226 DB, EDWARD SCHUNCK ON INDIGO-BLUE. 



composed, its watery solution od standing some time, even 

 at the ordinary temperature, depositing indigo-blue, I 

 think it will not be difficult to explain all the phenomena 

 hitherto observed by myself and others. 



On taking a plant of Polygonum tinctorium and making 

 incisions with a penknife in the leaves between the main 

 vessels, or crushing the soft parts of the leaves here and 

 there with an agate pestle, then, after a short time, 

 plunging the whole plant into boiling alcohol to remove 

 the chlorophyll, it will be found that those parts of the 

 leaf which have not been injured become white or retain 

 only a faint yellow tinge, while those parts that have been 

 cut, crushed, or otherwise injured, show a blue colour, the 

 coloration extending for some distance inwards from the 

 place where the lesion occurred, the most intense colour 

 being at the edge of the cut or bruise. So, too, in the 

 living plant, when some injury accidentally occurs to a 

 leaf, the part injured will appear blue. Nothing can be 

 more natural than to suppose that in these cases the blue 

 colouring-matter is formed by the action of the air, i. e. by 

 the oxidation of some substance which escapes from the 

 cells in consequence of organic lesion, just as the surface of 

 a freshly cut apple or pear becomes brown on exposure. 



If a plant of Polygonum tinctorium be immersed in water, 

 and the water be frozen by surrounding the vessel con- 

 taining it with a freezing -mixture, it will be found, after 

 complete thawing, that the leaves or parts of leaves which 

 have been thoroughly frozen appear of a dark colour and 

 are quite flaccid ; and if the plant be then immersed in 

 boiling alcohol so as to dissolve the chlorophyll and other 

 matters, those very parts show afterwards an intense blue 

 colour, while those portions which had remained unfrozen 

 appear almost colourless. This experiment, which had 

 already been made by Joly, was considered by him to prove 



