268 VEGETABLE PHYSIOLOGY 
what irregularly in the leaves, but it is mainly found along 
the veins and on the leaf-stalk. 
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. 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 
sereen 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 
bye-products of metabolism is not at all well defined. In 
many cases substances are included in the latter category 
because nothing is known as to their function, and the 
classification can therefore be regarded only as provisional. 
In many cases it cannot yet be determined whether 
