It had long been known that the growth of leguminous crops, unlike that of others, enriched the ground,[C] and Lawes and Gilbert had shown that this was due to an increase of soil nitrogen. But no explanation could be found till Hellriegel and Wilfarth solved the problem.[4] In studying the nitrogen nutrition of gramineous and leguminous crops, they discovered that the gramineous plants died in absence of nitrate, and in its presence made growth which increased regularly with nitrate supply; while leguminous plants sometimes died and sometimes flourished in absence of nitrate, and behaved equally erratically with increasing nitrate supply. When the plants flourished nodules were invariably present on the roots, but not otherwise. They concluded, therefore, that the nitrogen nutrition of leguminous plants differed from that of the gramineæ, and depended on some factor which sometimes came into their experiments and sometimes did not, and, in any case, was associated with the nodule. Knowing that the nodules on the roots of leguminous plants contained bacteria-like bodies, and remembering Berthelot’s results, they explored the possibility of bacterial fixation. They sterilised the sand and found that peas invariably failed to develop nodules and died, but after adding a little garden soil nodules were found and vigorous growth was obtained. [C] “Of the leguminous plants the bean best reinvigorates the ground ... because the plant is of loose growth and rots easily, wherefore the people of Macedonia and Thessaly turn over the ground when it is in flower” (i.e. dig it into the ground if the soil is poor). Theophrastus, “Enquiry into Plants,” bk. viii. 2, and bk. ix. I. This book is of profound interest to agriculturists and botanists. An excellent translation by Sir Arthur Hort is now available. (Loeb’s Classical Library.) Chemical analysis showed considerable fixation of gaseous nitrogen, which Hellriegel associated with the nodule organism. This has proved to be correct, and the fixation of nitrogen by bacteria is now a well-recognised process, the conditions of which are being thoroughly worked out. Two types of organisms are known--those associated with leguminous plants, and those living in a free and independent state in the soil. Of the latter the Clostridium, isolated by Winogradsky, is anaerobic, and the Azotobacter of Beijerinck is aerobic. The essential conditions are that a source of energy must be supplied--usually given as sugar--that the medium must not be acid, and that sufficient phosphate must be present.