60 PRACTICAL COURSE IN BOTANY 
tion, and so we may conclude that (with the possible addition 
of chlorine) they form the indispensable elements of plant 
food. Carbon, hydrogen, oxygen, nitrogen, sulphur, and 
phosphorus compose the structure of which the plant is built. 
The other four ingredients do not enter into the substance as 
component parts, but aid in the chemical processes by which 
the life functions of the plant are carried on, and are none 
the less essential elements of its food. Figure 74 shows the 
difference between a plant grown in a solution where all 
the food elements are present, and others in which some of 
them are lacking. 
63. How plants obtain their food material. — Plants 
obtain their supply of the various mineral salts from solu- 
tions in the soil water which 
they absorb through their 
roots. With a few doubtful 
exceptions, they cannot as- 
similate their food unless it 
is in a liquid or gaseous form. 
Of the gases, carbon dioxide, 
oxygen, and hydrogen can 
be freely absorbed from the 
air, or from water with va- 
rious substances in solution, 
but most plants are so .con- 
stituted that they cannot absorb free nitrogen from the air ; 
they can take it only in the form of compounds from nitrates 
dissolved in the soil, and hence the importance of ammonia 
and other nitrogenous compounds in artificial fertilizers. 
Some of the pea family, however, bear on their roots little 
tubers formed by minute organisms called bacteria, which 
have the power of extracting nitrogen directly from the 
free air mingled with the soil; and hence the soil in which 
these tuber-bearing legumes decay is enriched with nitrogen 
in a form ready for use. 
Fic. 75.— Roots of soy bean bearing 
tubercle-forming bacteria. 
