136 THE SIGNIFICANCE OF ANTHOCYANINS [CH. 



One item of special interest he also notes, namely, that red blotches 

 on leaves give off less water (tested by cobalt paper method) than 

 green ; this he found to be due to the fact that the red parts contained 

 fewer stomata. Stahl, however,, seems to think this compatible with 

 his views, as too greatly increased transpiration might otherwise occur. 

 Branches of the red-leaved Beech and Hazel he finds to transpire more 

 strongly than branches of the ordinary species when both were placed 

 in the shade, but in the sun and a dry atmosphere, the opposite was the 

 case. 



As regards the coloration of young leaves, Stahl considers that 

 reddening in temperate regions assists metabolic processes at low 

 temperatures. In the tropics, on the other hand, the most intense 

 colouring, he says, frequently occurs in the densest shade forests, and is 

 then an adaptation for promoting transpiration by heating up of the leaf ; 

 a view completely opposed to that held by Keeble (403). 



Ewart's (406) work and criticisms are so much connected with both 

 the screen hypothesis and that of Stahl, that it will be more convenient 

 to consider the arguments in the order which he follows in his papers. 

 He first concludes, from a number of observations and experiments, 

 that strong insolation may bring about inhibition of photosynthesis, 

 and hence too bright light may be injurious in this way rather than 

 in the destruction of chlorophyll, and this is especially so in the case of 

 shade plants. He is of the opinion, moreover, that anthocyanin does 

 act as a protection in such cases, for he apparently trusts Pringsheim's 

 observations, in preference to Reinke's, that concentrated blue or green 

 sunlight will kill and bleach chlorophyll grains in five minutes, whereas 

 concentrated red rays (the heat rays being eliminated) will only 

 cause bleaching after twenty minutes. Anthocyanin absorbs 70-90 % 

 of the green and 50 % of the blue, that is just those rays which are most 

 harmful to chloroplast activity, while rejecting those useful for photo- 

 synthesis. If, according to Stahl's view, its function were tha.t of a 

 heat absorber it would, instead of absorbing the green and yellow rays 

 (of which the heating effect is comparatively slight), show a marked 

 absorption of the dark heat rays, and this is not the case. But he 

 admits that Stahl has obtained a rise in temperature of one or two 

 degrees in red leaves by exposure to dark heat rays. This higher tem- 

 perature, he considers, to be a disadvantage, and since the red leaves 

 might thus be liable to transpire too much, excess of transpiration is 

 prevented by development of fewer stomata. Stahl has, as we have 

 seen, shown that red leaves, or areas, have fewer stomata than green, 



