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 pomt 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 hay 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 
