130 THE SIGNIFICANCE OF ANTHOCYANINS [CH. 



which are most absorbed by chlorophyll and are, at the same time, 

 most injurious. If, however, we accept Pringsheim's results that 

 the green portion of the spectrum is most harmful, then it is possible 

 to consider the question as to whether anthocyanin is a screen 

 or not. 



The matter was again brought up by Kny (397) in 1892. To test 

 the efficiency of anthocyanin as a light screen, Kny placed a solution 

 of chlorophyll behind a parallel-walled glass vessel which was filled in 

 one case with an extract of red, in another with an extract of white, 

 Beet-root. Behind the red extract, the chlorophyll retained its colour 

 much longer than behind the white. This experiment is of little value 

 since chlorophyll in a dull light (such as would be the case behind the 

 red solution) invariably retains its colour much longer than in a bright 

 light. 



Again, in 1894, Wiesner (400) supported the screen theory for the 

 protection of chlorophyll, especially in young leaves. 



In 1895 Keeble (403) published a paper of some significance on the 

 hanging foliage of tropical trees. The characteristic pendent position 

 and red coloration of young leaves of some of these trees has been 

 mentioned in Chapter u. Keeble is of the opinion that, though the 

 development of red pigment is not universal among them, yet it may 

 serve as a protection against strong insolation, not only as a screen 

 but also by protecting the leaf from too great heating. This view is 

 based on an experiment carried out with red and green leaflets of 

 Amherstia nobilis. These were laid side by side in the sun, and the 

 temperature taken by thermometers placed upon the upper surfaces 

 of the leaves; the thermometers were then placed under the leaves 

 and the temperature again taken. His results and conclusions are 

 expressed as follows: "...when young thin red and older tougher 

 green leaves of Amherstia are exposed side by side to the direct rays of 

 the sun, the temperature, as registered by the thermometer, is higher 

 to the extent of 1 C. at the upper surface of the red leaves : conversely, 

 of the temperatures registered at the lower surfaces of the leaves (i.e. 

 behind them), that beneath the green is higher than that beneath the 

 red, by a similar amount. 



"Put in general terms, the surface-layers of the red leaf reflect 

 more heat than those of the green leaf. The green leaf, conversely, 

 absorbs more of the sun's thermal rays than the red. Now, the two 

 sets of leaves differ in two respects : first, in colour ; then in that the 

 green has a thicker, more developed cuticle (and mesophyll) than the 



