100 CHEMISTRY OF PLANT LIFE 



Examples of both types of protective action have recently been 

 reported. 



It is obvious that the different forms of tannins may play 

 different roles in plant life, and the same tannin substance may 

 possibly serve different purposes under different conditions. 



BIOLOGICAL SIGNIFICANCE OF TANNINS IN FRUITS 



The presence of tannins in fruits and the changes which they 

 undergo during the ripening process cannot fail to attract atten- 

 tion to their biological significance in serving to protect the fruit 

 from premature consumption as food by animals. 



Tannins are of frequent occurrence in green fruits, imparting 

 to them their characteristic astringent taste. They nearly 

 always disappear as the fruit ripens. The fact that during the 

 ripening process both sugars and fruit esters, as well as attractive 

 surface pigments, are developed has led certain investigators to 

 the conclusion that tannins serve as mother-substances for these 

 materials in the green fruits and are converted into these attractive 

 agencies during ripening. There is nothing in the chemical com- 

 position of tannins which indicates, however, that they are pre- 

 cursors of sugars or fruit esters, although (as has been pointed out) 

 they may give rise to anthocyan pigments. 



Further, recent researches concerning the tannin of persim- 

 mons (the best-known and most striking example of the phenomena 

 under discussion) clearly show that the tannin is not actually used 

 up during the ripening process; that instead it remains in the ripe 

 fruit in practically undiminished quantity; but that when the 

 fruit is ripe, the tannin is enclosed in certain special large cells or 

 sacs, which are surrounded by an insoluble membrane, so that 

 when the fruit is eaten by animals the astringent tannin, enveloped 

 in these insoluble sacs, passes by the organs of taste of the animal 

 without causing any disagreeable effects. This walling-off of the 

 astringent tannins can be stimulated in partially ripe fruits by 

 treating them, with several different chemical agents, the simplest 

 ' method being that of placing the unripe fruit in an atmosphere of 

 carbon dioxide gas for a short period. The artificial " processing " 

 of persimmons to render them edible for a longer period before 

 they become naturally fully ripe and subject to decay is now a 

 commercial enterprise. This process is of interest because of 



