366 Introduction. [Book hi. 



reached a certain stage of development without any explanation 

 of the phenomena of vegetation from physics or chemistry, and 

 even in spite of erroneous theories on those subjects. What 

 Malpighi, Hales, and to some extent Du Hamel produced, was 

 really vegetable physiology, and of a better kind than some 

 moderns are inclined to believe; and their knowledge was 

 derived from observations on living plants, and not from the 

 chemical and physical theories of their time. The discovery 

 even of important facts,, for example, that green leaves only 

 can form the food suitable to effect the growth and formation 

 of new organs, was made a hundred years before that of the 

 decomposition of carbon dioxide by the green parts of plants, 

 at a time indeed when chemistry knew nothing of carbon 

 dioxide and oxygen. A whole series of physiological discoveries 

 might be mentioned, which were distinctly opposed to chemical 

 and physical theories, and even served to correct them. We 

 may give as examples, the establishment of the facts that roots 

 absorb water and the materials of food without giving up any- 

 thing in return, which seemed quite unintelligible on the 

 earlier physical theory of the endosmotic equivalent ; and that 

 the so-called chemical rays of the physicists are of subordinate 

 importance in vegetable assimilation, while contrary to the pre- 

 vailing notions of physicists and chemists the yellow portions 

 of the spectrum and those adjacent to it actively promote the 

 decomposition of carbon dioxide. From what doctrines of the 

 physicists could it have been concluded, that the downward 

 growth of roots and the upward growth of stems was due to 

 gravitation, as Knight proved in 1806 by experiments on living 

 plants * or could optics have foreseen that the growth of plants 

 is retarded by light, and that growing parts are curved under 

 its influence. Our best knowledge of the life of plants has 

 been obtained by direct observation, not deduced from chemi- 

 cal and physical theories. After these preliminary remarks we 

 may proceed to give a rapid survey of the progress of vegetable 

 physiology. 



