68 DISTRIBUTION IN NATURE IX 



of tie.'o main pigments, xanthophyll itself and xanthophyll epoxide, was 

 neglected. 



Literature references regarding the occurrence of carotenoids in green 

 plants, especially green leaves, are so numerous that no attempt will be made 

 to summarise them. Only the most important investigations in this field can, 

 therefore, be mentioned^. 



Non-green leaves 



Numerous investigations have been carried out concerning the pigments 

 which are responsible for the yellow colour of etiolated leaves^. However, 

 most of these studies belong to the early period of carotenoid research. More 

 recent investigations^ show that besides water-soluble pigments, carotenoids, 

 especially xanthophyll, are present in etiolated leaves. 



Our knowledge concerning the pigments of yellow leaves (aurea varieties) 

 is rather deficient. The last investigation of these pigments was made by 

 WiLLSTATTER and Stoll^ and showed the presence of carotenoids and of 

 water-soluble pigments. As the quantity of carotenoids was very small, however 

 it appears doubtful whether they are, in fact, responsible for the yellow colour 

 of the leaves. 



As a result of further studies, our knowledge regarding the pigments of yellow 

 attt'umn leaves is somewhat better, and it has been possible to recognise different 

 stages in the rather complicated pigment metabolism. The carotenoids 

 (possibly also the anthocyanins'^) remain in the leaves after the degradation 

 of the chlorophyll in the autumn and produce the well-known, striking colour- 

 ations. Gradually the polyene pigments are ako degraded and carotene seems 

 to be decomposed more rapidly than xa:nthophyll. In the last phases of the 

 necrobiosis, the well-known brown pigments are produced, to which fallen 

 leaves owe their brown colouration. These brown pigments appear to be 

 oxidation and decomposition products; they are soluble in water and produce 

 dark yellow to brown colourations with alkalis. 



An important question is whether only carotenoids already present in the 

 green leaf are involved in the course of necrobiosis or whether new pigments are 

 formed. According to Willstatter and Stoll^, the total quantity of pigments 

 in autumnal leaves is about equal to that in green leaves. These findings are 

 in agreement with the investigations of Goerrig**. It is interesting that whereas 

 the amount of carotene decreases in the course of necrobiosis, the quantity of 

 xanthoph^/ll is said to increase^". The yellow colouration of dead leaves is due, 

 according to Tswett^^ to epiphasic carotenoid pigments which, in contrast to 

 carotene, can be adsorbed on calcium carbonate from petroleum ether solution. 

 Tswett named these carotenoids "autumn xanthophylls". According to Kuhn 

 References p. io8. 



