274 VEGETABLE PHYSIOLOGY 



spring or in cold weather, the anthocyan may appear some- 

 what irregularly in the leaves, but it is found mainly along 

 the veins and on the leaf-stalk. It is probable that the 

 colour is produced by the oxidation of an antecedent colour- 

 less aromatic substance. 



The function of anthocyan is not well understood. 

 Many facts point to the probability that it aids in the 

 transformation of starch into sugar in the leaves in which 

 it occurs, rendering translocation more rapid. Engelmann 

 has showed that it absorbs the rays of light complementary 

 to those absorbed by chlorophyll. It has been found 

 that the red rays of the solar spectrum which it allows to 

 pass are instrumental in the formation of leaf -diastase from 

 its antecedent zymogen. The pigment, while allowing 

 these red rays to pass into the leaf, acts as a screen preventing 

 the passage of the violet ones, which have a very destructive 

 effect upon this enzyme. 



Other views as to the significance of this pigment have 

 been advanced. It has been suggested that it effects a 

 conversion of light rays into heating ones, so facilitating 

 the metabolic processes of the plant. Another hypothesis 

 regards it as a protective screen to the chloroplasts and to 

 the protoplasm, preserving them from injury from too 

 intense light. Neither of these views can, however, be 

 regarded as entirely satisfactory. 



In many cases it acts beneficially by absorbing the dark 

 heat rays and so facilitating transpiration as well as general 

 metabolism. 



Anthocyan appears to be a derivative of tannin, an 

 aromatic substance which is very widely distributed in 

 the vegetable organism. This substance has not generally 

 been included among the secretions of plants, but rather 

 as a bye-product of metabolism. It is not impossible that 

 it may in some cases be a definite secretion for some 

 particular purpose. 



The distinction between definite processes of. secretion 

 and such reactions as lead to the formation of the so-called 



