284 LEAVES 



cocoanut butter from the oil of cocoanuts. From various nuts 

 used as food much fat is obtained. Castor oil from the castor 

 bean is valuable as a medicine and as a lubricant for gasoline 

 engines, especially those of flying-machines, and is used as an. 

 illuminant and in making dyes. The extensive use of olive oil 

 and peanut butter are other examples of the usefulness of the 

 vegetable fats. 



In connection with plant substances used as food by animals 

 there are the vitamines, substances not understood but of im- 

 mense importance. Animals die regardless of the kind of diet if 

 vitamines are withheld. In leaves and stems, and other parts of 

 plants vitamines are obtained. Dairy products are an important 

 source of vitamines for man, but the cow gets them from the 

 vegetation she eats. 



Enzymes. — The most general substances made by the proto- 

 plasm are enzymes. They occur dissolved in the cell sap or in 

 intimate relation with the protoplasm. They are not well under- 

 stood but seem to be protein-like in structure. Their function 

 is to cause chemical changes. Apparently nearly all chemical 

 processes in both plants and animals depend upon them, and the 

 kinds of enzymes are about as numerous as the kinds of chemical 

 actions that occur in connection with cells. Proteases, comprising 

 ereptases and peptases, act upon proteins, causing them to break 

 down into proteoses, peptones, and amino-compounds. In these 

 soluble forms proteins are translocated and used as food. Bro- 

 melin in the Pineapple and papain in the papaw (Carica papaya) 

 are two well known peptases, but there are others and they are 

 present in all parts of all plants. Papain is used in making diges- 

 tive tablets. A number of enzymes, such as diastase, invertase, 

 maltase, zymase, and cytase act upon carbohydrates. Starch is 

 changed to sugar by diastase, a common enzyme in germinating 

 seeds. Invertase converts cane sugar into glucose and fructose, 

 and maltase converts maltose into glucose. Zymase, a well 

 known secretion of Yeast plants, converts sugar into alcohol and 

 carbon dioxide. • Cytase breaks cellulose into simpler compounds. 



The fat splitting enzymes are known as lipases. They change 

 fats into glycerine and fatty acids, thus making them soluble and 

 transportable. 



Substances are not only broken down but also built up through 

 the activity of enzymes. Their relation to metabolism is so 



