THE CELL 39 



solid and gaseous elements must combine in order to 

 produce a certain amount of vegetable matter. When 

 we pass from the combustible organic part of a plant 

 to study the ash, we find that a greater number of 

 elements enter into the composition of the latter. 

 We shall here enumerate only the principal ones, having 

 to return to the closer study of them in our fourth 

 lecture. 



The first four elements of the ash form acids, which 

 with the four metals mentioned in the second column 

 form salts. 



When once we know of which elements a plant is 

 composed, and knowing also that elements are incapable 

 of transformation one into the other, we can say before- 

 hand what are the sources from which these sub- 

 stances have been derived. 



In the air, in the atmosphere, a plant comes into 

 touch with free oxygen and nitrogen, and with small 

 quantities of carbonic acid — a gas composed of carbon 

 and oxygen — and also with very small quantities of 

 nitrogen combined with oxygen and hydrogen. In 

 the soil, besides the substances just mentioned, the 

 plant comes into touch with others, which, owing to 

 their non-volatility, cannot exist in the air ; these are 

 salts which contain the other elements found in the 

 plant. Some of these salts are dissolved in the water 

 of the soil, and so form part of the liquid environment 

 of the plant ; others exist in solid form. 



So far we have only disentangled the chemical 

 elements of which the body of a plant is composed; 



