CHAP, ii.] the Nutrition of Plants. 447 



the movement of the sap, and generally into the whole internal 

 economy of plants, depends on a knowledge of the fact, that it 

 is only the cells which contain chlorophyll, and therefore in 

 the higher plants the leaves chiefly as consisting largely of 

 such cells, which have the power of converting the gaseous 

 food supplied by the atmosphere into the substance of the 

 plant with the aid of the materials taken up from the soil. 

 This fact is of fundamental importance to the whole theory of 

 the nutrition of plants ; it is only by a knowledge of it that we 

 can explain the movement of material connected with nutrition 

 and growth, the dependence of vegetation on light, and to a 

 great extent also the function of the roots. 



But this principle could not be discovered till the new 

 chemical system founded by Lavoisier took the place of the 

 old phlogistic chemistry, and it is remarkable that the dis- 

 coveries, which laid the foundation of modern chemistry in the 

 period between 1760 and 1780, contributed essentially to the 

 establishment at the same time of the modern doctrine of the 

 nutrition of plants. Ingen-Houss, in reliance on Lavoisier's 

 antiphlogistic views on the composition of air, water, and the 

 mineral acids, succeeded in proving that all parts of plants are 

 continually absorbing oxygen and forming carbon dioxide, but 

 that the green organs at the same time under the influence of 

 light absorb carbon dioxide and exhale oxygen ; and as early 

 as 1796 he considered it probable that plants obtain the whole 

 mass of their carbon from the carbon dioxide of the atmosphere. 

 Soon after (1804) de Saussure proved, that plants, while they 

 decompose carbon dioxide, increase in weight by a greater 

 amount than that of the carbon which they retain, and that 

 this is to be explained by the fact that they at the same time 

 fix the elements of water. He likewise showed that the small 

 quantities of saline compounds, which plants take up from the 

 soil, are a necessary part of their food, and that it was at least 

 probable, that the nitrogen of the atmosphere does not contribute 

 to the formation of nitrogenous substances in plants. Setiebier 



