viii] THE SIGNIFICANCE OF ANTHOCYANINS 137 



and that green leaves also give off more vapour in sunlight and dry air 

 than red ones. 



The development of anthocyanin in young leaves and many tropical 

 shade-loving plants is not considered by Ewart to be an adaptation 

 for promoting transpiration. Its purpose, on the other hand, is to 

 guard against assimilatory inhibition to which shade plants may be 

 especially liable if accidentally exposed to intense light. He points out 

 that in trees and shrubs with young red foliage, individuals growing 

 in shady positions are less red than those in the sun, whereas, according 

 to Stahl, it should be the reverse. Further, though it is true that 

 certain plants growing in the shade form a considerable amount of 

 pigment, yet of these, again, individuals growing in the more exposed 

 positions have more colour than those in the deepest shade. Ewart 

 infers that such plants are extremely sensitive to, and are injured by, 

 light of marked intensity. He further points out that in many plants 

 which have anthocyanin on the under surfaces, the ventral surface of 

 the young leaves is exposed to light ; in one variety of Musa, for instance, 

 anthocyanin develops on the under surface when the young leaves are 

 vertical and rolled up, but it disappears as the leaf unrolls and expands 

 horizontally. In Uncaria sclerophylla the young leaves are so folded 

 that the ventral surface is most exposed and this develops anthocyanin 

 which disappears as the leaf expands. Several other instances are 

 also quoted, one of the most interesting being Mimosa pudica, in which 

 red coloration is developed on the parts of the under surfaces of the 

 leaflets which are exposed when folded. Ewart, however, does admit 

 that, in the case of certain Begonias which have horizontally expanded 

 leaves with red under surfaces, Stahl's view of the function of antho- 

 cyanin may be the correct, one, and there are other cases where it is 

 difficult, he says, to find any use at all for the pigment. 



That the distribution of anthocyanin does not conform to Stahl's 

 hypothesis is emphasised in the following passages from E wart's paper. 

 "If the primary function of the red dye in the tropics were to increase 

 the amount of transpiration, then it would be only natural to expect 

 that it would be formed in greatest abundance where the temperature is 

 lower and the air more nearly saturated with water-vapour. The very 

 opposite is however the case. Thus at the foot and sides of the volcanic 

 mountain of Gedeh, and in the valleys around, very many plants 

 have a reddish colour, especially in the young leaves. As one ascends 

 this becomes less marked, until at Tjibodas and in the forests above it 

 (4500 ft. to 6500 ft.), the number of plants showing a red colouration, 



