254 CAROTINOIDS AND RELATED PIGMENTS 



colorimeter should serve the purpose just as well. The writer has 

 found the Kober colorimeter very satisfactory, using daylight as the 

 source of illumination and the black glass cups with the colorless, op- 

 tical glass bottoms for holding the solutions. 



Calling h c the height of the unknown solution required to match the 

 color of 100 mm. of standard carotin solution and h x the height of the 

 unknown solution required to match the color of 100 mm. of stand- 

 ard xanthophyll solution the amount of carotin or xanthophyll in 1 

 kg. of fresh leaves can be calculated from the amount obtained from 

 20 grams by the method of Willstatter and Stoll by means of the 

 following formulae: 



Carotin equals 50 x 0.00536 x -^x and 



2i n c 



Xanthophyll equals 50 x 0.00568 x^- x - 



-' n x 



If the standard potassium dichromate solutions have been used in 

 place of the pure carotin and xanthophyll, the same formulae are 

 used because the dichromate is used at a depth of color corresponding 

 to 100 mm. of the carotinoid solutions. These need not necessarily be 

 set at the values corresponding to 100 mm. of the carotinoid solutions, 

 but, if desired, can be set at the values corresponding to 50 or 25 mm. 

 of the standard carotinoid solutions. In fact, the writer believes that 

 more accurate determinations are secured by averaging the results 

 obtained with the standards set at the equivalents of 100, 50 and 25 

 mm. of pure carotinoid solutions. 



Results by Willstatter and Stall's method. Table 19 shows some 

 of the results obtained by Willstatter and Stoll using their own 

 method. The data are averages of duplicate determinations reported 

 in full by these investigators and show the difference between the 

 carotin and xanthophyll content of leaves exposed to the light and 

 those which are heavily shaded, both being obtained from the same 

 plant. The fresh leaves which were in the shadow were in some cases 

 appreciably lower in carotinoids than the leaves exposed to the light, 

 but this difference appears to be due, in part, to a higher moisture 

 content in the fresh shaded leaves. 



The quantitative results of Willstatter and Stoll show a very dif- 

 ferent proportion between carotin and xanthophylls than was ob- 

 tained by Monteverde and Lubimenko, which can not be due entirely 

 to the fact that different plants were used in the two studies. The 



