THE FOOD OF PLANTS. 123 



in Lecture V. (p. 75) that oxygen is absorbed in considerable 

 quantity by all plants, and that carbon dioxide is absorbed 

 by green plants. It has been found that these gases are 

 essential to the life of the plants in question, and we may 

 therefore conclude that carbon and oxygen are important 

 constituents of their food. Further, water is essential to the 

 nutrition of plants ; hence hydrogen must be regarded as a 

 constituent of the food. With regard to the salts absorbed 

 by plants, the determination of their essential or non-essential 

 character has been effected by the method of water-culture 

 (fig. 21) to which allusion was made in a previous lecture 



(P. 60). 



The following may be taken as an example of a solution for water- 

 cultures. 



Distilled water, 1000 grammes. 



Potassium nitrate, i'o 



Calcium sulphate, 0*5 



Magnesium sulphate, 0*5 



Calcium phosphate \ 

 Ferrous sulphate J 



By varying the salts of the solution in a number of experi- 

 ments, and observing the effect produced upon the plants by 

 the presence or absence of certain salts, their importance to 

 plants, and that of their constituent elements, may be readily 

 ascertained. 



The elements which have been found to be essential to 

 the life of plants are the following ; Carbon, Hydrogen, 

 Oxygen, Nitrogen, Sulphur, Phosphorus, Potassium, Calcium, 

 Magnesium, Iron (in the case of green plants), and in certain 

 cases apparently also Chlorine. 



Inasmuch as the so-called organic substances which are 

 found in plants, such as fats and other hydrocarbons, starch, 

 cellulose and other carbohydrates, proteids and their allies, 

 consist of C and H, or of C, H, and O, or of C, H, O, and N, 

 or of these together with S, or P, it is clear that these 

 elements are of importance to the plant because they com- 

 pose the substances of which the structure of the plant is built 

 up. Further they are of importance because the complex 



