SUBSTANCES PRODUCED BY PLANTS. 73 



of the plant the same elements may exist in the form of a 

 sweet and soluble sugar, and in another of a tasteless and 

 insoluble starch. The changes by which albumen, casein, 

 and the various compounds containing nitrogen, may be 

 produced from the food which the plant receives, ai*e not so 

 easily understood, and their explanation would require details 

 which, however interesting to the scientific agriculturalist, 

 would be unsuitable in an elementary work. In another 

 place I shall have occasion to allude to the influence which 

 it has been found that an artificial supply of substances, rich 

 in nitrogen, exercises on the production of these valuable 

 compounds by the crops that you cultivate. 



