134 VEGETABLE PHYSIOLOGY 
humble organisms have the power of converting the 
ammonia compounds into nitrites, and the latter into 
nitrates, in which form they are taken up. This process 
of nitrification is the special property of two different 
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 made use of only in 
very exceptional cases. Certain 
lowly Alge are said to have the 
power of using it, but the process 
is not fully understood. Some of 
the bacteria in the soil appear to 
be able to cause the nitrogen of 
the air to enter into some form 
of combination, probably yielding 
either nitrates or compounds of 
: ammonia. A few green plants can 
Es also use atmospheric nitrogen, but 
‘iN their power depends upon the 
r association with their roots of 
certain fungi or bacteria which 
N infest the cortical tissues and 
{ 
\ 
generally develop peculiar tuber- 
cular structures upon the roots 
(fig. 82). The power was first 
observed among the members of 
the Natural Order Leguminosae, 
Fic. 82.—Roor or a Lecum- but it has since been found to be 
xous Euams suowste ‘at possessed by plants of other fami- 
Py hoot ANP TO 18 lies and seems to be more wide- 
spread than was at first imagined. 
The actual mode of absorption in these cases also is obscure ; 
