5<D2 Theory of the Nutrition [Book hi. 



tained even till after 1830 respecting the necessity of the 

 constituents of the ash to vegetation. 



It was known in de Saussure's time that nitrogen entered into 

 the substance of living plants; the question was, whence it was 

 obtained. As it was known that four-fifths of the atmosphere 

 consists of nitrogen, it was natural to suppose that it is this 

 which the plant makes use of for forming its nitrogenous sub- 

 stance. De Saussure endeavoured to settle the question by the 

 volumetric method, which, as was afterwards discovered, was 

 not in this case to be trusted. Nevertheless he arrived at the 

 right conclusion, that plants do not assimilate the nitrogen of 

 the atmosphere ; this gas must therefore be taken up by the 

 roots in some form of chemical combination. He made no 

 experiments on growing plants to decide what that form was, 

 but contented himself with the conjecture that vegetable and 

 animal matter in the soil and ammoniacal exhalations from it 

 supply the nitrogen in plants. This question, first ventilated 

 certainly by de Saussure, and afterwards the subject of protracted 

 discussion, was finally settled fifty years later by the experi- 

 ments of Boussingault. 



In connection with his researches into the importance of the 

 constituents of the ash, de Saussure proposed the question 

 whether roots take up the solutions of salts and other substances 

 exactly in the form in which they offer themselves. He found 

 first of all that very various and even poisonous matters are 

 absorbed by them, and that there is therefore no such power of 

 choice, as Jung had once supposed ; on the other hand, it 

 appeared that the solutions do not enter unchanged into the 

 roots, for in his experiments in every case the proportion of 

 water to the salt absorbed was greater than the proportion 

 between them in the solution, and that some salts enter the 

 plant in larger, some in smaller quantities, under circum- 

 stances in other respects the same. But at this time, and for 

 a long time after, it was not possible to understand and rightly 

 explain these facts; the theory of diffusions was not yet known, 



