CHAPTER XIL 



THE COMPOSITION OF -PhkS'i^— [continued). 



I. The elementary constituents of plants. — Chemical analysis 

 has shown that the following elements are always present in the 

 compounds which form the body of a healthy green plant, 

 namely, carbon, hydrogen, oxygen, nitrogen, silicon, sulphur, 

 phosphorus, chlorine, potassium, sodium, calcium, magnesium 

 and iron. 



In sea-weeds bromine and iodine are usually present, and 

 many other elements, such as aluminium, zinc and copper, have 

 been occasionally discovered in small quantities in certain 

 species of plants. 



On burning the dry matter of a plant, the carbon, hydrogen, 

 oxygen and nitrogen within it escapes into the air in the form 

 of water, carbon dioxide, free nitrogen and other volatile 

 compounds : the other elements are left in the ash. 



While chemical analysis enables us to determine the particular 

 elements of which the body of a plant is composed, it does 

 not furnish a means of deciding which and how many of these 

 elements are necessary for the plant's existence. 



Since the majority of plants contain no zinc, tin or lead, it is 

 clear that these elements and others which are only occasionally 

 present are not necessary for plant-growth. On the other hand, 

 that carbon, hydrogen, oxygen and nitrogen are absolutely 

 essential may be inferred from the fact that these elements are 

 essential components of the organic compounds of which the 

 cell-walls and protoplasm are constructed. It does not, however, 



