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 
