Page:History of botany (Sachs; Garnsey).djvu/547

] Moreover, he was able to rest on long-accepted facts in just those points which were the most important, and on these he had only to throw the light of his chemical knowledge to dispel the previous darkness. In accordance with his main purpose, which was to apply organic chemistry and vegetable physiology to the service of agriculture, Liebig directed the severity of his criticism first of all against the humus-theory constructed by chemists and agriculturists and thoughtlessly adopted by various physiologists ; this was the first thing that must be got rid of, if the question was to be answered, of what substances does the food of plants consist, for the humus-theory was at once incorrect, and the product of a want of reflection which overlooked facts which lay before men's eyes. Liebig showed that what was known as humus is not diminished but constantly increased by vegetation, that the quantity in existence would not suffice for any length of time for the support of a vigorous vegetation, and that it is not taken up by plants. This once established, and Liebig's calculations left no doubt on the point, there remained one source only for the carbon of the plant, namely, the carbon dioxide of the atmosphere, with regard to which it was shown by a very simple calculation resting on eudiometric results that its quantity is sufficient to supply the vegetation of the whole earth for countless generations. It is true that Liebig in his zeal went much too far, when he found something contradictory in the true respiration of plants, because it is connected with the elimination of carbon dioxide, and simply denied its reality. On the other hand the theoretical significance of the fact established by de Saussure, that the elements of water are assimilated at the same time as the carbon, was now for the first time clearly explained. Liebig was better able to realise the importance of this fact for the theory of nutrition than de Saussure had been. But these weighty points were not the ones which attracted most attention with the adherents and opponents of Liebig; the practical tendency of his book made