Other experiments were carried out of a similar nature by Professor S. W. Johnson, the different kinds of nitrogen experimented with being _uric acid_, _hippuric acid_, and _guanine_. But here, again, no definite conclusion can be drawn, as no analyses were made of the plants. More recently, however, Dr Hampe has carried out experiments with _urea_, _uric acid_, _hippuric acid_, and _glycocoll_. These experiments may be held as demonstrating the fact that at least one organic compound of nitrogen is capable of being assimilated, as urea was actually identified as being present in the plants experimented with. From further experiments, carried out by Dr Paul Wagner and Wolff, _glycin_, _tyrosin_, and _kreatin_ are able to be assimilated by the plant. _Plants able to absorb certain Forms of Organic Nitrogen._ We may conclude, then, from these interesting experiments, that plants are able to absorb certain organic forms of nitrogen. That they do so in nature to any extent is extremely improbable, such organic forms of nitrogen being rarely present in the soil, or if present, being converted into ammonia or nitrate salts before assimilation. _Nature of Humus in the Soil._ While on the subject of organic nitrogen, reference may be briefly made to that substance known as _humus_,--the name applied to the organic portion of soils,--a substance which figures so largely in early theories of plant-nutrition. The most elaborate investigation of the composition of humus has been carried out by Mulder. According to Mulder, it is composed of a number of organic bodies, and he has identified the following substances--ulmin, humin, ulmic, humic, geic acids, &c. These bodies are composed of carbon, hydrogen, and oxygen, which are invariably associated with nitrogen. Detmer and Simon have further investigated the subject. The true function of humus, it would seem, in addition to its numerous mechanical properties, is to furnish, by its decomposition, carbonic acid and nitrogen--in the form of ammonia and nitric acid--to the soil; the former acting as a solvent of the mineral food, the latter as the source of the plant's nitrogen. The old theory, therefore, that the presence of humus in a soil is a condition of fertility, is not so far removed from the truth. Where there is an abundance of humus in the soil there is likely also to be an abundance of nitrogen. _Relation of Ammonia to the Plant._