448 History of the Theory of [Book hi. 



had before insisted on the fact, that the decomposition of 

 carbon dioxide under the influence of light only takes place in 

 green organs. 



Thus the most important points in the nutrition of plants 

 were, discovered by Ingen-Houss, Senebier and de Saussure. 

 But, as often happens in the case of discoveries of such 

 magnitude, their ideas were for a long time exposed to great 

 misunderstanding. They were better appreciated in France 

 than in any other country ; Dutrochet and De Candolle were 

 able to see the importance of the interchange of gases in the 

 green organs to the general nutrition and respiration ; but 

 others, and especially German botanists, were not content with 

 these simple chemical processes as the foundation of the whole 

 system of nutrition and consequently of the whole life of the 

 plant ; the theory of the vital force, which was elaborated in con- 

 nection with the nature-philosophy during the first years of the 

 19th century, and was generally accepted by philosophers and 

 physiologists, chemists and physicists, preferred to supply the 

 plant with a mysterious substance for its food, which had its 

 source in the life itself and which it called humus. The most 

 obvious considerations, which must at once have shown that 

 this humus-theory was absurd, were entirely overlooked ; and 

 thus in the face of de Saussure's results the food of plants was 

 once more referred entirely to the soil and the roots, as it was 

 in the earliest times ; one of the consequences of this humus- 

 theory in combination with the vital force was that the ash- 

 constituents of plants were supposed to be merely accidental 

 admixtures or stimulants, or to be directly produced in the 

 plant by the vital force. 



In the period between 1820 and 1840 the reaction set in 

 from different quarters against the theory of vital force ; 

 chemists succeeded in producing by artificial means certain 

 organic compounds, which had hitherto been regarded as 

 products of that force ; Dutrochet discovered in endosmose a 

 process, which served to refer various vital phenomena in 



