An example of how a tree may exert energy. This rock has been split by the growing tree.
Foods serve exactly the same purposes in plants and in animals; they either build living matter or they are burned (oxidized) to furnish energy (power to do work). If you doubt that a plant exerts energy, note how the roots of a tree bore their way through the hardest soil, and how stems or roots of trees often split open the hardest rocks, as illustrated in the figure above.
Starch-Making and its Relation to Human Welfare.—Leaves which have been in darkness show starch to be present soon after exposure to light. A corn plant sends 10 to 15 grams of reserve material into the ears in a single day. The formation of fruit, and especially the growth of the grain fields, show the economic importance of this fact. Not only do plants make their own food and store it away, but they make food for animals as well. And the food is stored in such a stable form that it may be sent to all parts of the world in the form of grain or other fruits. Animals, herbivorous and flesh-eating, man himself, all are dependent upon the starch-making processes of the green plant for the ultimate source of their food. When we remember that in 1913 in the United States the total value of all farm crops was over $6,000,000,000, and when we realize that these products came from the air and soil through the energy of the sun, we may begin to realize why as city boys and girls the study of plant biology is of importance to us.
Experiment to show that oxygen is given off by green plants in the sunlight.
Green Plants give off Oxygen in Sunlight.—In still another way green plants are of direct use to us in the city. During this process of starch-making oxygen is given off as a by-product. This may easily be proven by the following experiment.[15] Place any green water plant in a battery jar partly filled with water, cover the plants with a glass funnel and mount a test tube full of water over the mouth of the funnel. Then place the apparatus in a warm sunny window. Bubbles of gas are seen to rise from the plant. After two or three hours of hot sun, enough of the gas can be obtained by displacement of the water to make the oxygen test.
That oxygen is given off as a by-product by green plants is a fact of far-reaching importance. City parks are true "breathing spaces." The green covering of the earth is giving to animals an element that they must have, while the animals in their turn are supplying to the plants carbon dioxide, a compound used in food-making. Thus a widespread relation of mutual helpfulness exists between plants and animals.
Respiration by Leaves.—All living things require oxygen. It is by means of the oxidation of food materials within the plant's body that the energy used in growth and movement is released. A plant takes in oxygen largely through the stomata of the leaves, to a less extent through the lenticels or breathing holes in the stem, and through the roots. Thus rapidly growing tissues receive the oxygen necessary for them to perform their work. The products of oxidation in the form of carbon dioxide are also passed off through these same organs. It can be shown by experiment that a plant uses up oxygen in the darkness; in the light the amount of oxygen given off as a by-product in the process of starch-making is, of course, much greater than the amount used by the plant.
Summary.—From the above paragraphs it is seen that a leaf performs the following functions: (1) breathing, or the taking in of oxygen and passing off of carbon dioxide; (2) starch-making, with the incidental passing out of oxygen; (3) formation of proteins, with their digestion and assimilation to form new tissues; and (4) the transpiration of water.
[14] The "rubber plant" leaf is an easily obtainable and excellent demonstration.
[15] Immediate success with this experiment will be obtained if the water has been previously charged with carbon dioxide.
Reference Books
elementary
Hunter, Laboratory Problems in Civic Biology. American Book Company.
Andrews, A Practical Course in Botany, pages 160-177. American Book Company.
Coulter, A Textbook of Botany, pages 5-40. D. Appleton and Company.
Coulter, Plant Life and Plant Uses. American Book Company.
Dana, Plants and their Children, pages 135-185. American Book Company.
Sharpe, A Laboratory Manual in Biology, pages 90-102. American Book Company.
Stevens, Introduction to Botany, pages 81-99. D. C. Heath and Company.
advanced
Clement, Plant Physiology and Ecology. Henry Holt and Company.
Coulter, Barnes, and Cowles, A Textbook of Botany, Part II, and Vol. II. American Book Company.
