169 



ASSIMILATION OF NITROGEN BY PLANTS. 



A great deal of attention has lately been paid to the origin of nitro- 

 gen in the soil and its assimilation by plants. At first sight the subject 

 seems to belong to the domain of agriculture, but it has such a deep 

 bearing upon the physiology of plants, and the discoveries recently 

 made in connection with it, throw so much light upon the chemical pro- 

 cesses which are accomplished on a grand scale in nature, that the che- 

 mist, the botanist, the agriculturist, and the student of bacteriology are 

 equally interested in it, and discuss it from their own special points of 

 view. Perhaps it is the more necessary, therefore, to consider the 

 whole matter under its general aspects. 



The questions at issue are plain enough. A seed has been put in the 

 soil ; there it grows first on the food that has been stored up within the 

 seed itself by the mother plant. Later on the seedling sends its root- 

 lets in search of food in the soil, while its leaves, waved in the air and 

 bathing in sunshine, absorb another part of the necessary food from the 

 atmosphere. The mineral matters required by the plant are found in a 

 soluble state in the soil, or may be easily supplied to it, while oxygen, 

 hydrogen, and carbon are borrowed either from the atmosphere or from 

 the air and water which permeate the soil, and both contain some car- 

 bonic acid. But with nitrogen which is as necessary for the life of the 

 plant, as it is for the life of the animal, the difficulties come in. There 

 is plenty of it both in the atmosphere and in the soil, but it cannot be 

 absorbed from the atmosphere by the leaves, and out of the nitrogen 

 contained by an unmanured soil only an imperceptible amount is in 

 such state that it can be taken in by the roots. Whence, then, does the 

 plant take it ? 



That plants do not absorb free nitrogen from the air through the 

 leaves was proved fifty years ago by Boussingault, and still more deci- 

 sively in 1861, by J. B. Lawes, Dr. Gilbert and Dr. Pugh. Their me- 

 moir upon this subject has become classical, and it at once won a world- 

 wide reputation to the then modest farm of Rothamstead. They estab- 

 lished beyond doubt that the higher plants — with the exception, per- 

 haps, of the Leguminosce or Papilionacece (peas, vetches, lupins and so 

 on) — borrow their nitrogen supplies from some other source than the at- 

 mosphere. And yet Gr. Yille, another agriculturist of great repute, has 

 not ceased during the last fifty years to bring forward no less conclusive 

 experiments, proving that in some way unknown small quantities of 

 nitrogen always find their way from the atmosphere into a vigorous 

 plant. Even when the plant is grown under a glass bell, and the soil 

 is thus prevented from receiving the small amount of nitrogen which 

 might be brought down by rain in the shape of ammonia or nitric acid 

 formed in the atmosphere after a thunderstorm — even then some nitro- 

 gen of the air penetrates into the plant. Both sets of experiments are 

 equally conclusive, and for fifty years their contradictory results re- 

 mained unexplained. 



A similar difficulty was experienced with regard to the nitrogen in 

 the soil. Of course there is plenty of it, even in a poor soil : the pre- 

 vious generations of plants have laid it in stock. There is so much of 

 it that at a time when Liebig's chemical theories ruled agriculture he 

 could teach in some such terms as these : " Never mind the nitrogen" 



