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 lipases 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- Vie. 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. 
