86 LIGHT, VEGETATION AND CHLOROPHYLL 



diminishes. However, investigations do not seem to have 

 been made to determine whether certain regions of the 

 spectrum are more particularly active in stimulating this 

 production. 



The formation of anthocyanins has been studied in more 

 detail. Some plants effect their synthesis in darkness, but it 

 seems certain that hght, if not always indispensable, is at 

 least a very active supporting factor. 



The reddening of Macintosh apples can be produced, 

 according to experiments made in natural light or in artificial 

 Ught coloured by optical filters, under the action of all the 

 visible radiations of wave-length shorter than 6,000 A. But 

 the blue and violet rays are the most efficacious ; the optimum 

 is situated in the neighbourhood of 4,100 A, i.e., in the 

 extreme violet. 



Similar results have been obtained in other cases; thus 

 flowers, whose colour is often partly attributable to antho- 

 cyanins, lose their brilHant colouring proportionately as the 

 rays of short wave-length are suppressed from the illumination 

 received during their formation. 



Red lettuce, red-leafed beetroots and begonias tend to 

 lose their colour in the same conditions and redden again 

 when they are re-exposed to complete solar radiation. This 

 is one of the rare examples which apparently demonstrates the 

 action of the ultra-violet near the visible; this radiation of 

 short wave-length is useful for the formation of anthocyanins. 



These few examples prove that the action of light is not 

 confined to photosynthesis. It extends more or less directly 

 to all the chemical processes of the plant, both for the 

 utilization of the mineral substances in the soil and for the 

 elaboration of the infinitely varied organic compounds whose 

 mysterious operation enables the plant to five. 



In the field of organic chemistry, we have mentioned some 

 experiments on the formation of anthocyanins. In the chapter 

 on phototropism we shall discuss other studies on auxins — • 

 substances whose chemical nature and composition are now 

 fairly well known and whose striking effects on growth have 



