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 pasteurianum, which is anaerobic, and can fix nitrogen only if protected from oxygen by aerobic species. It is very probable that numerous symbiotic fermentations in the soil are due to this co-operation of oxygen-protecting species with anaerobic ones, e.g. Tetanus. Astonishment has been frequently expressed at the powerful activities of bacteria—their rapid growth and dissemination, the extensive and profound decompositions and fermentations induced by them, the resistance of their spores to dessication, heat, &c.—but it is worth while to ask how far these properties are really remarkable when all the data for comparison with other organisms are considered. In the first place, the extremely small size and isolation of the vegetative cells place the protoplasmic contents in peculiarly favourable circumstances for action, and we may safely conclude that, weight for weight and molecule for molecule, the protoplasm of bacteria is brought into contact with the environment at far more points and over a far larger surface than is that of higher organisms, whether—as in plants—it is distributed in thin layers round the sap-vacuoles, or—as in animals—is bathed in fluids brought by special mechanisms to irrigate it. Not only so, the isolation of the cells facilitates the exchange of liquids and gases, the passage in of food materials and out of enzymes and products of metabolism, and thus each unit of protoplasm obtains opportunities of immediate action, the results of which are removed with equal rapidity, not attainable in more complex multi-cellular organisms. To put the matter in another way, if we could imagine all the living cells of a large oak or of a horse, having given up the specializations of function impressed on them during evolution and simply carrying out the fundamental functions of nutrition, growth, and multiplication which mark the generalized activities of the bacterial cell, and at the same time rendered as accessible to the environment by isolation and consequent extension of surface, we should doubtless find them exerting changes in the fermentable fluids necessary to their life similar to those exerted by an equal mass of bacteria, and that in proportion to their approximation in size to the latter. Ciliary movements, which undoubtedly contribute in bringing the surface into contact with larger supplies of oxygen and other fluids in unity of time, are not so rapid or so extensive when compared with other standards than the apparent dimensions of the microscopic field. The microscope magnifies the distance traversed as well as the organism, and although a bacterium which covers 9–10 cm. or more in 15 minutes—say 0.1 mm. or 100 per second—appears to be darting across the field with great velocity, because its own small size—say 5 × 1 —comes into comparison, it should be borne in mind that if a mouse 2 in. long only, travelled twenty times its own length, i.e. 40 in., in a second, the distance traversed in 15 minutes at that rate, viz. 1000 yards, would not appear excessive. In a similar way we must be careful, in our wonder at the marvellous rapidity of cell-division and growth of bacteria, that we do not exaggerate the significance of the phenomenon. It takes any ordinary rodlet 30-40 minutes to double its length and divide into two equal daughter cells when growth is at its best; nearer the minimum it may require 3-4 hours or even much longer. It is by no means certain that even the higher rate is greater than that exhibited by a tropical bamboo which will grow over a foot a day, or even common grasses, or asparagus, during the active period of cell-division, though the phenomenon is here complicated by the phase of extension due to intercalation of water. The enormous extension of surface also facilitates the absorption of energy from the environment, and, to take one case only, it is impossible to doubt that some source of radiant energy must be at the disposal of those prototrophic forms which decompose carbonates and assimilate carbonic acid in the dark and oxidize nitrogen in dry rocky regions where no organic materials are at their disposal, even could they utilize them. It is usually stated that the carbon dioxide molecule is here split by means of energy derived from the oxidation of nitrogen, but apart from the fact that none of these processes can proceed until the temperature rises to the minimum cardinal point, Engelmann's experiment shows that in the purple bacteria rays are used other than those employed by green plants, and especially ultra-red rays not seen in the spectrum, and we may probably conclude that “dark rays”—i.e. rays not appearing in the visible spectrum—are absorbed and employed by these and other colourless bacteria. The purple bacteria have thus two sources of energy, one by the oxidation of sulphur and another by the absorption of “dark rays.” Stoney (Scient. Proc. R. Dub. Soc., 1893, p. 154) has suggested yet another source of energy, in the bombardment of these minute masses by the molecules of the environment, the velocity of which is sufficient to drive them well into the organism, and carry energy in of which they can avail themselves.

.—General: Fischer, The Structure and Functions of Bacteria (Oxford, 1900, 2nd ed.), German (Jena, 1903); Migula, System der Bakterien (Jena, 1897); and in Engler and Prantl, Die natürlichen Pflanzenfamilien, I. Th. 1 Abt. a; Lafar, Technical Mycology (vol. i. London, 1898); Mace, Traité pratique de bakteriologie (5th ed. 1904). Fossil bacteria: Renault, “Recherches sur les Bactériacées fossiles,” ''Ann. des Sc. Nat.'', 1896, p. 275. Bacteria in Water: Frankland and Marshall Ward. “Reports on the Bacteriology of Water,” ''Proc. R. Soc.'', vol. li. p. 183, vol. liii. p. 245, vol. lvi. p. 1; Marshall Ward, “On the Biology of B. ramosus,” ''Proc. R. Soc.'', vol. lviii. p. 1; and papers on Bacteria of the river Thames in ''Ann. of Bot.'' vol. xii. pp. 59 and 287, and vol. xiii. p. 197. Cell-membrane, &c.: Bütschli, Weitere Ausführungen über den Bau der Cyanophyceen und Bakterien (Leipzig, 1896); Fischer, ''Unters. über den Bau der Cyanophyceen und Bakterien (Jena, 1897); Rowland, “Observations upon the Structure of Bacteria,” Trans. Jenner Institute'', 2nd ser. 1899, p. 143, with literature. Cilia: Fischer, “Unters. über Bakterien,” ''Pringsh. Jahrb.'' vol. xxvii.; also the works of Migula and Fischer already cited. Nucleus: Wager in ''Ann. Bot.'' vol. ix. p. 659; also Migula and Fischer, l.c.; Vejdovsky, “Über den Kern der Bakterien und seine Teilung,” ''Cent. f. Bakt.'' Abt. II. Bd. xi. (1904) p. 481; ibid. “Cytologisches über die Bakterien der Prager Wasserleitung,” ''Cent. f. Bakt.'' Abt. II. Bd. xv. (1905); Mencl, “Nachträge zu den Strukturverhältnissen von Bakterium gammari” in Archiv f. Protistenkunde, Bd. viii. (1907), p. 257. Spores, &c.: Marshall Ward, “On the Biology of B. ramosus,” ''Proc. R. Soc.'', 1895, vol. lviii. p. 1; Sturgis, “A Soil Bacillus of the type of de Bary's B. megatherium,” ''Phil. Trans''.