252 PIGMENTS 



Questions relating to the energy relationship between this 

 and other pigments and chlorophyll are outside the scope of 

 the present consideration ; it may be mentioned, however, 

 that it has been stated that leaves containing anthocyanin 

 have relatively less chlorophyll than those which have no red 

 pigment. 



According to Pick and others, anthocyanin is commonly 

 associated with tannins, for a red sap is characteristic of 

 tannin-containing plants, and the precipitate appearing in the 

 palisade cells of Hydrocharis on treatment with caffeine and 

 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,! 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,| 

 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. 



* Ewart: " Journ. Linn. Soc., Lond., Bot.," 1895-7, 31, 445 ; "Ann. Bot., r 

 (897, ii, 461. 



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

 ^Wulff; " Botanische Beobachtungen aus Spitzbergen," Lund., 1902, 



