In the petrified tissues of fossil plants there are occasionally found small spherical vesicles, with delicate limiting membranes, in the cavities of parenchymatous cells or in the elements of vascular tissue. Some of these spherical inclusions have been described as possibly simple forms of endophytic algae[206], such as we are now familiar with in species of the Cyanophyceae and other algae. So far, however, no recorded instance of such fossil endophytic algae is entirely satisfactory. Some of the cells figured by Williamson as possibly algae, endophytic in the tissues of Coal-Measure plants, are no doubt thin-walled vesicles which formed part of a highly vacuolated cell-contents. Examples of such vesicles in living and fossil cells are shown in fig. 42. The fact that the contents of living plant tissues have been erroneously described as endophytic organisms, should serve as a warning against describing fossil endophytes without the test of good evidence to support them. The description of a fossil _Nostoc_ by the late Prof. Heer[207] from the Tertiary rocks of Switzerland cannot be accepted as a trustworthy example of a fossil plant, much less of a genus of recent algae. The application of recent generic names to fossils which are possibly not even organic must do more harm than good. B. SCHIZOMYCETES (Bacteria). It is impossible to draw a sharp line between the two subdivisions of the Schizophyta. The so-called Fission-Fungi or Bacteria differ from the Schizophyceae or Fission-Algae in the cell-contents being either colourless, blood-red or green, but never blue-green. We may regard the Bacteria, generally, as the lowest forms of plants; they are extremely simple organisms which have been derived from some primitive types which possessed the power of independent existence and contained chlorophyll—that important substance which enables a plant to obtain its carbon first-hand from the carbon dioxide of the atmosphere. Bacteria may be briefly described as single-celled plants, and as de Bary suggested comparable in shape to a billiard ball, a lead pencil or a corkscrew[208]. A single spherical or cylindrical cell measures about 1 µ in diameter[209]. They occur either singly or in filaments, or as masses of various shapes consisting of numberless bacterial cells. The nature and manner of life of Bacteria, and their extraordinary power of successfully resisting the most unfavourable conditions, render it probable that they constitute an extremely ancient group of organisms.