods for a more or less lengthy period. If food is frozen, as in cold storage, it might keep without growth of bacteria for years. But fruits and vegetables cannot be frozen without spoiling their flavor. And all foods after freezing seem particularly susceptible to the bacteria of decay. For that reason products taken from cold storage must be used at once.
Ptomaines.—Many foods get their flavor from the growth of molds or bacteria in them. Cheese, butter, the gamey taste of certain meats, the flavor of sauerkraut, are all due to the work of bacteria. But if bacteria are allowed to grow so as to become very numerous, the ptomaines which result from their growth in foods may poison the person eating such foods. Frequently ptomaine poisoning occurs in the summer time because of the rapid growth of bacteria. Much of the indigestion and diarrhœa which attack people during the summer is doubtless due to this kind of poisoning.
Preservatives.[21]—This leads us to ask if we may not preserve food in ways other than those mentioned so as to protect ourselves from danger of ptomaine poisoning. Many substances check the development of bacteria and in this way they preserve the food. Preservatives are of two kinds, those harmless to man and those that are poisonous. Of the former, salt and sugar are examples; of the latter, formaldehyde and possibly benzoic acid.
Sugar.—We have noted the use of sugar in canning. Small amounts of sugar will be readily attacked by yeasts, molds, and bacteria, but a 40 to 50 per cent solution will effectually keep out bacteria. Preserves are fruits boiled in about their own weight of sugar. Condensed milk is preserved by the sugar added to it; so are candied and, in part, dried fruits.
Salt.—Salt has been used for centuries to keep foods. Meats are smoked, dried, and salted; some are put down in strong salt solutions. Fish, especially cod and herring, are dried and salted. The keeping of butter is also due to the salt mixed with it. Vinegar is another preservative. It, like salt, changes the flavor of materials kept in it and so cannot come into wide use. Spices are also used as preservatives.
Harmful Preservatives.—Certain chemicals and drugs, used as preservatives, seem to be on the border line of harmfulness. Such are benzoic acid, borax, or boracic acid. Such drugs may be harmless in small quantities, but unfortunately in canned goods we do not always know the amount used. The national government in 1906 passed what is known as the Pure Food Law, which makes it illegal to use any of these preservatives (excepting benzoic acid in very small amounts). Food which contains this preservative will be so labeled and should not be given to children or people with weak digestion. Unfortunately people do not always read the labels and thus the pure food law is ineffective in its working. Infrequently formaldehyde or other preservatives are used in milk. Such treatment renders milk unfit for ordinary use and is an illegal process.
Disinfectants.[22]—Frequently it becomes necessary to destroy bacteria which cause diseases of various kinds. This process is called disinfecting. The substances commonly used are carbolic acid, formalin or formaldehyde, lysol, and bichloride of mercury. Of these, the last named is the most powerful as well as the most dangerous to use. As it attacks metal, it should not be used in a metal pail or dish. It is commonly put up in tablets which are mixed to form a 1 to 1000 solution. Such tablets should be carefully safeguarded because of possible accidental poisoning.
Formaldehyde used in liquid form is an excellent disinfectant. When burned in a formalin candle, it sets free an intensely pungent gas which is often used for disinfecting sick rooms after the patient has been removed.
This shows how organic matter is broken down by bacteria so it may be used again by green plants.
Carbolic acid is perhaps the best disinfectant of all. If used in a solution of about 1 part to 25 of water, it will not burn the skin. It is of particular value to disinfect skin wounds, as it heals as well as cleanses when used in a weak solution. Its rather pleasant odor makes it useful to cover up unpleasant smells of the sick room.
The fumes of burning sulphur, which are so often used for disinfecting, are of little real value.
Bacteria cause Decay.—Let us next see in what ways the bacteria directly influence man upon the earth. Have you ever stopped to consider what life would be like on the earth if things did not decay? The sea would soon be filled and the land covered with dead bodies of plants and animals. Conditions of life would become impossible and living things on the earth would cease to exist.
