SECRETIONS 



285 



A number of enzymes, such as diastase, invertase, maltase, 

 zymase, and cytase, are involved in the digestion of the carbo- 

 hydrates. Diastase, which is especially abundant in germinating 

 seeds, changes starch into sugar. Invertase converts cane sugar 

 into glucose and fructose, and maltase converts maltose into 

 glucose. Zymase, well known as a secretion of the Yeast plant, 

 converts sugar into alcohol and carbon dioxide. Cytase breaks 

 cellulose into simpler compounds. 



The Upases digest the fats by changing them into fatty acids 

 and glycerine, in which form the fats can be moved in the plant 

 and consumed as food. 



For the glucosides there is also a group of enzymes. For 

 amygdalin there is amygdalase, 

 and for other glucosides certain 

 other enzymes which decom- 

 pose them. 



Although we know that en- 

 zymes have much to do with 

 the metabolic changes in plants, 

 our knowledge of enzymes is 

 comparatively meager. Inves- 

 tigations on the kinds of en- FlG - 253. - Cells containing crystals 



, ,, . , . , of calcium oxalate. 



zymes and their particular 



functions are much needed and are receiving much attention 

 by plant chemists. 



Other secretions so far omitted from our list are the acids 

 and the tannins. Malic acid, oxalic acid, citric acid, tartaric acid, 

 and a number of others, known as fruit acids, which function in 

 determining the taste of fruits, are very important to man and 

 assist in seed dissemination when they make the fruit more 

 pleasing to the taste. Some of the organic acids often form 

 compounds with minerals and form crystals. Crystals of cal- 

 cium oxalate are quite common in plant cells. (Fig. 253.) The 

 tannins are bitter astringent substances as any one who has 

 tasted a green Persimmon well knows. They occur throughout 

 plants, but are more abundant in the bark. Tannins harden the 

 gelatine in skins, and before the chemists provided substitutes bark 

 was extensively used in tanning leather. Due to their astringent 

 and antiseptic properties, it is thought that they protect the plant 

 against the action of organisms, such as Fungi and Bacteria. 



