THE SCIENTIFIC PRINCIPLES 



evolved resulted from the decomposition of the 

 carbonic acid taken up. 



So far, attention had been more prominently- 

 directed to the influence of plants upon the media 

 with which they were surrounded, than to that of 

 those media in contributing to the increased sub- 

 stance of the plants themselves. 



It was, too, towards the end of the last century, 

 and in the beginning of the present one, that De 

 Saussure followed up these enquiries ; and, in his 

 work already alluded to, he may be said to have 

 indicated, if not indeed established, some of the most 

 important facts with which we are yet acquainted 

 regarding the sources of the constituents of the 

 growing plant. He illustrated experimentally, and 

 even to a certain extent quantitatively, the fact that 

 in sun-light plants increase in carbon, hydrogen, and 

 oxygen, at the expense of carbonic acid and of 

 water ; and in his main experiment on the point he 

 found the increase in carbon, and in the elements of 

 water, was very closely in the proportion in which 

 they are known to exist in the chief non-nitrogenous 

 constituents of plants, the carbohydrates — starch, 

 gum, sugar, cellulose, etc. 



With regard to the nitrogen which plants had 

 already been shown to contain, Priestley and Ingen- 

 housz thought their experiments indicated that they 

 absorbed free nitrogen from the atmosphere ; but 

 Sennebier and Woodhouse arrived at an opposite 

 conclusion. De Saussure, again, thought that his 

 experiments showed rather an evolution of nitrogen 

 at the expense of the substance of the plant than 

 any assimilation of it from gaseous media. He 

 further concluded that the source of the nitrogen of 

 plants was more probably the nitrogenous compounds 



