BOOK in.] Introduction. 369 



and thus obtain the carbon which plants accumulate in organic 

 combinations, but that all parts of plants also absorb at all 

 times smaller quantities of oxygen, and exhale carbon dioxide, 

 and so perform a process of respiration exactly corresponding to 

 that of animals. He was soon followed by Theodore de Saussure 

 with more thorough investigation of these processes, and with 

 proofs that the ash-constituents of a plant are no chance or un- 

 important addition to its food, as had been hitherto commonly 

 supposed (1804). The influence also of general physical forces 

 on vegetation was established in some important points, though 

 not yet submitted to searching examination. Thus Senebier 

 showed in the period between 1780 and 1790 the great effect 

 which light exercises on the growth and green colour of plants, 

 and De Candolle at a later date discovered its operation in the 

 case of leaves and flowers that show periodic movements. 

 Still more important was Knight's discovery in 1806 that the 

 upright growth of stems and the downward direction of the 

 main roots are determined by gravitation. 



3. This second period of important discoveries was also 

 followed by a relapse, and again doubts were raised as to the 

 correctness of the very facts which had been best established ; 

 attempts were made under the influence of preconceived 

 opinions to invalidate or ignore these facts, and to substitute 

 for them theories that wore the guise of philosophy. The so- 

 called nature-philosophy, which had long been a great hindrance 

 to morphology, proved in like manner injurious to vegetable 

 physiology ; the doctrine of the vital force especially stood in 

 the way of every attempt to resolve the phenomena of life into 

 their elementary processes, to discern them as a chain of causes 

 and effects. The ash-constituents of plants, and even their 

 carbon, were traced to this vital force, and misty notions con- 

 nected with the word polarity were used to explain the direction 

 of growth and much beside. In like manner the influence of 

 the nature-philosophy was brought to bear on the established 

 results of the sexual theory to the destruction of all sound 



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