Fortunately, bacteria cause decay. All organic matter, in whatever form, is sooner or later decomposed by the action of untold millions of bacteria which live in the air, water, and soil. These soil bacteria are most numerous in rich damp soils containing large amounts of organic material. They are very numerous around and in the dead bodies of plants and animals. To a considerable degree, then, these bacteria are useful in feeding upon these dead bodies, which otherwise would soon cover the surface of the earth to the exclusion of everything else. Bacteria may thus be scavengers. They oxidize organic materials, changing them to compounds that can be absorbed by plants and used in building protoplasm. Without bacteria and fungi it would be impossible for life to exist on the earth, for green plants would be unable to get the raw food materials in forms that could be used in making food and living matter. In this respect bacteria are of the greatest service to mankind.
Microscopic appearance of ordinary milk, showing fat globules and bacteria which cause the souring of milk.
Relation to Fermentation.—They may incidentally, as a result of this process of decay, continue the process of fermentation begun by the yeasts. In making vinegar the yeasts first make alcohol (see page 135) which the bacteria change to acetic acid. The lactic acid bacteria, which sour milk, changing the milk sugar to an acid, grow very rapidly in a warm temperature; hence milk which is cooled immediately and kept cool or which is pasteurized and kept in a cool place will not sour readily. Why? These same lactic acid bacteria may be useful when they sour the milk for the cheese maker.
Other Useful Bacteria.—Certain bacteria give flavor to cheese and butter, while still other bacteria aid in the "curing" of tobacco, in the production of the dye indigo, in the preparation of certain fibers of plants for the market, as hemp, flax, etc., in the rotting of animal matter from the skeletons of sponges, and in the process of tanning hides to make leather.
A field of alfalfa, a plant which harbors the nitrogen-fixing bacteria.
Nitrogen-fixing Bacteria.—Still other bacteria, as we have seen before, "change over" nitrogen in organic material in the soil and even the free nitrogen of the air so that it can be used by plants in the form of a compound of nitrogen. The bacteria living in tubercles on the roots of clover, beans, peas, etc., have the power of thus "fixing" the free nitrogen in the air found between particles of soil. This fact is made use of by farmers who rotate their crops, growing first a crop of clover or other plants having root tubercles, which produce the bacteria, then plowing these in and planting another crop, as wheat or corn, on the same area. The latter plants, making use of the nitrogen compounds there, produce a larger crop than when grown in ground containing less nitrogenous material.
Bacteria cause Disease.—The most harmful bacteria are those which cause diseases of plants and animals. Certain diseases of plants—blights, rots, and wilts—are of bacterial nature. These do much annual damage to fruits and other parts of growing plants useful to man as food. But by far the most important are the bacteria which cause disease in man. They accomplish this by becoming parasites in the human body. Millions upon millions of bacteria exist in the human body at all times—in the mouth, on the teeth, in the blood, and especially in the lower part of the food tube. Some in the food tube are believed to be useful, some harmless, and some harmful; others in the mouth cause decay of the teeth, while a few kinds, if present in the body, may cause disease.
Tubercles on the roots of the soy bean. They contain the nitrogen-fixing bacteria. (Fletcher's Soils.) Copyright by Doubleday, Page and Company.
It is known that bacteria, like other living things, feed and give off organic waste from their own bodies. This waste, called a toxin, is poison to the host on which the bacteria live, and it is usually the production of this toxin that causes the symptoms of disease. Some forms, however, break down tissues and plug up the small blood vessels, thus causing disease.
Diseases caused by Bacteria.—It is estimated that bacteria cause annually over 50 per cent of the deaths of the human race. As we will later see, a very large proportion of these diseases might be prevented if people were educated sufficiently to take the proper precautions to prevent their spread. These precautions might save the lives of some 3,000,000 of people yearly in Europe and America. Tuberculosis, typhoid fever, diphtheria, pneumonia, blood poisoning, syphilis, and a score of other germ diseases ought not to exist. A good deal more than half of the present misery of this world might be prevented and this earth made cleaner and better by the coöperation of the young people now growing up to be our future home makers.
A single cell scraped from the roof of the mouth and highly magnified. The little dots are bacteria, most of which are harmless. Notice the comparative size of bacteria and cell.
How we take Germ Diseases.—Germ or contagious diseases either enter the body by way of the mouth, nose, or other body openings, or through a break in the skin. They may be carried by means of air, food, or water, but are usually transmitted directly from the person who has the disease to a well person. This may be done through personal contact or by handling articles used by the sick person or by drinking or eating foods which have received some of the germs. From this it follows that if we know the methods by which a given disease is communicated, we may protect ourselves from it and aid the civic authorities in preventing its spread.