Darwin, Insectivorous Plants. D. Appleton and Company.
Duggar, Plant Physiology. The Macmillan Company.
Goodale, Physiological Botany, pages 337-353 and 409-424. American Book Company.
Green, Vegetable Physiology. J. and A. Churchill.
Lubbock, Flowers, Fruits, and Leaves, last part. The Macmillan Company.
MacDougal, Practical Textbook of Plant Physiology. Longmans, Green, and Company.
Report of the Division of Forestry, U.S. Department of Agriculture, 1899.
Ward, The Oak. D. Appleton and Company.
Problem.—How green plants store and use the food they make.
(a) What are the organs of circulation?
(b) How and where does food circulate?
(c) How does the plant assimilate its food?
Laboratory Suggestions
Laboratory exercise.—The structure (cross section) of a woody stem.
Demonstration.—To show that food passes downward in the bark.
Demonstration.—To show the condition of food passing through the stem.
Demonstration.—Plants with special digestive organs.
The Circulation and Final Uses of Foods in Green Plants.—We have seen that cells of green plants make food and that such cells are mostly in the leaves. But all parts of the bodies of plants grow. Roots, stems, leaves, flowers, and fruits grow. Seeds are storehouses of food. We must now examine the stem of some plant in order to see how food is distributed, stored, and finally used in the various parts of the plant.
The Structure of a Woody Stem.—If we cut a cross section through a young willow or apple stem, we find it shows three distinct regions. The center is occupied by the spongy, soft pith; surrounding this is found the rather tough wood, while the outermost area is bark. More careful study of the bark reveals the presence of three layers—an outer layer, a middle green layer, and an inner fibrous layer, the latter usually brown in color. This layer is made up largely of tough fiberlike cells known as bast fibers. The most important parts of this inner bark, so far as the plant is concerned, are many tubelike structures known as sieve tubes. These are long rows of living cells, having perforated sievelike ends. Through these cells food materials pass downward from the upper part of the plant, where they are manufactured.
Section of a twig of box elder three years old, showing three annual growth rings. The radiating lines (m) which cross the wood (w) represent the pith rays, the principal ones extending from the pith in the center to the cortex or bark. (From Coulter's Plant Relations.)
In the wood will be noticed (see Figure) a number of lines radiating outward from the pith toward the bark. These are thin plates of pith which separate the wood into a number of wedge-shaped masses. These masses of wood are composed of many elongated cells, which, placed end to end, form thousands of little tubes connecting the leaves with the roots. In addition to these are many thick-walled cells, which give strength to the mass of wood. The bundles of tubes with their surrounding hard walled cells are the continuation of the bundles of tubes which are found in the root. In sections of wood which have taken several years to grow, we find so-called annual rings. The distance between one ring and the next (see Figure) usually represents the amount of growth in one year. Growth takes place from an actively dividing layer of cells, known as the cambium layer. This layer forms wood cells from its inner surface and bark from its outer surface. Thus new wood is formed as a distinct ring around the old wood.
Use of the Outer Bark.—The outer bark of a tree is protective. The cells are dead, the heavy woody skeletons serving to keep out cold and dryness, as well as prevent the evaporation of fluids from within. The bark also protects the tree from attack of other plants or animals which might harm it. Most trees are provided with a layer of corky cells. This layer in the cork oak is thick enough to be of commercial importance. The function of the corky layer in preventing evaporation is well seen in the case of the potato, which is a true stem, though found underground. If two potatoes of equal weight are balanced on the scales, the skin having been peeled from one, the peeled potato will be found to lose weight rapidly. This is due to loss of water, which is held in by the skin of the unpeeled potato (see right hand figure below).
There are also small breathing holes known as lenticels scattered through the surface of the bark. These can easily be seen in a young woody stem of apple, beech, or horse-chestnut.
Experiment to show that the skin of the potato (a stem) retards evaporation.
