FOODS IN PLANTS 39 
some of the compounds of nitrogen, potassium, phosphorus, or 
other substances, these things may be made into more complex 
foods known as proteins. 
Many stages of the process is 
Een me 
of making these more com- 
plex foods are not known, a 
and these are too intricate for ex- 
tended discussion at this time. The 
leaf or other parts of the plant may 
be used as the place where proteins 
are made. They may be made im- 
mediately after the carbohydrates, 
or later, but sooner or later some 
protein food is as necessary to the 
continued life of plants as of animals. 
The soil is the usual source of the 
nitrogen, potassium, phosphorus, 
and other substances that are used 
in addition to the carbohydrates in 
making proteins. Although the air 
is 78 per cent nitrogen, this atmos- 
pheric nitrogen is not available to 
plants, except to certain bacteria 
which are to be discussed later. 
From the soil, compounds contain- 
ing nitrogen and other substances 
may be dissolved in water and then 
carried into the plants. Fertile soils 
are those which contain in available 
form large quantities of the things 
which plants use for food-making. 
Replenishment and growth of new 
parts can take place only by means 
of foods, and since the plant makes 
its own supply, the importance of 
the process is very great. 
ing oxygen from working plants 
Water plants are submerged with 
one end in the mouth of the grad- 
uate. Bubbles of oxygen pass 
upward from the cut ends of the 
stems and crowd out some water 
from the previously filled grad- 
uate. The ordinary test for oxy- 
gen—witha burning stick — will 
determine whether it is present. 
In such an experiment care must 
be taken to see that there is 
plenty of space about the collect- 
ing tube, to permit free passage 
of the gases that are in the water. 
After Ganong 
