IT SUPPLIES NITROGEN. 189 



atmosphere, furnishes nitrogen to a plant in quan- 

 tity sufficient for its normal growth. Now its 

 growth must be considered as normal, when it pro- 

 duces a single seed, capable of reproducing the same 

 plant in the following year. Such a normal condi- 

 tion would suffice for the existence of plants, and 

 prevent their extinction, but they do not exist for 

 themselves alone ; the greater number of animals 

 depend on the vegetable world for food, and by 

 a wise adjustment of nature, plants have the 

 remarkable power of converting, to a certain 

 degree, all the nitrogen offered to them into nutri- 

 ment for animals. 



We may furnish a plant with carbonic acid, and 

 all the materials which it may require, we may 

 supply it with humus in the most abundant quan- 

 tity, but it will not attain complete development 

 unless nitrogen is also afforded to it ; an herb will 

 be formed, but no grain, even sugar and starch 

 may be produced, but no gluten. 



But when we give a plant nitrogen in con- 

 siderable quantity, we enable it to attract with 

 greater energy, from the atmosphere, the carbon 

 which is necessary for its nutrition, when that in 

 the soil is not sufficient ; we afford to it a means 

 of fixing the carbon of the atmosphere in its 

 organism. 



We cannot ascribe much of the power of the 

 excrements of black cattle, sheep, and horses, to 

 the nitrogen which they contain, for its quantity is 