Proof that Food passes down the Stem.—If freshly cut willow twigs are placed in water, roots soon begin to develop from that part of the stem which is under water. If now the stem is girdled by removing the bark in a ring just above where the roots are growing, the latter will eventually die, and new roots will appear above the girdled area. The food material necessary for the outgrowth of roots evidently comes from above, and the passage of food materials takes place in a downward direction just outside the wood in the layer of bark which contains the bast fibers and sieve tubes. This experiment with the willow explains why it is that trees die when girdled so as to cut the sieve tubes of the inner bark. The food supply is cut off from the protoplasm of the cells in the part of the tree below the cut area. Many of the canoe birches of our Adirondack forest are thus killed, girdled by thoughtless visitors. In the same manner mice and other gnawing animals kill fruit trees. Food substances are also conducted to a much less extent in the wood itself, and food passes from the inner bark to the center of the tree by way of the pith plates. This can be proved by testing for starch in the pith plates of young stems. It is found that much starch is stored in this part of the tree trunk.
Experiment to show that food material passes down in the inner bark.
In what Form does Food pass through the Stem?—We have already seen that materials in solution (those substances which will dissolve in the water) will pass from cell to cell by the process of osmosis. This is shown in the experiment illustrated in the figure. Two thistle tubes are partly filled, one with starch and water, the other with sugar and water, and a piece of parchment paper is tied over the end of each. The lower ends of both tubes are placed in a glass dish under water. After twenty-four hours, the water in the dish is tested for starch, and then for sugar. We find that only the sugar, which has been dissolved by the water, can pass through the membrane.
Experiment to show osmosis of sugar (right hand tube) and non-osmosis of starch (left hand tube).
Digestion.—Much of the food made in the leaves is stored in the form of starch. But starch, being insoluble, cannot be passed from cell to cell in a plant. It must be changed to a soluble form, for otherwise it could not pass through the delicate cell membranes. This is accomplished by the process of digestion. We have already seen that starch is changed to grape sugar in the corn by the action of a substance (an enzyme) called diastase. This process of digestion seemingly may take place in all living parts of the plant, although most of it is done in the leaves. In the bodies of all animals, including man, starchy foods are changed in a similar manner, but by other enzymes, into soluble grape sugar.
The food material may be passed in a soluble form until it comes to a place where food storage is to take place, then it can be transformed to an insoluble form (starch, for example); later, when needed by the plant in growth, it may again be transformed and sent in a soluble form through the stem to the place where it will be used.
In a similar manner, protein seems to be changed and transferred to various parts of the plant. Some forms of protein substance are soluble and others insoluble in water. White of egg, for example, is slightly soluble, but can be rendered insoluble by heating it so that it coagulates. Insoluble proteins are digested within the plant; how and where is but slightly understood. In a plant, soluble proteins pass down the sieve tubes in the bast and then may be stored in the bast or medullary rays of the wood in an insoluble form, or they may pass into the fruit or seeds of a plant, and be stored there.
Diagram to show the areas in a plant through which the raw food materials pass up the stem and food materials pass down.
What forces Water up the Stem.—We have seen that the process of osmosis is responsible for taking in soil water, and that the enormous absorbing surface exposed by the root hairs makes possible the absorption of a large amount of water. Frequently this is more than the weight of the plant in every twenty-four hours.
Experiments have been made which show that at certain times in the year this water is in some way forced up the tiny tubes of the stem. During the spring season, in young and rapidly growing trees, water has been proved to rise to a height of nearly ninety feet. The force that causes this rise of water in stems is known as root pressure.
The greatest factor, however, is transpiration of water from leaves. This evaporation of water in the form of vapor seems to result in a kind of suction on the column of water in the stem. In the fall, after the leaves have gone, much less water is taken in by roots, showing that an intimate relation exists between the leaves and the root.
