There are also types of “free-living,” as distinguished from these symbiotic, bacteria which absorb the free nitrogen of the air and aid materially in keeping up this supply under natural conditions. One of the most important of these types is the aërobic “Azotobacter” (Fig. 76), while another is the anaërobic _Clostridium pasteurianum_. The nitrogen which is absorbed is built up into the protein material of the cell body and this latter must in all probability be “worked over” by various types of decomposition bacteria and by the nitrous and nitric organisms and be converted into utilizable nitrates just as other protein material is, as has been discussed in Chapter X. At any rate there is as yet no definite knowledge of any other method of transformation. Up to the present no intentional practical utilization of this valuable property of these free-living forms has been made. =Nitrogen Nutrition of Green Plants.=--It is the belief of botanists that green plants obtain their nitrogen chiefly in the form of nitrates, though ammonium salts may be utilized to some extent by certain plants at least. Exceptions to this general rule are those plants provided with root tubercles (and the bog plants and others which have mycorrhiza?). These plants obtain their nitrogen in the form of organic compounds made for them by the bacteria growing in the tubercles. That nitrogen circulates throughout the structure of plants in organic combination is certain. There does not appear to be any reason why similar compounds which are soluble and diffusible (amino-acids?) should not be taken up through the roots of plants and utilized as such. _It seems to the author that this is very probably the case._ Arguments in favor of this view are: (1) The nitrogen nutrition of leguminous and other plants with root nodules. (2) The close symbiosis between “Azotobacter” and similar nitrogen-absorbing bacteria and many species of algæ in sea water at least. (3) The vigorous growth of plants in soils very rich in organic matter, which inhibits the production of nitrates by the nitrous-nitric bacteria when grown in culture, and possibly (?) in the soil, so that nitrates may not account for the vigorous growth. (4) The effect of nitrate fertilizers is to add an amount of nitrogen to the crop much in excess of the amount added as nitrate. (5) The most fertile soils contain the largest numbers of bacteria. The doctrine that nitrates furnish the only nitrogen to plants was established before the activities of bacteria in the soil were suspected, and, so far as the author is aware, has not been supported by experiments under conditions rigidly controlled as to sterility. It would seem that one of the chief functions of soil bacteria is to prepare soluble organic compounds of nitrogen for the use of green plants and thus to make a “short cut” in the nitrogen cycle (p. 107), as now believed in, direct from the “decomposition bacteria” to green plants.