vm] THE SIGNIFICANCE OF ANTHOCYANINS 131 



red. This might, therefore, be expected to oppose a more solid resistance 

 to the heat-rays than the thin immature cuticle of the red leaf. That 

 such is not the case must be regarded as due to the fact that the different 

 colouring-matters have different powers of reflection and absorption 

 of heat, and that this difference is of such a nature that the red colouring- 

 matter more effectually cuts off heat-rays from the body of the leaf 

 than does the green. That is to say, the red colouring-matter 

 acts as a screen by which the thermal effects of the sun's rays are 

 moderated." In conclusion Keeble says: "In addition, then, to its 

 value as a screen, the red or reddish coloured saps of such trees as 

 Amherstia nobilis have the capability of affording to the young leaf, 

 if necessary, a protection against too great heating effects of the rays 

 of a tropical sun." 



Later, Keeble's conclusions have been adversely criticised by both 

 Stahl (405) and Smith (420) : the latter remarks, alluding to Keeble : 

 "From his somewhat rough experiments he drew the novel conclusion, 

 which... is exactly the opposite of the conclusion logically to be drawn 

 from his observations, that the red colour is a protection against too 

 great heating up of the leaf." 



In 1895 a long paper appeared by Ewart (401) on ' Assimilatory 

 Inhibition in Plants.' Ewart maintains that leaves, especially of shade 

 plants, when exposed for a long time to strong illumination, suffer from 

 inhibition of photosynthetic power. He considers the development 

 of authocyanin, on exposure to light, to be an adaptation for protection 

 against this light rigor, rather than a protection against the destruction 

 of chlorophyll. We shall return later to further criticisms by 

 Ewart. 



We will now pass on to another suggestion as to the uses of antho- 

 cyanin, though it was the first in point of time. It is that made, in 

 1883, by Pick (391), who maintained that red light increases the rate 

 of hydrolysis and trauslocation of starch, while it allows photosynthesis 

 to go on unhindered. Hence its frequent occurrence in sterns and 

 petioles ; also in young leaves where there is active metabolism and in 

 autumnal leaves from which there is a migration of substances which 

 may be useful for storage. Pick brings forward certain evidence, 

 namely that in red leaves the palisade parenchyma contains less starch 

 than the spongy, on account of the increased translocation under the 

 influence of the red light. He, moreover, gives the results of an experi- 

 ment with a view to proving the same point. Of the lobes of a large 

 leaf of Ricinus, one was illuminated by light coming through orange 



92 