Summary of the Functions of Green Plants.—The processes which we have just described (with the exception of food making) are those which occur in the lives of any plant or animal. All plants and animals breathe, they oxidize their foods to release energy, carbon dioxide being given off as the result of the union of the carbon in the foods with the oxygen of the air. Both plants and animals digest their food; plants may do this in the cells of the root, stem, and leaf. Digestion must always occur so that food can be moved in a soluble condition from cell to cell in the plant's body.
Leaf of sundew closing over a captured insect.
The Venus fly trap, showing open and closed leaves.
Plants with Special Digestive Organs.—Some plants have special organs of digestion. One of these, the sundew, has leaves which are covered on one side with tiny glandular hairs. These attract insects and later serve to catch and digest the nitrogenous matter of these insects by means of enzymes poured out by the same hairs. Another plant, the Venus fly trap, catches insects in a sensitive leaf which folds up and holds the insect fast until enzymes poured out by the leaf slowly digest it. Still others, called pitcher plants, use as food the decayed bodies of insects which fall into their cuplike leaves and die there. In this respect plants are like those animals which have certain organs in the body set apart for the digestion of food.
Assimilation.—The assimilation of foods, or making of foods into living matter, is a process we know very little about. We know it takes place in the living cells of plants and animals. But how foods are changed into living matter is one of the mysteries of life which we have not yet solved.
Excretion.—The waste and repair of living matter seems to take place in both plants and animals. When living plants breathe, they give off carbon dioxide. In the process of starch-making, oxygen might be considered the waste product. Water is evaporated from leaves and stems. The leaves fall and carry away waste mineral substances which they contain.
The embryos of (a) the morning glory, (b) the barberry, (c) the potato, (d) the four o'clock, showing the position of their food supply. (After Gray.)
Reproduction.—Finally, both plants and animals have organs of reproduction. We have seen that the flower gives rise, after pollination, to a fruit which holds the seeds. These seeds hold the embryo. Thus the young plant is doubly protected for a time and is finally thrown off in the seed with enough food to give it a start in life. In much the same way we will find that animals reproduce, either by laying eggs which contain an embryo and food to start it in life or, as in the higher animals, by holding and protecting the embryo within the body of the mother until it is born, a helpless little creature, to be tenderly nourished by the mother until able to care for itself.
The Life Cycle.—Ultimately both plants and animals grow old and die. Some plants, for example the pea or bean, live but a season; others, such as the big trees of California, live for hundreds of years. Some insects exist as adults but a day, while the elephant is said to live almost two hundred years. The span of life from the time the plant or animal begins to grow until it dies is known as the life cycle.
Reference Books
elementary
Hunter, Laboratory Problems in Civic Biology. American Book Company.
Andrews, A Practical Course in Botany, pages 112-127. American Book Company.
Atkinson, First Studies of Plant Life, Chaps. IV, V, VI, VIII, XXI. Ginn.
Coulter, Plant Life and Plant Uses, Chap. V. American Book Company.
Dana, Plants and their Children, pages 99-129. American Book Company.
Mayne and Hatch, High School Agriculture. American Book Company.
Hodge, Nature Study and Life, Chaps. IX, X, XI. Ginn and Company.
MacDougal, The Nature and Work of Plants. The Macmillan Company.
advanced
Apgar, Trees of the United States, Chaps. II, V, VI. American Book Company.
Coulter, Barnes, and Cowles, A Textbook of Botany, Vol. I. American Book Company.
Duggar, Plant Physiology. The Macmillan Company.
Ganong, The Teaching Botanist. The Macmillan Company.
Goebel, Organography of Plants, Part V. Clarendon Press.
Goodale, Physiological Botany. American Book Company.
Gray, Structural Botany, Chap. V. American Book Company.
Kerner-Oliver, Natural History of Plants. Henry Holt and Company.
Strasburger, Noll, Schenck, and Karston, A Textbook of Botany. The Macmillan Company.
Ward, The Oak. D. Appleton and Company.
