544 TRANSACTIONS AND PROCEEDINGS OF THE [Sess. lx. 



of anthocyan {i.e. strong sunlight and diminished tempera- 

 ture), make it appear very doubtful whether we have not 

 to do with indirect, rather than direct environmental effects; 

 while the hypothesis of universal adaptation is negatived 

 by such facts as that fruits, e.g. apples, redden only on the 

 side exposed to the sun. We shall return to this subject 

 after considering the origin of anthocyan ; but it may be 

 noted meantime that the evidence points to the conclusion 

 that anthocyan arises primarily as a consequence of par- 

 ticular states of metabolism, and that any function which 

 it may possess is secondary. 



As to the origin of the anthocyan series, there is a 

 general consensus of opinion that it arises from tannin or 

 a related substance. Gautier (4), who analysed the 

 colouring matters of the vine, calls these tannic acids. 

 Wigand (19) says that the anthocyan colours arise from 

 a colourless chromogen which is directly derived from 

 tannin, and that the disappearance of the colours is 

 associated with the reformation of tannin. Pick (13) 

 confirms this, adding the interesting remark that the 

 colours are notably absent from plants with little or no 

 tannin. Hansen, as quoted by Krukenberg (9), found a 

 colourless chromogen in the leaves of the aloe, which took 

 up water and oxygen, and became converted into a pigment 

 resembling the anthocyan series; but no opinion is ex- 

 pressed as to the nature of the chromogen. An origin 

 from tannin seems, on the whole, to be fairly well estab- 

 lished, and is of great interest, because it connects these 

 pigments with many other plant pigments. Thus the 

 phlobaphenes, the colouring matters of bark and of many 

 injured tissues, e.g. cut apples, are almost certainly produced 

 by the direct oxidation of tannins. The yellow or brown 

 colours of some autumnal leaves, e.g. oak and beech, are 

 produced in the same way. See Sorby (18). Again, 

 according to Vines, the colouring matters of woods, such as 

 ha^matoxylin, santalin, brasilin, etc., are probably derived 

 from tannins. Further, since most tannins are giucosides, 

 it seems not unnatural to associate with the chromogens 

 mentioned above, those well-known giucosides which, out- 

 side the organism, act the part of chromogens. Thus 

 there is reason to suppose that the process by which 



