190 PLANT PHYSIOLOGY 



the presence of the tannins in the unripe fruit keeps it from being 

 eaten until the seeds are ready for dispersal. 



5. According to Van Wisselingh, tannins play an important 

 role in conjugation and cell wall formation in Spirogyra. Cells 

 about to conjugate are always rich in tannins, which diminish as 

 conjugation proceeds. During the formation of the cell walls after 

 nuclear division, the tannin also diminishes. If the tannin was 

 precipitated during the early stages of cell division, nuclear division 

 continued normally but the cell wall was not formed. Such func- 

 tions for tannin in higher plants have not been proposed. 



6. Tannins play an important role in cork formation, which 

 explains their presence in bark. That such is the case may be 

 concluded from three lines of evidence, (a) They occur abun- 

 dantly in corky regions, (b) If a stream of carbon dioxide is passed 

 through a mixture of formaldehyde and tannic acid, condensation 

 products are formed which have many of the characteristics of 

 cork, (c) Cork may be decomposed, yielding among other things 

 gallic and tannic acid. The chain of evidence is thus seen to be 

 quite complete. The formation of cork is important to the plant 

 because it protects it against water loss and also against invasion 

 by fungi after wounding. When trees are cut or pruned, a corky 

 layer, which prevents infection, is soon formed over the cut surface. 



7. The tannins may aid in some cases to maintain the turgor of 

 the cell, the same as all substances in solution. Their presence in 

 the vicinity of nectaries may be explained on this basis; and in 

 the guard cells they may play a part in the opening and closing 

 of the stomata. In evergreen leaves they may thus help to prevent 

 undue evaporation in the winter and to retain the turgor of the 

 cells in this time of physiological drought. 



8. Finally, the tannins have an antiseptic value. Although 

 fungi may use tannins as food in weak solutions, the concentration 

 in the bark and leaves is generally high enough to retard or inhibit 

 completely the growth of injurious fungi, especially parasites, 

 which are more sensitive to tannins than saprophytes. 



Pigments. — In addition to the chlorophyll and the carotinoids 

 associated with it, other pigments are found in plants giving the 

 characteristic colors of flowers, fruits, etc. These may be classified 

 on the basis of their chemical structure as the anthocyans, antho- 

 xanthins, and protein pigments. 



The anthocyans are the soluble pigments in the cell sap to which 