Deaths from tuberculosis compared with other contagious diseases in the city of New York in 1908.
Tuberculosis.—The one disease responsible for the greatest number of deaths—perhaps one seventh of the total on the globe—is tuberculosis. It is estimated that of all people alive in the United States to-day, 5,000,000 will die of this disease. But this disease is slowly but surely being overcome. It is believed that within perhaps one hundred years, with the aid of good laws and sanitary living, it will be almost extinct.
This curve shows a decreasing death rate from tuberculosis. Explain.
Tuberculosis is caused by the growth of bacteria, called the tubercle bacilli, within the lungs or other tissues of the human body. Here they form little tubers full of germs, which close up the delicate air passages in the lungs, while in other tissues they give rise to hip-joint disease, scrofula, lupus, and other diseases, depending on the part of the body they attack. Tuberculosis may be contracted by taking the bacteria into the throat or lungs or possibly by eating meat or drinking milk from tubercular cattle. Especially is it communicated from a consumptive to a well person by kissing, by drinking or eating from the same cup or plate, using the same towels, or in coming in direct contact with the person having the germs in his body. Although there are always some of the germs in the air of an ordinary city street, and though we may take some of these germs into our bodies at any time, yet the bacteria seem able to gain a foothold only under certain conditions. It is only when the tissues are in a worn-out condition, when we are "run down," as we say, that the parasite may obtain a foothold in the lungs. Even if the disease gets a foothold, it is quite possible to cure it if it is taken in time. The germ of tuberculosis is killed by exposure to bright sunlight and fresh air. Thus the course of the disease may be arrested, and a permanent cure brought about, by a life in the open air, the patient sleeping out of doors, taking plenty of nourishing food and very little exercise. See also Chapter XXIV.
This figure shows how sewage from a cesspool (c) might get into the water supply: lm, layer of rock; w, wash water.
Typhoid Fever.—One of the most common germ diseases in this country and Europe is typhoid fever. This is a disease which is conveyed by means of water and food, especially milk, oysters, and uncooked vegetables. Typhoid fever germs live in the intestine and from there get into the blood and are carried to all parts of the body. A poison which they give off causes the fever so characteristic of the disease. The germs multiply very rapidly in the intestine and are passed off from the body with the excreta from the food tube. If these germs get into the water supply of a town, an epidemic of typhoid will result. Among the recent epidemics caused by the use of water containing typhoid germs have been those in Butler, Pa., where 1364 persons were made ill; Ithaca, N. Y., with 1350 cases; and Watertown, N. Y., where over 5000 cases occurred. Another source of infection is milk. Frequently epidemics have occurred which were confined to users of milk from a certain dairy. Upon investigation it was found that a case of typhoid had occurred on the farm where the milk came from, that the germs had washed into the well, and that this water was used to wash the milk cans. Once in the milk, the bacteria multiplied rapidly, so that the milkman gave out cultures of typhoid in his milk bottles. Proper safeguarding of our water and milk supply is necessary if we are to keep typhoid away.
Blood Poisoning.—The bacterium causing blood poisoning is another toxin-forming germ. It lives in dust and dirt and is often found on the skin. It enters the body through cuts or bruises. It seems to thrive best in less oxygen than is found in the air. It is therefore important not to close up with court-plaster wounds which such germs may have entered. It, with typhoid, is responsible for four times as many deaths as bullets and shells in time of battle. The wonderfully small death rate of the Japanese army in their war with Russia was due to the fact that the Japanese soldiers always boiled their drinking water before using it, and their surgeons always dressed all wounds on the battlefield, using powerful antiseptics in order to kill any bacteria that might have lodged in the exposed wounds.
This figure shows how a milk route might be instrumental in spreading diphtheria. X is a farm on which a case of diphtheria occurred that was responsible for all the cases along milk routes A and F in Hyde Park, Dorchester, and Milton. How would you explain this?
Other Diseases.—Many other diseases have been traced to bacteria. Diphtheria is one of the best known. As it is a throat disease, it may easily be conveyed from one person to another by kissing, putting into the mouth objects which have come in contact with the mouth of the patient, or by food into which the germs have been carried. Another disease which probably causes more misery in the world than any other germ disease is syphilis. Hundreds of thousands of new-born babies die annually or grow up handicapped by deformities from this dread scourge. Syphilis and gonorrhea, both diseases of the same sort and contracted in the same manner, hand down to innocent wives and still more innocent children a heritage of disease "even unto the third and fourth generation." Grippe, pneumonia, whooping cough, and colds are believed to be caused by bacteria. Other diseases, as malaria, yellow fever, sleeping sickness, and probably smallpox, scarlet fever, and measles, are due to the attack of one-celled animal parasites. Of these we shall learn later in Chapter XV.
