504 Theory of the Nutrition [BOOK in.
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. 1804-1840.
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