8. Cells in other tissues. It has been demonstrated that nerve tissue, muscle tissue, and other building materials of the body are all composed of cells (Fig. 3). A tissue may now be defined as a building material of the body, composed of cells of the same kind.

CHAPTER II

MICROORGANISMS AND THEIR RELATION TO HUMAN

WELFARE

I. STRUCTURE AND FUNCTIONS OF BACTERIA

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9. Bacteria:1 their microscopical appearance and size. In the preceding chapter we considered to some extent the organs, tissues, and cells of the human body. However, before we discuss further the structure and functions of these various parts of our bodies, we shall study in some detail certain microscopic plants which have a most intimate relation to human welfare. Chief among these are the tiny organisms known as 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, we usually see many different forms of living things. Some of them appear relatively large, and these, as we have already seen (A. B., Chapter VI), are single-celled animals. A closer examination will disclose countless numbers of very minute,

1 The substance of this section, and several of those that follow, appear in Part I, "Plant Biology." Many teachers, however, find it impracticable to discuss bacteria until the work in human biology is taken up; hence the repetition of this material in this volume.

colorless organisms; these are the bacteria. A careful study of many kinds of bacteria shows that they have several characteristic shapes (see Fig. 7), by means of which they may be roughly classified. Some are rod-shaped (like a firecracker), some are spherical, or egg-shaped,

rod-shaped
bacteria
(bacilli)

cleus 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 kinds of 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 cell through the water. The spiral bacteria roll over and over, and advance in a spiral path like a corkscrew.

It is very difficult to get any clear notion of the extreme minuteness of bacteria. It means little to say that the rod-shaped forms are 50% 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.

10. Microscopic study of bacteria. - Laboratory demonstration.

Place on a glass slide a drop of the scum found on the surface of a hay infusion, and cover with a cover glass. Examine with the highest powers of the compound microscope. 1. Describe the source of the material you are examining. 2. What is the apparent color of the tiny bodies (bacteria) that you see?

3. Which of the different forms of bacteria shown in Fig. 7 do you find? Draw enlarged figures of each of the shapes that you find.

4. Do any of the bacteria seem to be in motion? If so, describe the motion.

When conditions are

11. Reproduction of bacteria. favorable, the production of new cells goes on with marvelous rapidity. The process is something as follows: the tiny cells take in through the cell wall some of the food materials 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 (Fig. 7). If the daughter cells cling together, a chain or a mass is formed. Oftentimes they separate entirely from each other. In either case the whole mass of bacteria is called a colony.

It usually takes about an hour for the division to take

place. Suppose, then, we start at ten o'clock some morning with a single healthy bacterium. If conditions are favorable, there would be two cells at eleven o'clock, and by twelve o'clock each of these two daughter cells would form two granddaughter cells; the colony would then number four individuals. Should this process continue for twentyfour hours or until ten o'clock on the day after the single bacterium began its race, the colony would number 16,777,216 bacteria. "It has been calculated by an eminent biologist," says Dr. Prudden,1 "that if the proper conditions could be maintained, a rodlike bacterium, which would measure about a thousandth of an inch in length, multiplying in this way, would in less than five days make a mass which would completely fill as much space as is occupied by all the oceans on the earth's surface, supposing them to have an average depth of one mile."

12. Spore formation in bacteria. Such startling possibilities as those suggested in the preceding section fortunately can never become realities, for favorable conditions soon cease to exist and the cells either die or cease to multiply. Sometimes, when food or moisture begins to fail, the protoplasm within each cell rolls itself into a ball and covers itself with a much thickened wall. This protects it until it again meets with conditions favorable for growth. The process we have been describing is known as spore formation; the tiny protoplasmic sphere is called a spore, and its dense covering a spore wall (Fig. 7). In this condition bacteria may be blown hither and yon as a part of the dust. They may be heated even above the temperature of boiling water without being killed. When at length they settle down on

"The Story of the Bacteria," by Dr. T. Mitchell Prudden. G. P. Putnam's Sons, New York.