Immunity.—It has been found that after an attack of a germ disease the body will not soon be again attacked by the same disease. This immunity, of which we will learn more later, seems to be due to a manufacture in the blood of substances which fight the bacteria or their poisons. If a person keeps his body in good physical condition and lives carefully, he will do much toward acquiring this natural immunity.
Acquired Immunity.—Modern medicine has discovered means of protecting the body from some contagious diseases. Vaccination as protection against smallpox, the use of antitoxins (of which more later) against diphtheria, and inoculation against typhoid are all ways in which we may be protected against diseases.
Methods of fighting Germ Diseases.—As we have seen, diseases produced by bacteria may be caused by the bacteria being directly transferred from one person to another, or the disease may obtain a foothold in the body from food, water, or by taking them into the blood through a cut or a wound or a body opening.
It is evident that as individuals we may each do something to prevent the spread of germ diseases, especially in our homes. We may keep our bodies, especially our hands and faces, clean. Sweeping and dusting may be done with damp cloths so as not to raise a dust; our milk and water, when from a suspicious supply, may be sterilized or pasteurized. Wounds through which bacteria might obtain foothold in the body should be washed with some antiseptic such as carbolic acid (1 part to 25 water), which kills the germs. In a later chapter we shall learn more of how we may coöperate with the authorities to combat disease and make our city or town a better place in which to live.[23]
[17] Experiments on conditions favorable to growth of mold should be introduced here.
[18] An experiment to show conditions unfavorable for growth of molds should be shown at this point.
[19] See Goff and Mayne, First Principles of Agriculture, page 59, for formula of Bordeaux mixture.
[20] For directions for making a culture medium, see Hunter, Laboratory Problems in Civic Biology. Culture tubes may be obtained, already prepared, from Parke, Davis, and Company or other good chemists.
[21] Perform experiment here to determine the value of different preservatives. Use sugar, salt, vinegar, boracic acid, benzoic acid, formaldehyde, and alcohol.
[22] Experiment to determine the most effective disinfectants. Use tubes of bouillon containing different strength solutions of formaldehyde, lysol, iodine, carbolic acid, and bichloride of mercury. Results. Conclusions.
[23] Teachers may take up parts or all of Chapter XXIV at this point. I have found it advisable to repeat much of the work on bacteria after the students have taken up the study of the human organism.
elementary
Hunter, Laboratory Problems in Civic Biology. American Book Company.
Bigelow, Introduction to Biology. The Macmillan Company.
Conn, Bacteria, Yeasts, and Molds in the Home. Ginn and Company.
Conn, Story of Germ Life. D. Appleton and Company.
Davison, The Human Body and Health. American Book Company.
Frankland, Bacteria in Daily Life. Longmans, Green, and Company.
Overton, General Hygiene. American Book Company.
Prudden, Dust and its Dangers. G. P. Putnam's Sons.
Prudden, The Story of the Bacteria. G. P. Putnam's Sons.
Ritchie, Primer of Sanitation. World Book Company.
Sharpe, Laboratory Manual in Biology, pages 123-132. American Book Company.
advanced
Conn, Agricultural Bacteriology. P. Blakiston's Sons and Company.
Coulter, Barnes, and Cowles, A Textbook of Botany, Vol. I. American Book Company.
De Bary, Comparative Morphology and Biology of the Fungi, Mycetozoa, and Bacteria. Clarendon Press.
Duggar, Fungous Diseases of Plants. Ginn and Company.
Hough and Sedgwick, The Human Mechanism. Ginn and Company.
Hutchinson, Preventable Diseases. Houghton, Mifflin and Company.
Lee, Scientific Features of Modern Medicine. Columbia University Press.
Muir and Ritchie, Manual of Bacteriology. The Macmillan Company.
Newman, The Bacteria. G. P. Putnam's Sons.
Sedgwick, Principles of Sanitary Science and Public Health. The Macmillan Company.
Problems.—To determine the general biological relations existing between plants and animals.
