BOOK HI.] Introduction. 373 



nutrition in plants with Saussure's results. Agricultural 

 chemists were chiefly engaged till nearly 1860 with the 

 questions, whether all or certain constituents of the ash of 

 a plant are indispensable parts of its food, and whence these 

 constituents are derived, and with cognate considerations on 

 the exhaustion of the soil by cultivation and its remedy by 

 suitable manuring. In France Boussingault had undertaken 

 experimental and analytical investigations on these subjects 

 before 1840, and it was he who in the course of the next 

 twenty years made the most valuable physiological discoveries ; 

 of these the most important was the fact that plants do not 

 make use of free atmospheric nitrogen as food, but take up 

 compounds of nitrogen for the purpose. In Germany the 

 interest in such questions was increased by the instrumentality 

 of Justus Liebig, who gathered from the knowledge that had 

 been accumulated up to 1840 all that was fundamental and 

 of real importance, and drew attention to the great practical 

 value of the theory of the nutrition of plants in agriculture 

 and in the management of woods and forests ; considerable state 

 provision was soon made for investigations of the kind, but 

 these often wandered from the right path for the reason, that 

 being designed to promote practical interests they lost sight 

 of the inner connection between all vital phenomena. Still 

 a great mass of facts was accumulated, which careful sifting 

 might afterwards render serviceable to pure science. Some of 

 the best agricultural chemists deserve .the credit of vindicating 

 purely scientific as well as practical points of view, and 

 explained in comprehensive works the general subject of the 

 nutrition of plants, so far as it was possible to do so without 

 going deeply into their organisation ; among these were Bous- 

 singault and the Germans Emil Wolff and Franz Schulze. 

 But the questions of the nutrition of plants, which are con- 

 nected with the chemical processes of assimilation and meta- 

 bolism within them, remained still undecided, though some 

 valuable preliminary work on these points dates from this time. 



