Leaves and Their Work 131 



of an egg. There is albumin in the Hquid part of blood, 

 and there is albumin in the seeds and juices of plants. 

 There is fibrin in flesh and in the thick red part of blood; 

 and there is vegetable fibrin, a sticky substance usually 

 called gluten, in flour. There is casein in milk and 

 cheese, and there is casein in beans. 



These various substances are all jelly-like, and are very 

 similar in composition, whether they are obtained from 

 animal or from vegetable matter. They are called nitro- 

 genous because it is the nitrogen they contain which gives 

 them their especial characteristics and value as food; but 

 the nitrogen in them forms less than a seventh part of 

 their substance, more than half of which consists of car- 

 bon. In addition to this large amount of carbon, most of 

 the nitrogenous compounds contain sulphur, besides hydro- 

 gen and oxygen, and most of them phosphorus as well. 

 They could not therefore be formed without the help of 

 mineral matter from the soil, and of carbon from the air; 

 the nitrogen being obtained both from the nitrates in the 

 soil and from the ammonia in the air. 



All the corn crops, as well as the clovers, beans, peas, 

 and other leguminous plants, require much nitrogen, espe- 

 cially when their seeds are forming and ripening; but by 

 the time they have done blossoming, they have taken up 

 all they want, and it is being prepared and compounded 

 by the leaves, to be gradually passed on by them to the 

 growing seeds as they need it. Grass, therefore, is not 

 cut for hay until after it has blossomed, because it then 

 contains most nitrogen and is most nutritious. For the 

 nitrogenous compounds are the flesh-forming part of all 

 food, animal or vegetable. 



They are very much alike in composition, as has been 



