188 PLANT PHYSIOLOGY 



teristics. They are amorphous, colloidal, astringent materials, 

 which precipitate gelatin and combine with tissues containing 

 gelatin to form insoluble compounds. For this reason they are 

 used in tanning to convert hides and skins into leather. They 

 all give blue or green compounds with iron salts, and were for this 

 reason used formerly in the manufacture of inks, but in both the 

 tanning and the ink industries the tannins have been largely re- 

 placed by synthetic substitutes. The tannins are compounds of 

 protocatechuic and gallic acids, and are divided into two groups 

 on this basis. 



Tannins are very widely distributed in nearly all plants, but 

 woody perennials of the temperate zones are especially rich in 

 them. The leaves and bark of the oaks, chestnuts, and sumacs 

 contain large quantities of tannin, which is also found in abundance 

 in unripe fruits and in the pathological growths known as galls. 

 The astringent taste of green persimmons, of poorly prepared tea, 

 and of bad wine, is due to the presence of tannins. Although found 

 throughout the plant, they tend to accumulate in dead regions 

 such as the outer bark. 



Many theories have been proposed to explain the presence of 

 the tannins, and since they vary in different species it is not sur- 

 prising that several ideas concerning their use have been suggested. 

 What may be true for one tannin may not be true for all, and the 

 same tannin may serve different purposes under different condi- 

 tions. 



1. Although the tannins are not a direct photosynthetic product, 

 they are dependent upon light and carbon dioxide for their for- 

 mation, which is probably the result of their glucoside nature. 

 Oak seeds, when germinated in the dark, show an increase in tan- 

 nin as the glucosides are hydrolyzed; and the mulberry shows 

 more tannin in the twigs in the morning than at night (Ghirlanda, 

 1920). The tannins behave in this respect like reserve foods, and 

 this is the function assigned them by many workers, including 

 Michel-Durand, who places in the reserve food category especially 

 those derived from protocatechuic acid. 



Although fungi and perhaps a few other plants can use tannin 

 as a source of food in the absence of anything better, the evidence 

 is against the normal use of tannins as a reserve. An argument 

 against their use as reserves is their distribution; they are not 

 found in the sieve tubes or ray cells, where other reserve foods are 



