256 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the air ; so, too, sections of stems and roots, of leaves, and fleshy 

 fruits which acquire a brown colour on exposure. Little has been 

 ascertained in regard to the physiology of these changes. They 

 obviously depend upon the oxidation of certain constituents ; this is 

 seen, for instance, on -exposing grated potatoes to the air, when the 

 uppermost layer assumes a brown colour, which by frequent turning 

 over of the mass may be communicated throughout. The same is 

 seen in the case of the expressed juice of the potato. Putrefaction or 

 fermentation, and reducing agents, such as sulphurous or hydro- 

 sulphuric acid, decolorize these fluids. The juice of the white sugar- 

 beet is even more sensitive, becoming on exposure to the air immedi- 

 ately of a dirty wine-red colour, then violet, brown, and finally 

 almost black. These facts indicate the presence in plants of easily 

 oxidizable bodies, and inasmuch as the products of their oxidation do 

 not occur within the uninjured cells, it follows that there is either no 

 free oxygen in the latter, or that these oxidizable substances are 

 accompanied by other reducing substances, which hinder their oxida- 

 tion, or again, that in the protoplasm oxidation afibrds other uncoloured 

 products. Upon which of these three factors the colourless state of 

 the protoplasm and cell-sap of living plants depends is not yet decided. 



In the study of oxidation processes in the living plant-cell, 

 an important question presents itself, as to whether substances occur 

 in the cell, which at ordinary temperatures unite with atmospheric 

 oxygen without the essential co-operation in this process of the living 

 protoplasm. Difficult as the problem is, the isolation and determina- 

 tion of the constitution of these easily oxidizable substances forms an 

 indispensable preliminary step. It may be conjectured that they 

 belong to the aromatic series. In this connection the numerous 

 hydroxy benzene derivatives claim attention, of which many are 

 known to be easily oxidizable. Pyrogallol in alkaline solutions 

 greedily absorbs oxygen and becomes decomposed into carbonic 

 anhydride, acetic acid, and a brown body of unknown nature. The 

 dihydroxybenzenes (catechol, resorcinol, and quinol) are easily oxidi- 

 zable bodies, and their methyl derivative, orcinol, is coloured red 

 by the air. As regards derivatives of the anthraquinone series, 

 there is the change of indigo white into indigo blue, and the behav- 

 iour of Boletus luridus, the colourless section of which becomes at 

 once blue on exposure to the air. Lastly, there is a series of complex 

 plant-constituents, undoubtedly benzene derivatives, although their 

 constitution has not yet been ascertained, which exhibit many 

 analogies to the discoloration of plant-juices. Of these brazilin 

 may be named, the colourless aqueous solution of which becomes 

 first yellow, then reddish yellow in the air. 



J. Eeinke, in his endeavours to isolate the easily oxidizable 

 constituents of the sugar-beet and potato to which the discoloration 

 of their respective fluids is attributable, succeeded in the first 

 instance in isolating from the beet-root a chromogen which on 

 exposure to the air acquired a red colour. This substance he has 

 accordingly designated Bhodogen. The product of its oxidation he 

 terms heet-red, and he notes certain remarkable analogies between the 



