The fact that they can develop in media destitute of organic matter, is one of very great interest and importance to Vegetable Physiology. It implies that they can derive their carbon from carbonic acid--a power which it was believed was possessed by green plants alone among living structures. For organisms destitute of chlorophyll, the source of their protoplasmic carbon, it has been hitherto commonly believed, must be _organic matter_ of some sort. While it would appear that the nitrifying organisms can, when opportunity affords, feed upon organic matter, yet it has been proved beyond doubt that they can also freely develop in media entirely devoid of it, and are capable, under such circumstances, of deriving their carbon from a purely mineral source.[109] This fact, which is subversive of what was believed to be a fundamental law of Vegetable Physiology, is one of the most important of the many important and interesting facts which these nitrification researches have elicited.[110] CONDITIONS FAVOURABLE FOR NITRIFICATION. We may now proceed to discuss the conditions favourable for nitrification. _Presence of Food-constituents._ Among these conditions the first is the presence of certain food-constituents. To both animal and vegetable life alike a certain amount of mineral food is absolutely necessary. Among these phosphoric acid is one of the most important, and in the experiments on nitrification it has been found that the nitrifying organisms will not develop in any medium destitute of it. That other mineral food-constituents are necessary is highly probable, although the influence of their absence on the development of the process has not been similarly studied. Probably potash, magnesia, and lime salts are necessary. In the cultivating solutions used in the experiments on the subject, the mineral food-constituents added consisted of lime, magnesia, and potash salts and phosphoric acid.[111] As we have seen above, the presence of organic matter is not necessary for the process. In this respect these organisms are differentiated from all other ferments hitherto discovered. _Presence of a Salifiable Base._