Yearbook, U. S. Department of Agriculture, 1894, 1895, 1898-1910.
Problem.—Man's relations to forests.
(a) What is the value of forests to man?
(b) What can man do to prevent forest destruction?
Laboratory Suggestions
Demonstration of some uses of wood. Optional exercise on structure of wood. Method of cutting determined by examination. Home work on study of furniture trim, etc.
Visit to Museum to study some economic uses of wood.
Visit to Museum or field trip to learn some common trees.
A forest in North Carolina. (U. S. G. S.)
The Economic Value of Trees. Protection and Regulation of Water Supply.—Trees form a protective covering for parts of the earth's surface. They prevent soil from being washed away, and they hold moisture in the ground. The devastation of immense areas in China and considerable damage by floods in parts of Switzerland, France, and in Pennsylvania has resulted where the forest covering has been removed. No one who has tramped through our Adirondack forest can escape noticing the differences in the condition of streams surrounded by forest and those which flow through areas from which trees have been cut. The latter streams often dry up entirely in hot weather, while the forest-shaded stream has a never failing supply of crystal water.
Working to prevent erosion after the removal of the forest in the French Alps.
Erosion at Sayre, Pennsylvania, by the Chemung River. (Photograph by W. C. Barbour.)
The city of New York owes much of its importance to its position at the mouth of a great river with a harbor large enough to float the navies of the world. This river is supplied with water largely from the Adirondack and Catskill forests. Should these forests be destroyed, it is not impossible that the frequent freshets which would follow would so fill the Hudson River with silt and débris that the ship channels in the bay, already costing the government hundreds of thousands of dollars a year to keep dredged, would become too shallow for ships. If this should occur, the greatest city in this country would soon lose its place and become of second-rate importance.
The story of how this very thing happened to the old Greek city of Poseidonia is graphically told in the following lines:—
"It was such a strange, tremendous story, that of the Greek Poseidonia, later the Roman Pæstum. Long ago those adventuring mariners from Greece had seized the fertile plain, which at that time was covered with forests of great oak and watered by two clear and shining rivers. They drove the Italian natives back into the distant hills, for the white man's burden even then included the taking of all the desirable things that were being wasted by incompetent natives, and they brought over colonists—whom the philosophers and moralists at home maligned, no doubt, in the same pleasant fashion of our own day. And the colonists cut down the oaks, and plowed the land, and built cities, and made harbors, and finally dusted their busy hands and busy souls of the grime of labor and wrought splendid temples in honor of the benign gods who had given them the possessions of the Italians and filled them with power and fatness.
"Every once in so often the natives looked lustfully down from the hills upon this fatness, made an armed snatch at it, were driven back with bloody contumely, and the heaping of riches upon riches went on. And more and more the oaks were cut down—mark that! for the stories of nations are so inextricably bound up with the stories of trees—until all the plain was cleared and tilled; and then the foothills were denuded, and the wave of destruction crept up the mountain sides, and they, too, were left naked to the sun and the rains.
"At first these rains, sweeping down torrentially, unhindered by the lost forests, only enriched the plain with the long-hoarded sweetness of the trees; but by and by the living rivers grew heavy and thick, vomiting mud into the ever shallowing harbors, and the land soured with the undrained stagnant water. Commerce turned more and more to deeper ports, and mosquitoes began to breed in the brackish soil that was making fast between the city and the sea.
"Who of all those powerful landowners and rich merchants could ever have dreamed that little buzzing insects could sting a great city to death? But they did. Fevers grew more and more prevalent. The malaria haunted population went more and more languidly about their business. The natives, hardy and vigorous in the hills, were but feebly repulsed. Carthage demanded tribute, and Rome took it, and changed the city's name from Poseidonia to Pæstum. After Rome grew weak, Saracen corsairs came in by sea and grasped the slackly defended riches, and the little winged poisoners of the night struck again and again, until grass grew in the streets, and the wharves crumbled where they stood. Finally, the wretched remnant of a great people wandered away into the more wholesome hills, the marshes rotted in the heat and grew up in coarse reeds where corn and vine had flourished, and the city melted back into the wasted earth."[16]
Result of deforestation in China. This land has been ruined by erosion. (Carnegie Institution Research in China.)
