SOURCES OF THE NITROGEN OF PLANTS. 51 



The source of the nitrogen in plants is not so clear. 

 We know that four-fifths of the air surrounding all 

 plants is nitrogen, and yet it is proved that but little 

 if any of this nitrogen is absorbed through their leaves; 

 neither can it be shown to enter in any quantity 

 through their roots. We find, however, that the soil 

 is the place from which it comes, but that it is always 

 in some form chemically united with other bodies. 

 The two substances, ammonia and nitric acid, described 

 under a previous chapter as containing nitrogen, are 

 the chief sources of supply to plants : this fact partly 

 explains their great efficacy as manures, They are 

 both present in fertile soils, sometimes the one and 

 sometimes the other in largest quantity. Both are 

 soluble in water, and therefore can without difficulty 

 enter the roots. 



It will now be easily perceived that these organic 

 bodies to which attention has been so frequently called, 

 are indeed of very great importance. They constitute 

 the great bulk of vegetable life in all of its forms. 

 In the air and the soil, they are indispensable to life. 

 We cannot see them, yet depend on them for existence 

 itself. If half of the 2 5*0 o tn f carbonic acid present 

 in the atmosphere were withdrawn, nearly all valuable 

 plants would cease to flourish, and as a consequence 

 animal life too would gradually become extinct. 



