Bacteria are considered to belong to the plant kingdom not because of any one character they possess, but because they most nearly resemble organisms which are generally recognized as plants. While it is not difficult to distinguish between the higher plants and higher animals, it becomes almost, if not quite, impossible to separate the lowest, forms of life. It is only by the method of resemblances above mentioned that a decision is finally reached. It has even been proposed to make a third class of organisms neither plants nor animals but midway between in which the bacteria are included, but such a classification has not as yet been adopted.
In many respects the bacteria are most nearly related to the lowest algæ, since both are unicellular organisms, both reproduce by transverse division and the forms of the cell are strikingly similar. The bacteria differ in one important respect, that is, they do not contain chlorophyl, the green coloring matter which enables all plants possessing it to absorb and break up carbon dioxide in the light, and hence belong among the fungi. Bacteria average much smaller than even the smallest algæ.
Bacteria are closely connected with the fission yeasts and the yeasts and torulæ. All are unicellular and without chlorophyl. The bacteria, as has been stated, reproduce by division but the others characteristically by budding or gemmation, though the fission yeasts also by division.
There is a certain resemblance to the molds in their absence of chlorophyl. But the molds grow as branching threads and also have special fruiting organs for producing spores as a means of reproduction, neither of which characteristics is found among the true bacteria. The higher thread bacteria do show true branching and rudimentary fruiting bodies (Streptothrix) and appear to be a link connecting the true bacteria and the molds.
