iv] CARBON ASSIMILATION 39 



Two modifications (Schryver, 4) can be adopted in applying this test. First, in 

 testing for formaldehyde in pigmented solutions, the following course can be pursued. 

 The reaction mixture, after addition of phenylhydrazine, ferricyanide and hydro- 

 chloric acid, is diluted with water, and ether is added in a separating funnel. The 

 hydrochloride of the chromatogenic base is dissociated and the base is taken up by 

 the ether. The aqueous solution is run off, and on addition of strong hydrochloric 

 acid to the ether, the base passes into the acid as a coloured hydrochloride again. 

 By using a small quantity of acid, the sensitiveness of the test is increased, since the 

 colour is now distributed through a small quantity of liquid only. 



The second modification consists in warming the solution to be tested for 

 a short time with the phenylhydrazine hydrochloride before adding the other 

 reagents. In this way, formaldehyde can also be detected if it should be in a poly- 

 merized form. 



As a control, 10 c.c. of the colloidal solution of chlorophyll should be tested, 

 using both the above modifications. The remainder of the solution should be 

 exposed to sunlight (or the light from either an arc or mercury vapour lamp) in a 

 loosely corked vessel, until it is completely bleached. The bleached solution, on 

 testing, will be found to give a positive test for formaldehyde. 



THE YELLOW PLASTID PIGMENTS. 



These have already been mentioned in connexion with the leaf 

 pigments (pp. 28 and 29). In addition, however, they have a further 

 significance in that they constitute the pigments, located in plastids, 

 of most yellow and orange flowers and fruits. Sometimes also they occur 

 in other organs, i.e. root of Carrot (carotin). 



Carotin, C^H^, is an unsaturated hydrocarbon. It crystallizes in 

 lustrous rhombohedra which are orange-red by transmitted and blue by 

 reflected light. It is readily soluble in chloroform, benzene and carbon 

 bisulphide, but with difficulty in petrol ether and ether. 



One of its most characteristic properties is that it readily undergoes 

 oxidation in air, and becomes bleached. With concentrated sulphuric 

 acid it gives a deep blue colour. 



Xanthophyll, C^H^O^ also forms yellow crystals with a blue 

 lustre. It is soluble in chloroform and ether, but insoluble in petrol 

 ether. It is more soluble than carotin in methyl alcohol. It gives 

 a blue colour with sulphuric acid, and also oxidizes in air with bleaching. 



The separation of the two pigments (see Expt. 21) is based on the 

 fact that in a mixture of petrol ether and methyl alcohol containing a 

 little water, the carotin passes entirely into the petrol ether, whereas the 

 greater part of the xanthophyll remains in the methyl alcohol layer. 



