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. Bacteria are disseminated in water, as the germ of typhoid fever and cholera; in milk and other fluids; in the air; and on the bodies of flies, feet of birds, and otherwise. Bacteria are thought by many to have descended from algæ by the loss of chlorophyll and decrease in size due to the more specialized acquired saprophytic and parasitic habit. ALGÆ The algæ comprise most of the green floating “scum” which covers the surfaces of ponds and other quiet waters. The masses of plants are often called “frog spittle.” Others are attached to stones, pieces of wood, and other objects submerged in streams and lakes, and many are found on moist ground and on dripping rocks. Aside from these, all the plants commonly known as seaweeds belong to this category; these latter are inhabitants of salt water. The simplest forms of algæ consist of a single spherical cell, which multiplies by repeated division or fission. Many of the forms found in fresh water are filamentous, _i.e._ the plant body consists of long threads, either simple or branched. Such a plant body is termed a _thallus_. This term applies to the vegetative body of all plants that are _not differentiated into stem and leaves_. Such plants are known as _thallophytes_ (p. 181). All algæ contain chlorophyll, and are able to assimilate carbon dioxide from the air. This distinguishes them from the fungi. * * * * *