Page:History of botany (Sachs; Garnsey).djvu/543

] 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 concluded 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 assimilation 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 important 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 ascertaining 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 altogether. He too found the humus-theory, which had been constructed by the chemists, more convenient for his purpose, 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