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 Algae 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 

 power was first observed among 

 the members of the Natural 

 Order Leguminosce, 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 also 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 



Fia. 82. ROOT OF A LEGUMI- 

 NOUS PLANT, SHOWING THE 

 TUBERCLES ATTACHED TO THE 



MAIN ROOT AND TO ITS 

 BRANCHES. 



