(_b_) That this absorption of free nitrogen is not effected directly by the plant, but is the result, so to speak, of the joint action of certain micro-organisms present in certain soils and in the plant itself, (_symbiosis_). (_c_) That this fixation is connected with the formation of minute tubercles on the roots of the plants of the leguminous class; and that these tubercles may be the home of the fixing organism. (_d_) That these fixing micro-organisms are not present in all soils.[27] While the relation of free nitrogen to the plant has long been, and still is, a very obscure problem, it was early recognised that the combined nitrogen present in soils and manures was an important source of plant-food. Reference has already been made to the early theory of Sir Kenelm Digby regarding the value of nitrates.[28] De Saussure, as we have also already seen, was fully impressed with the importance of applying nitrogen to the soil as a manure. Liebig's early attitude on this question was to the effect, that to apply nitrogen in manures was quite unnecessary, as the plant had a sufficient source in the ammonia present in the air, which he erroneously supposed was sufficient in quantity to supply all the needs of the crops. Despite this early recognition of the value of combined nitrogen to the plant, it is only of recent years that we have obtained any definite knowledge as to the respective value of its different compounds as manures, or as to the form in which it is assimilated by the plant. It exists in three forms--(1) as organic nitrogen; (2) as ammonia salts; (3) as nitrates and nitrites. Much experimental work has during late years been devoted to studying the comparative action and merits of these three forms. _Relation of Organic Nitrogen to the Plant._ First, as to the relation of organic nitrogen to the plant. There is a large number of different organic compounds which contain nitrogen. That the plant is able to assimilate certain of these organic compounds, seems, from several experiments, to be extremely probable. From certain researches, carried out as far back as the year 1857, Sir Charles Cameron concluded that the plant could assimilate one of them--viz., _urea_. From what, however, we have subsequently learned regarding the process of "nitrification," it is quite probable that the nitrogen in these experiments was first converted into nitrates before being assimilated. At any rate, as the plants were not tested for urea, the experiments must be regarded as leaving the problem unsolved.