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.
Nostoc.—On wet rocks and damp soil dark, semitransparent irregular or spherical gelatinous masses about the size of a pea are often found. These consist of a colony of contorted filamentous algæ embedded in the jelly-like mass. The chain of cells in the filament is necklace-like. Each cell is homogeneous, without apparent nucleus, and blue-green in colour, except one cell which is larger and clearer than the rest. The plant therefore belongs to the group of blue-green algæ. The jelly probably serves to maintain a more even moisture and to provide mechanical protection. Multiplication is wholly by the breaking up of the threads. Occasionally certain cells of the filament thicken to become resting-spores, but no other spore formation occurs.
Fig. 264.—Filament of Oscillatoria, showing one dead cell where the strand will break.