vi] INFLUENCING THE FORMATION OF ANTHOCYANINS 87 



may greatly affect the sugar contents of the tissues, and hence may 

 in this way cause the reddening, apart from any more direct effect. 



Overton made some observations on the effect of temperature on 

 reddening of Hydrocharis leaves, and found that the higher the tempera- 

 ture the less anthocyanin is formed and vice versa, but obviously in 

 this case it is impossible to eliminate the effect of temperature on the 

 photosynthetic activity of the leaves, and on growth and respiration 

 in general with the resultant employment of synthetic materials. 

 Klebs also (360) gives an account of the effect of temperature on the 

 colour of the flowers of Campanula Trachelium. From cultures at 

 various times of the year in green-houses, etc., kept at different tempera- 

 tures, he obtained a variation in flower-colour from white (in heat) 

 through pale blue to deep blue (in cold). He also observed that Primula 

 sinensis produces tinged flowers in a hot-house and full-coloured flowers 

 in the cold. Klebs is of the opinion that the colour changes induced 

 by changes of temperature are not directly due to the effect of tempera- 

 ture on pigment formation but indirectly to the effect of temperature 

 on metabolism. At high temperatures, growth is so rapid that the 

 substances used in pigment formation are not present in sufficient 

 quantity. 



Effect of light. 



As regards the effect of light on anthocyanin formation, there have 

 been numerous observations, some more or less conflicting. As with 

 temperature, the main question at issue is again, whether light directly 

 affects anthocyanin formation, or whether its influence is only indirect, 

 in so far as it affects photosynthesis and the accumulation of the products 

 of this process, among which are the chromogens from which pigment 

 is formed 1 . 



The effect of the absence of light on the development of pigment 

 in flowers may first be considered. As early as 1799, Senebier (2) 

 noted that the Crocus and Tulip develop coloured flowers in the dark. 

 The same observations were recorded by Marquart (5) in 1835 for 

 Crocus sativus. Later Sachs made definite experiments on various 

 plants either by growing them entirely in the dark, or by enclosing 



1 I am indebted to Dr F. F. Blackman for drawing my attention to a fact which is 

 interesting in this connection, i.e., that the development of chlorophyll may be affected 

 by nutrition. The statement is founded on the observation that certain Algae develop 

 chlorophyll in the dark when provided artificially with protein; when supplied with 

 nitrates under the same conditions, however, the pigment does not appear. 



