664 Hie Sources of the Nitrogen of our Leguminous Cro))s. 
ourselves happened to be presiding, that Professor Hellriegel 
first announced his new results. Quite consistently, not 
only with common experience in. agriculture, but also with 
the direct experimental results of ourselves and others, Hellriegel 
found, in his experiments, that plants of the gramineous, 
the chenopodiaceous, the polygonaceous, and the cruciferous 
families depended on combined nitrogen supplied within the soil. 
On the other hand, he found that leguminous plants did not 
depend entirely on such supplies. His results were, indeed, 
not only very definite, but it is seen that they had a special 
bearing on the admittedly unsolved problem of the source of the 
whole of the nitrogen of leguminous crops. 
Hellriegel's experiments and results may be briefly described 
as follows : — In 1883 he commenced a comprehensive series of 
vegetation experiments in pots, in which he grew agricultural 
plants of various families in washed quartz sand. To all the 
pots nutritive solutions, but containing no nitrogen, were added. 
To one series nothing else was supplied ; to a second a fixed 
quantity of nitrogen as sodium-nitrate was added ; to a third 
twice as much ; and to a fourth, four times as much. The 
result was that, in the case of the Graminece, and some other 
plants, the growth was largely proportional to the combined 
nitrogen supplied, whilst in that of the Leguminosce it was not 
so. In the case of these plants — that of peas, for example — it 
was observed that, in a series of pots to which no nitrogen wa8 
added, most of the plants were apparently Limited in their 
growth by the amount of nitrogen which the seed supplied. 
Here and there, however, a plant growing under ostensibly the 
same conditions grew very luxuriantly ; and, on examination, 
it was found that whilst no nodules were developed on the roots 
of the plants of limited gi'owth, they were abundant on those of 
the luxuriantly grown plants. 
In view of this result, Hellriegel, with his colleague, Dr. 
Wilfarth, instituted experiments to determine whethe*, by the 
infection of the soil with appropriate organisms, the formation 
of the root-nodules and luxuriant growth could be induced ; and 
whether, by the exclusion of such infection, the result could be 
prevented. To this end, they added to some of a series of ex- 
perimental pots, 25 cubic centimetres, or sometimes 50 cubic 
centimetres, of the turbid watery extract of a fertile soil, made 
by shaking a given quantity of it with five times its weight of 
distilled water, and then allowing the solid matter to subside. 
In some cases, however, the extract was sterilised. In those in 
which it was not sterilised, there was almost always luxuriant 
growth, and abundant formation of root-nodules ; but with 
