M. Renault has recently described certain minute structures in a Palaeozoic coprolite to which he gives the name Gloioconis Borneti[204], and which he regards as a Permian gelatinous alga similar to the well-known recent genus Glœocapsa. The appearances revealed in a section of the coprolite are interpreted by this author as a collection of small colonies of a unicellular gelatinous alga in various stages of development. Renault’s figure shows a spherical group of faintly outlined and cloudy bodies, most of which include one or two small dark spots. The latter are regarded as the cells of the alga, and the surrounding cloudy substance is described as the gelatinous sheath. The absence of a nucleus in these extremely minute fossil cells (8–10 µ in diameter) is referred to as an argument in favour of referring the organism to the Cyanophyceae rather than to the Chlorophyceae. It is possible that the ill-defined structure described by Renault may be a petrified alga, but there is not sufficient evidence to warrant a decided opinion; the absence of nuclei can hardly be taken seriously in such a case as this as an argument in favour of the Cyanophyceae.
CYANOPHYCEAE.
Although our exact knowledge of fossil Cyanophyceae is extremely small, it is probable that such simple forms of plants existed in abundance during the past ages in the earth’s history. Several writers have expressed the opinion that the blue-green algae may be taken as the modern representatives of those earliest plants which first existed on an archaean land-surface. The living species possess the power of resisting unfavourable conditions in a marked degree, and are able to adapt themselves to very different surroundings. Their occurrence in hot springs proves them capable of living under conditions which are fatal to most plants, and suggests the possibility of their occurrence in the heated waters which probably constituted the medium in which vegetable life began. An interesting example of the growth of blue-green algae under unfavourable conditions was recorded in 1886 by Dr Treub[205] of the Buitenzorg Gardens, Java. In 1883 a considerable part of the island Krakatoa, situated in the Straits of Sunda, between Sumatra and Java, was entirely destroyed by a terrific volcanic explosion. What remained had been reduced to a lifeless mass of hot volcanic ashes. Three years later, Treub visited the island, and found that several plants had already established themselves on the volcanic rocks. Various ferns and flowering plants were recorded in Treub’s description of this newly established flora. It seemed that the barren rocky surface had been prepared for the more highly organised plants by the action of certain forms of Cyanophyceae, which were able to live under conditions which would be fatal to more complex types.
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.
The wonderful perfection of preservation of many fossil plants enables us to investigate the contents of petrified cells and to examine in minutest detail the histology of extinct plants. To those who are familiar with the possibilities of microscopical research as applied to silicified and calcified fossil tissues, it is by no means incredible that evidence has been detected of the existence of Bacteria as far back in the history of the earth as the Carboniferous and Devonian periods.
Were there no trustworthy records of the occurrence of Bacteria in Palaeozoic times, it would still be a natural supposition that these ubiquitous organisms must have been abundantly represented. It has been suggested as a probable conclusion that some forms of Bacteria, which produced chemical changes in the soil necessary for the nutrition of plants, must have existed contemporaneously with the oldest vegetation[210].