Prevention of Erosion by Covering of Organic Soil.—We have shown how ungoverned streams might dig out soil and carry it far from its original source. Examples of what streams have done may be seen in the deltas formed at the mouths of great rivers. The forest prevents this by holding the water supply and letting it out gradually. This it does by covering the inorganic soil with humus or decayed organic material. In this way the forest floor becomes like a sponge, holding water through long periods of drought. The roots of the trees, too, help hold the soil in place. The gradual evaporation of water through the stomata of the leaves cools the atmosphere, and this tends to precipitate the moisture in the air. Eventually the dead bodies of the trees themselves are added to the organic covering, and new trees take their place.
The forest regions of the United States.
Other Uses of the Forest.—In some localities forests are used as windbreaks and to protect mountain towns against avalanches. In winter they moderate the cold, and in summer reduce the heat and lessen the danger from storms. Birds nesting in the woods protect many valuable plants which otherwise might be destroyed by insects.
Forests have great commercial importance. Pyrogallic and other acids are obtained from trees, as are tar, creosote, resin, turpentine, and many useful oils. The making of maple sirup and sugar forms a profitable industry in several states.
The Forest Regions of the United States.—The combined area of all the forests in the United States, exclusive of Alaska, is about 500,000,000 acres. This seemingly immense area is rapidly decreasing in acreage and in quality, thanks to the demands of an increasing population, a woeful ignorance on the part of the owners of the land, and wastefulness on the part of cutters and users alike.
A glance at the map on page 109 shows the distribution of our principal forests. Washington ranks first in the production of lumber. Here the great Douglas fir, one of the "evergreens," forms the chief source of supply. In the Southern states, especially Louisiana and Mississippi, yellow pine and cypress are the trees most lumbered.
Transportation of lumber in the West. A logging train.
Which states produce the most hardwoods? From which states do we get most of our yellow pine, spruce, red fir, redwood? Where are the heaviest forests of the United States?
Transportation of lumber in the East. Logs are mostly floated down rivers to the mills.
Uses of Wood.—Even in this day of coal, wood is still by far the most used fuel. It is useful in building. It outlasts iron under water, in addition to being durable and light. It is cheap and, with care of the forests, inexhaustible, while our mineral wealth may some day be used up. Distilled wood gives wood alcohol. Partially burned wood is charcoal. In our forests much of the soft wood (the cone-bearing trees, spruce, balsam, hemlock, and pine), and poplars, aspens, basswood, with some other species, make paper pulp. The daily newspaper and cheap books are responsible for inroads on our forests which cannot well be repaired. It is not necessary to take the largest trees to make pulp wood. Hence many young trees of not more than six inches in diameter are sacrificed. Of the hundreds of species of trees in our forests, the conifers are probably most sought after for lumber. Pine, especially, is probably used more extensively than any other wood. It is used in all heavy construction work, frames of houses, bridges, masts, spars and timber of ships, floors, railway ties, and many other purposes. Cedar is used for shingles, cabinetwork, lead pencils, etc.; hemlock and spruce for heavy timbers and, as we have seen, for paper pulp. Another use for our lumber, especially odds and ends of all kinds, is in the packing-box industry. It is estimated that nearly 50 per cent of all lumber cut ultimately finds its way into the construction of boxes. Hemlock bark is used for tanning.
Diagrams of sections of timber. a, cross section; b, radial; c, tangential. (From Pinchot, U. S. Dept. of Agriculture.)
The hard woods—ash, basswood, beech, birch, cherry, chestnut, elm, maple, oak, and walnut—are used largely for the "trim" of our houses, for manufacture of furniture, wagon or car work, and endless other purposes.