(a) As shown in a balanced aquarium.
(b) As shown in hay infusion.
Suggestions for Laboratory Work
Demonstration of life in a "balanced" and "unbalanced" aquarium.—Determination of factors causing balance.
Demonstration of hay infusion.—Examination to show forms of animal and plant life.
Tabular comparison between balanced aquarium and hay infusion.
Some Ways in which Plants affect Animals.—We have been studying the life of plants in order better to understand the life of animals and men. We have seen first that green plants play indirectly a tremendous part in man's welfare by supplying him with food. We have found that the colorless plants directly affected his welfare by causing disease, and by causing decay, thus making usable the nitrogen locked up in dead bodies of plants and animals, and by some even supplying nitrogen from the atmosphere. The dependence of animals upon plants has been shown and the interdependence of plants on animals has also been seen in cross-pollination and in the supply of raw food materials to plants by animals.
Study of a Balanced Aquarium.—Perhaps the best way for us to understand the interrelation between plants and animals is to study an aquarium in which plants and animals live and in which a balance has been established between the plant life on one side and animal life on the other. Aquaria containing green pond weeds, either floating or rooted, a few snails, some tiny animals known as water fleas, and a fish or two will, if kept near a light window, show this relation.
A balanced aquarium. Explain the term "balanced."
We have seen that green plants under favorable conditions of sunlight, heat, moisture, and with a supply of raw food materials, give off oxygen as a by-product while manufacturing food in their green cells. We know the necessary raw materials for starch manufacture are carbon dioxide and water, while nitrogenous material is necessary for the making of proteins within the plant. In previous experiments we have proved that carbon dioxide is given off by any living thing when oxidation occurs in the body. The crawling snails and the swimming fish give off carbon dioxide, which is dissolved in the water; the plants themselves, at all times, oxidize food within their bodies, and so must pass off some carbon dioxide. The green plants in the daytime use up the carbon dioxide obtained from the various sources and, with the water taken in, manufacture starch. While this process is going on, oxygen is given off to the water of the aquarium, and this free oxygen is used by the animals there.
This diagram shows that plants and animals on the earth hold the same relation to each other as plants and animals in a balanced aquarium. Explain the diagram in your notebook.
But the plants are continually growing larger. The snails and fish, too, eat parts of the plants. Thus the plant life gives food to the animals within the aquarium. The animals give off certain nitrogenous wastes of which we shall learn more later. These materials, with other nitrogenous matter from the dead parts of the plants or animals, form part of the raw material used for protein manufacture in the plant. This nitrogenous matter is prepared for use by several different kinds of bacteria which first break the dead bodies down and then give it to the plants in the form of soluble nitrates. The green plants manufacture food, the animals eat the plants and give off organic waste, from which the plants in turn make their food and living matter. The plants give off oxygen to the animals, and the animals give carbon dioxide to the plants. Thus a balance exists between the plants and animals in the aquarium. Make a table to show this balance.
The carbon and oxygen cycle in the balanced aquarium. Trace by means of the arrows the carbon from the time plants take it in as CO2 until animals give it off. Show what happens to the oxygen.
The relations between green plants and animals.
Relations between Green Plants and Animals.—What goes on in the aquarium is an example of the relation existing between all green plants and all animals. Everywhere in the world green plants are making food which becomes, sooner or later, the food of animals. Man does not feed to a great extent upon leaves, but he eats roots, stems, fruits, and seeds. When he does not feed directly upon plants, he eats the flesh of plant eating animals, which in turn feed directly upon plants. And so it is the world over; the plants are the food makers and supply the animals. Green plants also give a very considerable amount of oxygen to the atmosphere every day, which the animals may use.
The nitrogen cycle. Trace the nitrogen from its source in the air until it gets back again into the air.
