rapidly over the surface, swelling the streams into torrents, which bring destruction and death as they flood the valleys and fields along their course. As the water flows over the surface of the land from which the trees have been cut, it carries along the richest part of the soil, thus causing loss of fertility in the uplands. The material thus carried away fills up the river beds and harbor mouths, and in many cases a heavy expense is entailed in its removal.

144. Dangers to forests. We have already called attention to the threatened destruction of our American forests by careless lum

bering. (See 141.)

This means not

only the wholesale cutting of large areas of trees, but the lack of forethought which lumbermen show in

leaving dead tree trunks and

FIG. 69.-Excessive erosion of land caused by de struction of forests. (Bailey.)

branches to become the prey of destructive forest fires, which, when once started, devastate wide areas. The annual loss of property from this cause is conservatively estimated at more than one hundred million dollars.

This forest débris of dead tree trunks and branches also furnishes breeding places for insects, which, when hatched, prey upon healthy trees. Another source of danger, especially to young forest growth, comes from permitting large flocks of grazing animals like cattle and sheep to feed upon and trariple down the small trees. If we are to preserve the

remnants of our once vast forest resources, a public sentiment must be thoroughly aroused which will compel the passage and enforcement of conservation laws.

145. Necessity for reforesting and for forest protection. Surely, enough has been said to show the necessity for forest

protection. For

tunately, laws are now being passed that will enable the National and some of the State governments to acquire large tracts of land for forest reservations. In many States these forest areas will protect the sources of large streams. There is great need of trained experts who will go through the forests, mark the trees which are mature enough should fall to do Again, large areas

to be cut, and decide which way they the least damage to the younger growth. now devastated should be replanted with young forest trees, and this is also being done to a considerable extent. In many

foreign countries, notably in Germany, the forests are so used that year after year they supply the requisite timber, and still continue to do their much needed work in conserving the rainfall. Such must be the policy in our country if we wish to escape most disastrous penalties that always result from forest destruction.

Another method of forest protection is that afforded by cutting trees in such a way as to form long, treeless strips of land known as fire lanes. Systems of telegraphic communication from one part of the forest reserves to another and fire wardens are necessary factors in efficient protection of forests.

III. FUNGI AND THEIR RELATION TO HUMAN WELFARE

146. Fungi. Thus far we have confined our attention to plants which are easily visible to the naked eye and which consist of roots, stems, and leaves. While we ordinarily think of these as the common plants, in reality the most common plant organisms are those which have neither roots, stems, nor leaves, and which in many cases are microscopic in size. The smallest and most numerous of these are known as bacteria, which are found all about us, in the soil, in the air we breathe, on the food we eat, and in the water we drink. Bacteria belong to a great group of plants called fungi. All fungi are characterized by the absence of chlorophyll, hence plants of this group cannot manufacture their carbohydrate food out of materials from the soil and air, but are dependent on foods made by green plants. More familiar to us, perhaps, than bacteria are the fungi known as mushrooms and toadstools, and the molds and mildews. Still other fungi are the yeasts, the rusts, and the smuts. Because of the enormous economic importance of many of these forms, we shall consider more or less in detail the structure, functions, and life-history of several of them.

147. Microscopical appearance and size of bacteria. Every one is familiar with the fact that if a bouquet of flowers is left for some time in a vase of water, the stems decay and disagreeable odors are given off. This is a common example of the action of bacteria, for all decay is due to the work of these organisms. When we come to examine the flower-stems or the putrid water, we find a slimy scum. If we put a drop of this scum on a slide, cover with a cover-glass, and examine with the highest powers of the microscope, usually we would see many different forms of living things. Some of them would probably appear relatively large, and these, as we shall see later (Chapter IV, "Animal Biology"), are single-celled animals. A closer examination will disclose countless numbers of very minute, colorless organisms; these are the bacteria. A careful study of many kinds of bacteria shows that they have several characteristic shapes (see Fig. 71) by means of which they can be roughly classified. Some are rod-shaped (like a firecracker), some are spherical, or egg-shaped, and still others are spiral-shaped. Each bacterium is a tiny bit of translucent protoplasm, inclosed in a cell wall of cellulose. Thus far no nucleus has been discovered in any kind of bacteria. Because of their cellulose walls, and because of their likeness to certain low forms of green plants, biologists now regard these organisms as plants rather than animals.

Some of the rod-shaped bacteria have one or more long, hairlike projections from the ends, called cil'i-a, which give the germs still further resemblance to firecrackers. These cilia lash about rapidly, and thus drive the cells through the

1 Because of the importance of bacteria in relation to sanitation, it may be found advisable to consider this whole topic in connection with human biology. Sections 148-154 will therefore be repeated in the book on human biology.

water. The spiral bacteria roll over and over, and advance in a spiral path like a corkscrew. Other forms have rapid movements, but it is not known how they are accomplished.

It is very difficult to get any clear notion of the extreme minuteness of bacteria. It means but little to say that the rod-shaped forms are 5 of an inch in length. The imagination may be somewhat assisted if we remember that fifteen hundred of them arranged in a procession end to end would scarcely equal the diameter of a pin head.

148. Reproduction of bacteria. -When conditions are favorable, the production of new cells goes

on with marvelous rapidity. The Ө

process is something as follows: The tiny cells take in through the

A

E

Various forms of bacteria.

A, a colony of spherical bacteria (coccus, plur. cocci); B, rod-shaped bacteria (bacillus, plur. bacilli); C, rodshaped bacteria dividing; D, rod-shaped bacteria, each containing a spore; E, spiral bacteria (spirillum, plur. spirilla) each with cilia.

cell wall some of the food materials FIG. 71. that are about them, change this food into protoplasm, and thus increase somewhat in size. The limit is soon reached, however, and the bacterium begins to divide crosswise into halves. The mother cell thus forms two daughter cells by making a cross partition (cell wall of cellulose) between the two parts. (See Fig. 71, C.) If the daughter cells cling to