[Illustration: FIG. 11.--A chain of spherical blue-green algæ.] [Illustration: FIG. 12.--A chain of spherical bacteria.] [Illustration: FIG. 13.--A pair of spherical blue-green algæ.] Further the _chemical composition_ of bacteria is more like that of other fungous plants than of any of the forms classed as animals. [Illustration: FIG. 14.--Spherical bacteria. Several pairs are shown.] [Illustration: FIG. 15.--Yeast cells. Some show typical budding.] The food of bacteria is always taken up in solution by diffusion through the outer covering of the cell as it is in all plants. Plant cells never surround and engulf particles of solid food and digest them within the cell as many single-celled animals do, and as the leukocytes and similar ameboid cells in practically all multicelled animals do.[2] [Illustration: FIG. 16.--A portion of the mycelium of a mold. Note the large size and the branching.] One of the most marked differences between animals and plants is with respect to their energy relationships. Plants are characteristically storers of energy while animals are liberators of it. Some bacteria which have the power of swimming in a liquid certainly liberate relatively large amounts of energy, and in the changes which bacteria bring about in the material which they use as food considerable heat is evolved (“heating” of manure, etc.). Nevertheless the evidence is good that the bacteria as a class store much more of the energy contained in the substances actually taken into the body cell as food than is liberated in any form. Bacteria do show some resemblance to the protozoa, or single-celled animal forms, in that the individuals of each group consist of one cell only and some bacteria have the power of independent motion from place to place in a liquid as most “infusoria” do, but here the resemblance ceases. Bacteria are among the smallest of organisms, so small that it requires the highest powers of the microscope for their successful study, and the use of a special unit for their measurement. This unit is the one-thousandth part of a millimeter and is called the micro-millimeter or micron. Its symbol is the Greek letter _mu_ (µ). The size varies widely among different kinds but is fairly constant in the same kind. The smallest described form is said to be only 0.18µ long by 0.06µ thick and is just visible with the highest power of the microscope, though it is possible and even probable that there are forms still smaller which cannot be seen. Some large rare forms may measure 40µ in length, but the vast majority are from 1µ to 4µ or 5µ long, and from one-third to one-half as wide. From the above description a bacterium might be said to be a _microscopic, unicellular plant, without chlorophyl, which reproduces by dividing transversely_. PART I. MORPHOLOGY CHAPTER II. CELL STRUCTURES.