4o8 SCIENCE PROGRESS. 



plants, and that the pigment of this sap arises from a colour 

 less highly refractive substance giving tannin reactions. 

 Kraus and Detmer (8) also hold the view that the reds 

 appearing in the autumnal coloration of leaves, in so far as 

 they are due to substances dissolved in cell sap, also origi- 

 nate from tannins. Pfeffer, (9) on the contrary, states that 

 in no case has this origin been actually proved. Kutscher 

 (10) and more recently Pick (11) claim to have demonstrated 

 the transition from tannin-reacting substance to red pigment 

 in many flowering plants. The latter author describes a 

 large number of cases of developing shoots whose cells, 

 which a little later are filled with red sap, abound in a 

 colourless, refractive, tannin-reacting substance, the dis- 

 appearance of which is followed />a?^i passu by the advent 

 of the red pigment. 



It has long been known that external conditions exert 

 an important influence on the production of the red colour- 

 ing matter. Senebier {12) observed that, in fruits partially 

 covered by tin foil, the red is only developed in the areas ex- 

 posed to the sun. Von Mohl notes in his Vermischte 

 Schriften that the red colour is called forth by illumination, 

 and that a low temperature favours its appearance. Landel 

 (13) has investigated the influence of light more completely, 

 and finds that variations of the intensity of the sun's rays 

 act uniformly, but in varying degrees, on the production of 

 red colours in stems and other parts. 



Sachs' (14) classical experiments, by which he found that 

 the colours of flowers are developed in the dark — albeit, ac- 

 cording to Askenasy (15), more feebly than in the light — 

 and also the facts that red sap may occur in dark grown and 

 underground parts, indicate, unless we suppose here a pro- 

 longed after-effect of light, that external conditions influence 

 the amount of formation rather than the actual formation of 

 the pigment. 



It may be well here to enumerate the chief localities in 

 which anthocyan may occur, in order that the intricacy of 

 the problem of its physiological significance may be borne 

 fully in mind. The following is largely based on Frank's 

 classification : — 



