[Illustration: 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.
[Illustration: 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.
IX. OUR FORESTS, THEIR USES AND THE NECESSITY FOR THEIR PROTECTION
_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.
[Illustration: 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.
[Illustration: Working to prevent erosion after the removal of the forest in the French Alps.]
[Illustration: 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 debris 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 Paestum. 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 Paestum. 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]
Footnote 16: Elizabeth Bisland and Anne Hoyt, _Seekers in Sicily_. John Lane Company.
[Illustration: 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.
[Illustration: 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.