110 A TEXTBOOK OF BOTANY [Ch. Ill, 13 



of the substances called gltjcosides, a very large and het- 

 erogeneous group, probably of diverse significance to the 

 plant, characterized chiefly by the chemical fact that they 

 consist of glucose (grape sugar) in union with another sub- 

 stance. Certain ones give the bitter taste to nut kernels, 

 and to the bark of many trees, and the peppery taste to 

 Nasturtium, Water Cress, and some other plants. 



The Enzymes are the most important of the plant secre- 

 tions. They are formed in small quantities but large numbers 

 of kinds in diverse parts of plants, where they are apparently 

 dissolved in the protoplasm. Chemically they are supposed 

 to be proteins, but this is not certain, for, while we know their 

 effects, we hardly yet know the enzymes themselves. This 

 is because of the great difficulty of extracting them in a pure 

 state from the complicated protoplasm. Their importance 

 depends upon the fact that, like the catalyzers of the chemist, 

 they cause chemical changes in various substances (each en- 

 zyme but one change in one substance, as a rule), without 

 themselves entering into the reaction ; and on this account 

 very small quantities of enzymes can change great quantities 

 of substance. It is apparently by the action of enzymes that 

 the majority of chemical changes in plants are brought 

 about. Thus an enzyme called diastase is active in diges- 

 tion, changing the insoluble starch into soluble sugar both in 

 germinating seeds and animal saliva; another, called zy- 

 mase, secreted by the Yeast Plant, changes sugar into al- 

 cohol and carbon dioxide, as will be described under fermen- 

 tation ; lipase converts fats to soluble fatty acids ; pepsin 

 changes insoluble proteins into soluble peptones both in seeds 

 and the animal stomach; and so with many others. No 

 phase of plant chemistry is now of such acute interest and 

 active investigation as that concerned with the enzymes. 



Other secretions are the following. The fruit acids, malic 

 and citric and others, give the tart taste to fruits, of functional 

 utility in connection with dissemination by animals, and pleas- 

 ing to man. The tannins occur chiefly in the bark of plants, 



