148 



VEGETABLE PHYSIOLOGY 



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 Algae 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 plants belong- 

 ing to the Natural Order Legumi- 

 noscB can also use atmospheric 

 nitrogen, but their power depends 

 upon the association with their 

 roots of certain fungi or bacteria 

 which infest the cortical tissues 

 and develop peculiar tubercular 

 structures upon the roots (fig. 84) . 

 The actual mode of absorption in 

 this case also is obscure ; the parts 

 Plo . 84. -ROOT OF A LEGUMI- played by the root and the fungus 

 NOUS PLANT, SHOWING THE Qr b ac terium respectively are not 

 at all determined. The atmo- 

 spheric nitrogen apparently is 

 made to enter into some form of combination, and is then 

 absorbed by the root, probably through the tissue of the 

 fungus. It is not absorbed by the leaves of the plant. 



Organic compounds of nitrogen are seldom presented to 



TUBERCLES ATTACHED TO THE 

 MAIN ROOT AND TO ITS 

 BRANCHES. 



