CAROTINOIDS IN THE PHANEROGAMS 63 



Kraus separation. In the author's opinion the pigments might better 

 have been called autumn carotins, for the behavior of the carotinoids 

 in the Kraus separation unquestionably depends primarily upon chemi- 

 cal composition, as Tswett (1911b) himself has pointed out, while 

 their relation to adsorbents is largely a colloidal phenomenon, as 

 already explained, and is not necessarily related to chemical com- 

 position. 



Tswett also made a careful study of the question of the alleged 

 (Sorby, 1871a, b, Kraus, 1872, Kohl, 1902) presence of water-soluble 

 yellow pigments in yellow autumn foliage, with most convincing re- 

 sults. He found that hot water decoctions of autumn leaves, obtained 

 with the exclusions of as much air as possible, were scarcely colored 

 at all, and that similar decoctions with dilute acetic acid were pale 

 yellow, while extractions with alkaline water were golden yellow to 

 brown or reddish brown. The more golden colors were destroyed by 

 acid, but the deeper ones persisted. Decoctions with water slightly 

 alkaline with carbonates were likewise golden yellow, and the extracts 

 acted towards acids, alkalies and air like the extracts with distilled 

 water. Tswett obtained further proof of the presence of colorless 

 water- and alcohol-soluble chromogens 7 in autumn leaves (he regards 

 the chromogens as present in green leaves also), which give golden 

 yellow salts with alkalies and oxidize to a dark brown color, by shak- 

 ing the diluted alcoholic extract of the yellow tulip and maple leaves 

 with chloroform. This removed the color, leaving a colorless hydro- 

 alcoholic layer which acted towards alkalies like the distilled water 

 decoctions from the leaves. 



Tswett holds, apparently rightly, that colored salts of the above 

 mentioned chromogens may at times play a part in the coloration of 

 autumn leaves during the necrobiotic period. It would appear that 

 the definiteness of the relation between this period and the true post- 

 mortal period of the leaf is the important factor in determining this 

 type of coloration for Tswett believes that the brown oxidation prod- 

 ucts of the yellow alkali salts of the colorless chromogen of the leaves 

 play the chief role in the postmortal coloration of leaves, a reaction 

 no doubt accelerated by oxidizing enzymes. 



Miss Goerrig's (1917) recent study of yellow autumn pigments was 

 an attempt to determine quantitatively the relation of the carotinoids 

 in the green and autumn leaf, just before and during the necrobiotic 



7 These substances are probably flavones which are characterized especially by their 

 yellow color reaction with alkalies. 