Methods of cutting Timber.—A glance at the diagram of the sections of timber shows us that a tree may be cut radially through the middle of the trunk or tangentially to the middle portion. Most lumber is cut tangentially. In wood cut in this manner the yearly rings take a more or less irregular course. The grain in wood is caused by the fibers not taking straight lines in their course in the tree trunk. In many cases the fibers of the wood take a spiral course up the trunk, or they may wave outward to form little projections. Boards cut out of such a piece of wood will show the effect seen in many of the school desks, where the annual rings appear to form elliptical markings. Quite a difference in color and structure is often seen between the heartwood, composed of the dead walls of cells occupying the central part of the tree trunk, and the sapwood, the living part of the stem.
Section of a tree trunk showing knot.
Knots.—Knots, as can be seen from the diagram, are branches which at one time started in their outward growth and were for some reason killed. Later, the tree, continuing in its outward growth, surrounded them and covered them up. A dead limb should be pruned before such growth occurs. The markings in bird's-eye maple are caused by buds which have not developed, and have been overgrown with the wood of the tree.
Destruction of the Forest.—By Waste in Cutting.—Man is responsible for the destruction of one of this nation's most valuable assets. This is primarily due to wrong and wasteful lumbering. Hundreds of thousands of dollars' worth of lumber is left to rot annually because the lumbermen do not cut the trees close enough to the ground, or because through careless felling of trees many other smaller trees are injured. There is great waste in the mills. In fact, man wastes in every step from the forest to the finished product.
By Fire.—Indirectly, man is responsible for fire, one of the greatest enemies of the forest. Most of the great forest fires of recent years, the losses from which total in the hundreds of millions, have been due either to railroads or to carelessness in making fires in the woods. It is estimated that in forest lands traversed by railroads from 25 per cent to 90 per cent of the fires are caused by coal-burning locomotives. For this reason laws have been made in New York State requiring locomotives passing through the Adirondack forest preserve to burn oil instead of coal. This has resulted in a considerable reduction in the number of fires. In addition to the loss in timber, the fires often burn out the organic matter in the soil (the "duff") forming the forest floor, thus preventing the growth of forest there for many years to come. In New York and other states fires are fought by an organized corps of fire wardens, whose duty it is to watch the forest and to fight forest fires.
A forest in the far west totally destroyed by fire and wasteful lumbering.
Other Enemies.—Other enemies of the forest are numerous fungus plants, insect parasites which bore into the wood or destroy the leaves, and grazing animals, particularly sheep. Wind and snow also annually kill many trees.
The forest primeval. Trees are killing each other in the struggle for light and air.
Forestry.—In some parts of central Europe, the value of the forests was seen as early as the year 1300 A.D., and many towns consequently bought up the surrounding forests. The city of Zurich has owned forests in its vicinity for at least 600 years and has found them a profitable investment. In this country only recently has the importance of preserving and caring for our forests been noted by our government. Now, however, we have a Forest Survey of the Department of Agriculture and numerous state and university schools of forestry which are rapidly teaching the people of this country the best methods for the preservation of our forests. The Federal government has set aside a number of tracts of mountain forest in some of the Western states, making a total area of over 167,000,000 acres. New York has established for the same purpose the Adirondack Park, with nearly 1,500,000 acres of timberland. Pennsylvania has one of 700,000 acres, and many other states have followed their example.
A German beech forest. The trees are kept thinned out so as to allow the young trees to get a start. Contrast this with the picture above.
