LECTURE VIII. 

 THE FOOD OF PLANTS. 



IF a plant be dried at a high temperature, the analysis of 

 the residue will afford, as pointed out in a previous lecture 

 (p. 60), an adequate account of the substances absorbed by 

 the plant during its life. The first step in the analysis would 

 be to incinerate the residue, and it would be found at the 

 close of this process that it had lost considerably in weight, a 

 proof that a portion of the dry solid of the plant had been 

 burnt up and given off in the form of gas. If this gas were 

 collected and analysed it would be found to consist princi- 

 pally of carbon dioxide, watery vapour, and nitrogen, the infer- 

 ence being that the combustible portion of the plant contained 

 the elements Carbon, Hydrogen, and Nitrogen. The incom- 

 bustible portion, the ash, would be found to be of a mineral 

 nature, containing a number of elements of which the principal 

 would be Sulphur, Phosphorus, Potassium, Calcium, Magne- 

 sium, Iron, Sodium, Chlorine, and Silicon. 



Of these elements, the Carbon, Hydrogen, Nitrogen, 

 Sulphur, and Phosphorus, are derived more especially from 

 the organised parts of the plant, such as the protoplasm and 

 the cell-wall, and from carbonaceous substances, such as sugar, 

 fats, acids, etc., which had been formed by the metabolic 

 activity of the protoplasm. These elements, together with 

 Oxygen, the presence of which cannot be directly ascertained 

 by analysis, are in consequence frequently, but somewhat 



