140 VEGETABLE PHYSIOLOGY 
bacteria, one of which forms nitrites from the ammonia 
compound, and the other transforms nitrites into nitrates. 
Certain fungi differ in their behaviour from green plants, 
absorbing ammonia compounds without such conversion. 
It is in the way described that a normal green plant 
absorbs all the nitrogen which it uses for the construction 
of food substances. The nitrogen 
of the air is utilised only by 
its being made to enter into 
some form of combination by 
bacteria in the soil. There is 
much greater activity in this 
fixation than was thought till re- 
cently, very considerable amounts 
of atmospheric nitrogen being 
made available for absorption 
by this instrumentality. Certain 
lowly Algze are said to have the 
power of using it, but the process 
is not fully understood. A few 
green plants can also use atmo- 
spheric nitrogen, but their power 
depends upon the association with 
their roots of certain fungi or 
bacteria which infest the cortical 
tissues and generally develop 
peculiar tubercular structures 
upon the roots (fig. 82). The 
Fe eee ertteMr power was first observed among 
Tuperoues arracuep to THE the members of the Natural 
eerie apm Order Leguminose, but it has 
since been found to be possessed 
by plants of other families and seems to be more widespread 
than was at first imagined. The actual mode of absorption 
in these cases algo is obscure ; the parts played by the root 
and the fungus or bacterium respectively are not at all 
determined. The atmospheric nitrogen apparently is made 
