282 VEGETABLE PHYSIOLOGY 



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 

 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 is beneficial 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 

 particular substances are formed by the direct decomposi- 

 tion of protoplasm, or by subsequent changes in the primary 

 products of such decomposition. Till quite recently the 

 formation of resin and allied bodies in the resin passages 

 of the Conifers and in many glandular hairs was con- 



