504 Theory of the Nutrition [Book hi. 



called in question one of the most weighty facts of vegetable 

 physiology, namely, that only cells which contain chlorophyll 

 eliminate oxygen. 



5. Vital force. Respiration and heat of plants. 

 Endosmose. i 804-1 840. 



During the twenty years that followed the appearance of 

 de Saussure's chemical researches the theory of the nutrition of 

 plants can scarcely be said to have been advanced in any one 

 direction, while much that had already been accomplished was 

 not even understood. Various circumstances worked together 

 to introduce misconceptions in this province of botany ; above 

 all others the inclination, more strongly pronounced than ever 

 at this period, to attribute to organisms a special vital principle 

 or force, which was supposed to possess a variety of wonderful 

 powers, so that it could even produce elementary substances, 

 heat, and other things out of nothing. Whenever any process 

 in such organisms was difficult to explain by physical or che- 

 mical laws, the vital force was simply called in to bring about 

 the phenomena in question in some inexplicable manner. It 

 was not that the question was now raised, which at a later time 

 engaged the attention of profounder thinkers, whether there 

 was a special agent operating in organic bodies beside the 

 general forces which govern inorganic nature ; for a careful 

 examination of this question would certainly have led to the 

 most earnest efforts to explain all the phenomena of life by 

 physical or chemical laws. On the contrary, it was found con- 

 venient to assume this vital force as proved, and to assign it as 

 the cause of a variety of phenomena, thus escaping the neces- 

 sity of explaining the way in which the effects were produced ; 

 in a word, the assumption of a vital force was not a hypothesis 

 to stimulate investigation, but a phantom that made all intel- 

 lectual efforts superfluous. 



Another hindrance to the progress of physiology, especially 



