CHAP, ii.] of Plants. Treviranus and Meyen. 523 



There is much in the details of Meyen's views on the 

 chemical processes in the nutrition of plants that is better than 

 what we find in Treviranus ; it is a great point that he con- 

 cluded from earlier experiments, that the salts which find their 

 way with the water into the roots are not merely ' stimulants ' 

 but food-material, and, as was before said, he explained the 

 respiration of oxygen by plants correctly in accordance with de 

 Saussure's observation. But he too stumbled over the assimi- 

 lation of carbon ; he, like so many before and after him, was 

 confused by the simple fact, that gaseous matter takes part 

 both in the nutrition and the respiration of the plant ; and 

 taking the processes in both cases for processes of respiration, 

 he considered the absorption of oxygen to be the only im- 

 portant and intelligible function, and the decomposition of 

 carbon dioxide in light to be a matter of indifference as 

 regards the internal economy of the plant. Instead of ascer- 

 taining by a simple calculation, whether the apparently small 

 quantity of carbon dioxide in the atmosphere was perhaps 

 sufficient to supply vegetation with carbon, he simply declared 

 it to be insufficient, and because plants will not flourish in 

 barren soil merely by being supplied with water containing 

 carbon dioxide, he gave up the importance of that gas alto- 

 gether. He too found the humus-theory, which had been 

 constructed by the chemists, more convenient for his pur- 

 pose, and like Treviranus derived the whole of the carbon in 

 plants from 'extract' of the soil, without any close attention 

 to the facts of the case ; he refused to believe that the soil 

 is rendered not poorer but richer in humus by the plants that 

 grow on it. It is obvious then that the account given by 

 Treviranus and Meyen of the chemical processes that take 

 place in the nutrition of plants, though correct in some of 

 the details, could afford no true general view of the processes 

 of nutrition, because it entirely misconceived the cardinal 

 points in the whole theory, namely the source of the carbon, 

 and the co-operation of light and of the atmosphere ; and 