The Nitrogen Cycle.—The animals in their turn supply much of the carbon dioxide that the plant uses in starch making. They also supply some of the nitrogenous matter used by the plants, part being given the plants from the dead bodies of their own relatives and part being prepared from the nitrogen of the air through the agency of bacteria, which live upon the roots of certain plants. These bacteria are the only organisms that can take nitrogen from the air. Thus, in spite of all the nitrogen of the atmosphere, plants and animals are limited in the amount available. And the available supply is used over and over again, perhaps in nitrogenous food by an animal, then it may be given off as organic waste, get into the soil, and be taken up by a plant through the roots. Eventually the nitrogen forms part of the food supply in the body of the plant, and then may become part of its living matter. When the plant dies, the nitrogen is returned to the soil. Thus the usable nitrogen is kept in circulation.[24]
Symbiosis.—We have seen that in the balanced aquarium the animals and plants, in a wide sense, form a sort of unconscious partnership. This process of living together for mutual advantage is called symbiosis. Some animals thus combine with plants; for example, the tiny animal known as the hydra with certain of the one-celled algæ, and, if we accept the term in a wide sense, all green plants and animals live in this relation of mutual give and take. Animals also frequently live in this relation to each other, as the crab, which lives within the shell of the oyster; the sea anemones, which are carried around on the backs of some hermit crabs, aiding the crab in protecting it from its enemies, and being carried about by the crab to places where food is plentiful.
Life in the late stage of a hay infusion. B, bacteria, swimming or forming masses of food upon which the one-celled animals, the paramœcia, are feeding; G, gullet; F.V., food vacuole; C.V., contractile vacuole; P, pleurococcus; P.D., pleurococcus dividing. (Drawn from nature by J. W. Teitz.)
A Hay Infusion.—Still another example of the close relation between plants and animals may be seen in the study of a hay infusion. If we place a wisp of hay or straw in a small glass jar nearly full of water, and leave it for a few days in a warm room, certain changes are seen to take place in the contents of the jar; after a little while the water gets cloudy and darker in color, and a scum appears on the surface. If some of this scum is examined under the compound microscope, it will be found to consist almost entirely of bacteria. These bacteria evidently aid in the decay which (as the unpleasant odor from the jar testifies) is beginning to take place. As we have learned, bacteria flourish wherever the food supply is abundant. The water within the jar has come to contain much of the food material which was once within the leaves of the grass,—organic nutrients, starch, sugar, and proteins, formed in the leaf by the action of the sun on the chlorophyll of the leaf, and now released into the water by the breaking down of the walls of the cells of the leaves. The bacteria themselves release this food from the hay by causing it to decay. After a few days small one-celled animals appear; these multiply with wonderful rapidity, so that in some cases the surface of the water seems to be almost white with active one-celled forms of life. If we ask ourselves where these animals come from, we are forced to the conclusion that they must have been in the water, in the air, or on the hay. Hay is dried grass and may have been cut in a field near a pool containing these creatures. When the pool dried up, the wind may have scattered some of these little organisms in the dried mud or dust. Some may have existed in a dormant state on the hay and the water awakened them to active life. In the water, too, there may have been some living cells, plants and animals.
At first the multiplication of the tiny animals within the hay infusion is extremely rapid; there is food in abundance and near at hand. After a few days more, however, several kinds of one-celled animals may appear, some of which prey upon others. Consequently a struggle for life takes place, which becomes more and more intense as the food from the hay is used up. Eventually the end comes for all the animals unless some green plants obtain a foothold within the jar. If such a thing happens, food will be manufactured within their bodies, a new food supply arises for the animals within the jar, and a balance of life may result.
[24] A small amount of nitrogen gas is returned to the atmosphere by the action of the decomposing bacteria on the ammonia compounds in the soil. (See figure of nitrogen cycle.)
Reference Books
elementary
Hunter, Laboratory Problems in Civic Biology. American Book Company.
Sharpe, A Laboratory Manual for the Solution of Problems in Biology, pp. 133-138. American Book Company.
advanced
Eggerlin and Ehrenberg, The Fresh Water Aquarium and its Inhabitants. Henry Holt and Company.
Furneaux, Life in Ponds and Streams. Longmans, Green, and Company.
Parker, Biology. The Macmillan Company.
Sedgwick and Wilson, Biology. Henry Holt and Company.
Problems.—To determine:
(a) How a one-celled animal is influenced by its environment.
(b) How a single cell performs its functions.
(c) The structure of a single-celled animal.
Laboratory Suggestions
Laboratory study.—Study of paramœcium under compound microscope in its relation to food, oxygen, etc. Determination of method of movement, turning, avoiding obstructions, sensitiveness to stimuli. Drawings to illustrate above points.
Laboratory demonstration.—Living paramœcium to show structure of cell. Demonstration with carmine to show food vacuoles, and action of cilia. Use of charts and stained specimens to show other points of cell structure. Laboratory demonstration of fission.