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 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 green plants 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 

 generally develop peculiar tuber- 

 cular structures upon the roots 

 (fig. 82). The power was first 

 observed among the members of 

 the Natural Order Leguminosce, 

 FIG. 82. BOOT OF A LEGUMI- but it has since been found to be 



TUBEKCXES N ATTACHrD N TO THE PO 886886 ^ ^J P^IltS Of Other fami- 



MAIN BOOT AND TO ITS }i es an( j seems to be more wide- 



BBANCHES. . . 



spread than was at first imagined. 

 The actual mode of absorption in these cases also is obscure ; 



