10 THE CHEMISTRY OP THE FARM. 



several other amide bodies. The facility, however, with 

 which ammonia, and other nitrogenous substances, are 

 converted into nitric acid in the soil is so great that 

 nitrates become by far the most important source of 

 nitrogen at a plant's disposal. 



Recent investigations have shown that in some cases 

 the feeding power of roots is modified to a very consider- 

 able extent by their union with another vegetable 

 organism. Thus, according to Frank, the absorption of 

 food from the soil is in the case of oak, beech, horn- 

 beam, hazel and chestnut, mainly accomplished through 

 the medium of a fungus, the mycelium of which com- 

 pletely covers the root, and is united with it. A remark- 

 able instance of such an action is afforded by legu- 

 minous plants. All the members of this family have 

 tubercles on their roots, unless the plant has been 

 grown from seed in a sterilised soil. These tubercles 

 are occasioned by the invasion of an organism, present 

 in the soil, the character of which has not yet been fully 

 studied. When the seeds of peas, lupins or vetches, 

 are sown in sterilised sand, containing the necessary 

 ash constituents of plants, but no nitrogen, only a 

 small, dwarfed growth is obtained, and the roots 

 are not furnished with tubercles. If, however, a minute 

 quantity of ordinary soil is added, tubercles appear on 

 the roots, and the plant now grows vigorously. At the 

 end of the experiment it is found that the quantity of 

 nitrogen in the crop is far greater where tubercles have 

 been formed than where they are absent : indeed, in the 

 former case, the quantity of nitrogen in the crop and soil 

 at harvest much exceeds that originally present in the 

 sand, seed, and added soil. This gain of nitrogen has 



