72 THE VEGETATIVE FUNCTIONS OF PLANTS 



chlorine. Calcium is not necessary for all fungi (e.g., 

 not for Aspergillus niger), nor for all algae. Three main 

 problems now confront us : 



1. What is the source of these food elements? 



2. Where, in the plant, are they elaborated into plant 

 food? 



3. How is the process accomplished? 



71. Source of the Food Elements. Since most plants 

 are fixed for life to a certain spot they must obtain their 

 food elements from their immediate surroundings; they 

 cannot, like animals, go in search of them. The carbon 

 and oxygen are obtained chiefly from the air, where the 

 carbon appears, in combination with oxygen, as carbon 

 dioxide. Free oxygen (as well as in combination with 

 carbon) is also obtained from the air. About four-fifths 

 of the air is nitrogen, and one might naturally infer 

 that the plant could obtain an abundant supply from 

 that source. We shall learn later, however, that most 

 plants cannot utilize the free nitrogen of the air, but 

 must have it supplied in chemical combination with 

 other elements, in the form of nitrates. These are 

 obtained from the soil. The calcium, potassium, mag- 

 nesium, phosphorus, sulphur, and iron are also obtained 

 from the soil in the form of soluble salts, such as phos- 

 phates, nitrates, sulphates, and carbonates. The hy- 

 drogen is obtained chiefly from water (H^O) . 



72. Seat of Elaboration of Carbohydrates. Careful 

 and thorough studies have established, beyond all doubt, 

 the fact that the inorganic food elements are combined 

 to form carbohydrates only in the green cells of plants, 

 either in leaves, stems, or other parts. This is one of the 

 most fundamental facts in all science. Among those 



