254 PIGMENTS 



the pigment in the petal coincides exactly with that of a per- 

 oxidase. In some varieties of this same plant there is no 

 need to add hydrogen peroxide, the characteristic reaction 

 being obtained by the oxidase reagent alone. 



Properties. 



The chief physical property of anthocyanin is its absorp- 

 tion spectrum. Engelmann found that it is complementary to 

 that of chlorophyll, the main absorption bands being in the 

 yellow and yellow-green, with minor ones in the blue end 

 of the spectrum. 



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,f who used Hyuh-ocharis and other plants. He found that, 

 in addition to the presence of sugar, light and temperature 



* Ewart : "Journ. Linn. Soc, Lond., Bot.," 1S95-7, 31, 445; "Ann. Bot.," 

 1897, II, 461. 



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



