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outside the plant. We have here a very interesting case of symbiosis as mentioned above. The green plant, however, always keeps the upper hand, restricting the development of the bacteria to the nodules and later absorbing them for its own use. It should be mentioned that different genera require different races of the bacterium for the production of nodules.

The important part that these bacteria play in agriculture led to the introduction in Germany of a commercial product (the so-called “nitragin”) consisting of a pure culture of the bacteria, which is to be sprayed over the soil or applied to the seeds before sowing. This material was found at first to have a very uncertain effect, but later experiments in America, and the use of a modified preparation in England, under the direction of Professor Bottomley, have had successful results; it is possible that in the future a preparation of this sort will be widely used.

The apparent specialization of these bacteria to the leguminous plants has always been a very striking fact, for similar bacterial nodules are known only in two or three cases outside this particular group. However, Professor Bottomley announced at the meeting of the British Association for the Advancement of Science in 1907 that he had succeeded in breaking down this specialization and by a suitable treatment had caused bacteria from leguminous nodules to infect other plants such as cereals, tomato, rose, with a marked effect on their growth. If these results are confirmed and the treatment can be worked commercially, the importance to agriculture of the discovery cannot be overestimated; each plant will provide, like the bean and vetch, its own nitrogenous manure, and larger crops will be produced at a decreased cost.

Fig. 17.—A plate-culture of a bacillus which had been exposed for a period of four
hours behind a zinc stencil-plate, in which the letters C and B were cut. The light had
to traverse a screen of water before passing through the C, and one of aesculin
(which filters out the blue and violet rays) before passing the B. The plate was then
incubated, and, as the figure shows, the bacteria on the C-shaped area were all killed,
whereas they developed elsewhere on the plate (traces of the B are just visible to the
right) and covered it with an opaque growth.  (H. M. W.) 

Another important advance is in our knowledge of the part played by bacteria in the circulation of carbon in nature. The enormous masses of cellulose deposited annually on the earth's surface are, as we know, principally the result of chlorophyll Cellulose-bacteria.action on the carbon dioxide of the atmosphere decomposed by energy derived from the sun; and although we know little as yet concerning the magnitude of other processes of carbon-assimilation—e.g. by nitrifying bacteria—it is probably comparatively small. Such cellulose is gradually reconverted into water and carbon dioxide, but for some time nothing positive was known as to the agents which thus break up the paper, rags, straw, leaves and wood, &c., accumulating in cesspools, forests, marshes and elsewhere in such abundance. The work of van Tieghem, van Senus, Fribes, Omeliansky and others has now shown that while certain anaerobic bacteria decompose the substance of the middle lamella—chiefly pectin compounds—and thus bring about the isolation of the cellulose fibres when, for instance, flax is steeped or “retted,” they are unable to attack the cellulose itself. There exist in the mud of marshes, rivers and cloacae, &c., however, other anaerobic bacteria which decompose cellulose, probably hydrolysing it first and then splitting the products into carbon dioxide and marsh gas. When calcium sulphate is present, the nascent methane induces the formation of calcium carbonate, sulphuretted hydrogen and water. We have thus an explanation of the occurrence of marsh gas and sulphuretted hydrogen in bogs, and it is highly probable that the existence of these gases in the intestines of herbivorous animals is due to similar putrefactive changes in the undigested cellulose remains.

Cohn long ago showed that certain glistening particles observed in the cells of Beggiatoa consist of sulphur, and Winogradsky and Beyerinck have shown that a whole series of sulphur bacteria of the genera Thiothrix, Chromatium, Spirillum, Monas,Sulphur bacteria. &c., exist, and play important parts in the circulation of this element in nature, e.g. in marshes, estuaries, sulphur springs, &c. When cellulose bacteria set free marsh gas, the nascent gas reduces sulphates—e.g. gypsum—with liberation of SH₂, and it is found that the sulphur bacteria thrive under such conditions by oxidizing the SH₂ and storing the sulphur in their own protoplasm. If the SH₂ runs short they oxidize the sulphur again to sulphuric acid, which combines with any calcium carbonate present and forms sulphate again. Similarly nascent methane may reduce iron salts, and the black mud in which these bacteria often occur owes its colour to the FeS formed. Beyerinck and Jegunow have shown that some partially anaerobic sulphur bacteria can only exist in strata at a certain depth below the level of quiet waters where SH₂ is being set free below by the bacterial decompositions of vegetable mud and rises to meet the atmospheric oxygen coming down from above, and that this zone of physiological activity rises and falls with the variations of partial pressure of the gases due to the rate of evolution of the SH₂. In the deeper parts of this zone the bacteria absorb the SH₂, and, as they rise, oxidize it and store up the sulphur; then ascending into planes more highly oxygenated, oxidize the sulphur to SO₃. These bacteria therefore employ SH₂ as their respiratory substance, much as higher plants employ carbohydrates—instead of liberating energy as heat by the respiratory combustion of sugars, they do it by oxidizing hydrogen sulphide. Beyerinck has shown that Spirillum desulphuricans, a definite anaerobic form, attacks and reduces sulphates, thus undoing the work of the sulphur bacteria as certain de-nitrifying bacteria reverse the operations of nitro-bacteria. Here again, therefore, we have sulphur, taken