158 Leaves and their Work 



reason why it is more nourishing is that it contains 

 more nitrogen, in the form of nitrogenous or albu- 

 minous compounds, commonly called albuminoids, 

 because they resemble the albumin, or white, of an 

 ^gg. There is albumin in the liquid 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 nitrogenous because it is the nitrogen they con- 

 tain 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 carbon. In addition to this 

 large amount of carbon, most of the nitrogenous com- 

 pounds contain sulphur, besides hydrogen 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, especially 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 pre- 

 pared 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 