Methods for Keeping and Protecting the Forests.—Forests should be kept thinned. Too many trees are as bad as too few. They struggle with one another for foothold and light, which only a few can enjoy. In cutting the forest, it should be considered as a harvest. The oldest trees are the "ripe grain," the younger trees being left to grow to maturity. Several methods of renewing the forest are in use in this country. (1) Trees may be cut down and young ones allowed to sprout from cut stumps. This is called coppice growth. This growth is well seen in parts of New Jersey. (2) Areas or strips may be cut out so that seeds from neighboring trees are carried there to start new growth. (3) Forests may be artificially planted. Two seedlings planted for every tree cut is a rule followed in Europe. (4) The most economical method is that shown in the lower picture on page 114, where the largest trees are thinned out over a large area so as to make room for the younger ones to grow up. The greatest dangers to the forests are from fire and from careless cutting, and these dangers may be kept in check by the efficient work of our national and state foresters.
We must protect our city trees. This tree was badly wounded by being gnawed by a horse.
A City's Need for Trees.—The city of Paris, well known as one of the most beautiful of European capitals, spends over $100,000 annually in caring for and replacing some of the 90,000 trees owned by the city. All over the United States the city governments are beginning to realize what European cities have long known, that trees are of great value to a city. They are now following the example of European cities by planting trees and by protecting the trees after they are planted. Thousands of city trees are annually killed by horses which gnaw the bark. This may be prevented by proper protection of the trunk by means of screens or wire guards. Chicago has appointed a city forester, who has given the following excellent reasons why trees should be planted in the city:—
(1) Trees are beautiful in form and color, inspiring a constant appreciation of nature.
(2) Trees enhance the beauty of architecture.
(3) Trees create sentiment, love of country, state, city, and home.
(4) Trees have an educational influence upon citizens of all ages, especially children.
(5) Trees encourage outdoor life.
(6) Trees purify the air.
(7) Trees cool the air in summer and radiate warmth in winter.
(8) Trees improve climate and conserve soil and moisture.
(9) Trees furnish resting places and shelter for birds.
(10) Trees increase the value of real estate.
(11) Trees protect the pavement from the heat of the sun.
(12) Trees counteract adverse conditions of city life.
Let us all try to make Arbor Day what it should be, a day for caring for and planting trees, for thus we may preserve this most important heritage of our nation.
[16] Elizabeth Bisland and Anne Hoyt, Seekers in Sicily. John Lane Company.
Reference Books
elementary
Hunter, Laboratory Problems in Civic Biology. American Book Company.
Mayne and Hatch, High School Agriculture. American Book Company.
Murrill, Shade Trees, Bul. 205, Cornell University Agricultural Experiment Station.
Pinchot, A Primer of Forestry, Division of Forestry, U. S. Department of Agriculture.
advanced
Apgar, Trees of the United States, Chaps. II, V, VI. American Book Company.
Coulter, Barnes, and Cowles, A Textbook of Botany, Part I and Vol. II. American Book Company.
Goebel, Organography of Plants, Part V. Clarendon Press.
Strasburger, Noll, Schenck, and Karston, A Textbook of Botany. The Macmillan Company.
Ward, Timber and Some of its Diseases. The Macmillan Company.
Yearbook, U. S. Department of Agriculture, Division of Forestry, Buls. 7, 10, 13, 16, 17, 18, 20, 26, 27.
Problems.—How green plants are useful to man.
(a) As food.
(b) For clothing.
(c) Other uses.
—How green plants are harmful to man.
Suggested Laboratory Work
If a commercial museum is available, a trip should be planned to work over the topics in this chapter. The school collection may well include most of the examples mentioned, both of useful and harmful plants.
A study of weeds and poisonous plants should be taken up in actual laboratory work, either by collection and identification or by demonstration.
Green Plants have a "Dollar and Cents" Value.—To the girl or boy living in the city green plants seem to have little direct value. Although we see vegetables for sale in stores and we know that fruits have a money value, we are apt to forget that the wealth of our nation depends more upon its crops than it does on its manufactories and business houses. The economic or "dollars and cents" value of plants is enormous and far too great for us to comprehend in terms of figures.
We have already seen some of the uses to mankind of the products of the forest; let us now consider some other plant products.