226 PIGMENTS 



With regard to the substances which immediately precede 

 chlorophyll, and from which chlorophyll is formed, nothing 

 definite is known. 



The chief physical properties of chlorophyll are well known ; 

 its solubility in various reagents, such as alcohol, acetone, ether, 

 and chloroform, its fluorescence, and its absorption spectrum. 

 In this connexion it should be noted that the positions of the 

 absorption bands in the spectrum of the fresh green leaf do not 

 correspond exactly to those given by a solution of chlorophyll. 

 The explanation of this is uncertain ; it is supposed to be due 

 either to the fact that the chlorophyll occurs in the chloroplast 

 dissolved in a solvent which has a high dispersive power, or 

 to the fact that the chlorophyll forms a precipitate as it were 

 within the chloroplast. This last opinion is supported by 

 Iwanowski,* who, by precipitating chlorophyll from an alco- 

 holic solution by the addition of water and a little magnesium 

 sulphate, produced a spectrum, the absorption bands of which 

 corresponded very closely in position to those of a green leaf. 

 He concludes that the positions of the bands in the latter case 

 are due to the displacement of the true absorption spectrum 

 of chlorophyll by the superposition upon it of the spectrum 

 of the light reflected from the chloroplasts, the amount of 

 displacement depending upon the size of the plastids. 



Mention must here be made of Tswett'sf observations. 

 As already mentioned, the pigments contained in the chlo- 

 roplasts may be dissolved out by many solvents, but the 

 character of the solution depends upon the nature of the 

 solvent and upon the conditions of extraction. For instance, 

 leaves extracted with hot petroleum ether yield a green 

 solution, whereas if the operation be carried out at a low 

 temperature the extract will contain relatively much larger 

 quantities of carotin. Alcohol, on the other hand, dissolves 

 all the pigments equally. Tswett found that if the green 

 extract be soaked up in filter paper which is then dried in 

 vacuo, the paper will part with its colouring matters in exactly 

 the same manner with different solvents at various tempera- 

 tures as do green leaves. 



* Iwanowski: " Ber. deut. bot. Gesells.," 1907, 25, 416. 

 tTswett: "Ber. deut. bot. Gesells.," 1906, 24, 316, 384, and "Ber. deut. 

 chem. Gesells.," 1908, 41, 1352, 



