CHAPTER XXIV
STUDIES IN CRYPTOGAMS
The pupil who has acquired skill in the use of the compound microscope may desire to make more extended excursions into the cryptogamous orders. The following plants have been chosen as examples in various groups. Ferns are sufficiently discussed in the preceding chapter.
Bacteria
If an infusion of ordinary hay is made in water and allowed to stand, it becomes turbid or cloudy after a few days, and a drop under the microscope will show the presence of minute oblong cells swimming in the water, perhaps by means of numerous hair-like appendages, that project through the cell wall from the protoplasm within. At the surface of the dish containing the infusion the cells are non-motile and are united in long chains. Each of these cells or organisms is a bacterium (plural, bacteria). (Fig. [135].)
Bacteria are very minute organisms,—the smallest known—consisting either of separate oblong or spherical cells, or of chains, plates, or groups of such cells, depending on the kind. They possess a membrane-like wall which, unlike the cell walls of higher plants, contains nitrogen. The presence of a nucleus has not been definitely demonstrated. Multiplication is by the fission of the vegetative cells; but under certain conditions of drought, cold, or exhaustion of the nutrient medium, the protoplasm of the ordinary cells may become invested with a thick wall, thus forming an endospore which is very resistant to extremes of environment. No sexual reproduction is known.
Bacteria are very widely distributed as parasites and saprophytes in almost all conceivable places. Decay is largely caused by bacteria, accompanied in animal tissue by the liberation of foul-smelling gases. Certain species grow in the reservoirs and pipes of water supplies, rendering the water brackish and often undrinkable. Some kinds of fermentation (the breaking down or decomposing of organic compounds, usually accompanied by the formation of gas) are due to these organisms. Other bacteria oxidize alcohol to acetic acid, and produce lactic acid in milk and butyric acid in butter. Bacteria live in the mouth, the stomach, the intestines, and on the surface of the skins of animals. Some secrete gelatinous sheaths around themselves; others secrete sulphur or iron, giving the substratum a vivid colour.
Were it not for bacteria, man could not live on the earth, for not only are they agents in the process of decay, but they are concerned in certain healthful processes of plants and animals. We have learned in Chapter VIII how bacteria are related to nitrogen-gathering.
Bacteria are of economic importance not alone because of their effect on materials used by man, but also because of the disease-producing power of certain species. Pus is caused by a spherical form, tetanus or lock-jaw by a rod-shaped form, diphtheria by short oblong chains, tuberculosis or “consumption” by more slender oblong chains, and typhoid fever, cholera, and other diseases by other forms. Many diseases of animals and plants are caused by bacteria. Disease-producing bacteria are said to be pathogenic.
The ability to grow in other nutrient substances than the natural one has greatly facilitated the study of these minute forms of life. By the use of suitable culture media and proper precautions, pure cultures of a particular disease-producing bacterium may be obtained with which further experiments may be conducted.
Milk provides an excellent collecting place for bacteria coming from the air, from the coat of the cow and from the milker. Disease germs are sometimes carried in milk. If a drop of milk is spread on a culture medium (as agar), and provided with proper temperature, the bacteria will multiply, each one forming a colony visible to the naked eye. In this way, the number of bacteria originally contained in the milk may be counted.