338 PIGMENTS 



antipyrine closely resembles the precipitates given by the 

 same reagents with tannin. Plants in which this particular 

 pigment does not occur are free from tannin. 



The appearance of anthocyanin is closely related to the 

 sugar-content of the tissues in which it occurs. 



Ewart * has pointed out that in the case of Elodea cana- 

 densis and other aquatic plants the red dye will appear pro- 

 vided the plants be immersed in a weak solution of sugar and 

 exposed to strong sunlight at ordinary temperatures, whilst 

 the red colour does not appear if the plants be grown in water 

 or in diffuse daylight. 



These experiments of Ewart were much extended by Over- 

 ton,t who used Hydrocharis and other plants. He found that, 

 in addition to the presence of sugar, light and temperature 

 were important factors. If the temperature be low, but above 

 freezing-point, then the formation of the red pigment will be 

 promoted, which accounts for the red colour prevalent in 

 alpine plants, since under their conditions of existence sugar 

 tends to accumulate rather than starch. This also is true for 

 arctic plants in which, according to the observations of Wulff, J 

 the leaves are very frequently sugar leaves, and are commonly 

 characterized by the presence of anthocyanin. 



In the case of Hydrocharis grown in water culture, Overton 

 found that when the temperature and the intensity of light 

 were so balanced that no colour was formed, the addition of 

 2 per cent of invert sugar caused its appearance in three days, 

 not only in the young leaves but also in the old ones. 



Other aquatic plants behave similarly, but in the case of 

 cut shoots of lilies the red pigment only developed provided 

 sugar were added to the culture solution. 



Further experiments showed that the red colour is not 

 formed in those plants, in which the pigment was restricted to 

 the epidermis, when cultivated in sugar solution. Success only 

 resulted in those cases where the colouring matter occurred in 

 the mesophyll. 



* Ewart : " Journ. Linn. Soc. Lond., Bot.," 1895-7, 3'» 445 '• " Ann. 

 Bot.," 1897, II, 461. 



t Overton : " Nature," 1899, 59, 296 ; " Jahrb. Wiss. Bot.," 1899, 33. 

 I Wulff : " Botanische Beobachtungen aus Spitzbergen," Lund., 1902. 



