living stuff in our bodies has a kind of life of its own, 

 somewhat as a county has its life within the state. Each 

 cell lives its own life, takes up its own food, grows, divides, 

 grows old, and may die. To be sure this is not the whole 

 story. Beside doing its own private work, it does some 

 things in relation with the other cells of the body of which 

 it is a part. It does special work, as secreting or absorbing 

 or contracting or feeling, for the other cells. It has tvvo 

 lives, so to speak: its own individual life, and its part of the 

 life of the whole organism. 



Now the simplest animals and plants are something like 

 one of these single, microscopic cells if it were turned 

 loose in nature to live an independent life. These 

 organisms are single cells. They are mostly very minute, 

 though they vary greatly in size. They must have, and 

 do have, all the powers necessary to sustain individual 

 life, and equally to reproduce and keep the species going. 

 They get food, and use it both to grow and to supply 

 energy. They are sensitive to all the important external 

 conditions of life. They are able to adjust themselves to 

 these conditions. 



2. Their Location and Interest. As one might expect, 

 these minute one-celled plants and animals must live in 

 moist places. Otherwise they would dry up and lose the 

 internal water that is absolutely necessary to active life. 

 Many of them can dry up for days and months and come 

 again into activity when water returns, but, of course, 

 they are not active during this period of dryness. We 

 find them in all waters, both fresh and salt; in the bodies 

 of larger plants and animals, where they may produce 

 diseases; in decaying organic matter, where they assist 

 the decay and live upon the products of it. 



They are found in all parts of the world and are among 

 the most interesting of all living things. They are usually 

 transparent under the microscope. Many of them are so 

 small, however, that they can scarcely be seen at all, even 

 by means of our microscopes of highest-powers. 



The great improvement of the compound microscope 

 has made these simplest organisms one of the most 

 interesting fields of study in the whole natural realm. 

 You are able to watch all their activities, and even to 

 see them passing through the various stages of their life 

 cycle. Sometimes this cycle is completed in a few hours. 



3. Their Importance in Nature. Aside from the mere 

 fact that they are interesting to look at and to study, 

 these lowly plants and animals are extremely important 

 to man, and to the other plants and animals that live on 

 the earth. Some of the plants, minute as they are, are 

 green like the leaves of trees. A good example of this is 

 the little Pleurococcus which you may see as a green 

 stain on the north side of fences and trees. Each plant 

 is a single cell, but their great numbers make up for their 

 smallness. This green color enables each of them to 

 manufacture sugars and starches out of the inorganic 

 materials that come to it. 



Other plants, as the bacteria, are not green and have 

 not the power of making starches and sugars, but they 

 have other powers equally important. They must get 

 their food chiefly by attacking other plants and animals, 

 either after they are dead or while they are still alive. 

 Those organisms which live at the expense of other living 

 things we call parasites. 



Finally, we have a great many animals that are one- 

 celled in their structure. These are like the bacteria in not 

 being able to make their own food. They depend on other 

 organisms living or dead. They are, however, usually 

 much larger than the bacteria and have more striking 

 activities, and rather better development. That is to say, 

 they are more complex. 



These lowly things may be very valuable or very 



hurtful to man and his interests. The simpler green plants 

 may make and store foods, which in turn may be 

 devoured to support more complex organisms. The bacteria 

 and one-celled animals attack dead matter, and by 

 hastening decomposition serve as scavengers. In this 

 way they destroy dead matter and return the elements 

 that compose it back to the earth and air where they may 

 again enter other plants and animals. But for their 

 action, the bodies of the great mass of animals and plants 

 that die every year would remain to vex us for a long 

 time. Within the moist soil itself they are at work 

 tearing down substances, and thus they prepare the 

 way for other life. Bacteria help to ripen butter and 

 cheese, to make vinegar, to prepare sponges, skins 

 (leather), flax, and some other fabric plants for their 

 commercial uses. 



The bacteria also do a good deal of damage to man 

 when they attack foods and other materials that he does 

 not want to have decay or sour, as milk, fruits, meats, etc. 

 The bacteria of course do not know what man wishes to 

 preserve and what he wants to get rid of, and it would 

 not make any difference to them if they did, since their 

 sole purpose is to make a living for themselves. They 

 attack therefore anything they can decompose, and 

 always find food there. 



The greatest harm done to man by these simple plants 

 and animals, however, is in the diseases they produce in 

 the human body or in the animals and plants that we 

 depend on. The most of our contagious and infectious 

 diseases, as tuberculosis, typhoid fever, yellow fever, 

 sleeping sickness, and scores of others are produced by 

 these one-celled organisms and the poisons they produce. 

 Similarly, diseases among hogs, cattle, horses, and 

 poultry; among garden vegetables, orchard fruits, and 

 flowers are caused by them. 



They do all thig damage in spite of the minuteness and 

 small powers of a single organism, simply because they 

 can reproduce so rapidly and effectively. 



4. The Method of Their Reproduction. This is often 

 just as simple as can be. When a bacterium grows up to 

 its full size by taking food, it may be a simple globe, or 

 rod, or spiral, depending on the kind. When this has 

 taken place, the cell may at once divide, by a partition 

 through the middle, into two cells, each having one-half 

 the material and one-half the size of the original cell. 

 Each of these daughter cells then grows up to the adult 

 size, and repeats the piocess. In some bacteria this 

 growth and division may take place in less than one hour. 

 This rapid reproduction is possible because the plant and 

 the process are both so simple. Because of the rapid 

 growth and reproduction it comes about that in forty- 

 eight hours the descendants of one bacterium, at this rate 

 of one division, in an hour, would be two raised to the 

 forty-seventh power. 



Most of the one-celled animals and plants have this 

 method of reproducing by simple division into two, 

 though few of them multiply so rapidly as the bacteria. 



Some of them vary this method sometimes by dividing 

 the protoplasm of the original cell into several offspring 

 instead of two. In this case each daughter gets a 

 correspondingly smaller share of the original body. 



5. What of the Parent? This method of reproduction 

 raises an interesting question. The original organism 

 divides into two equal daughters. There is no visible 

 difference between these. There is nothing that would 

 justify us in calling one of them the parent any more 

 than the other. The substance of the parent has gone 

 into them equally. We cannot say that the parent has 

 died in the ordinary sense. There is no corpse; and yet 

 there is no parent. The parent is destroyed completely, 



