i6 FORMATION IN PLANTS II 



b. The Function of Carotcnoids in the Visual Process 



Some carotenoids appear to play a part in the process of visional. Although 

 there has been a good deal of theoretical speculation as well as experimental 

 work in this connection, this field of research is still in an early stage of develop- 

 ment and it is not yet possible to draw any definite conclusions. The following 

 observations are merely meant to provide a brief summary. 



The suggestion that visual purple is a carotenoid was first made by Boll^^ 

 about 70 years ago. For some time afterwards this question was not further 

 investigated, presumably because science as a whole had not yet sufficiently 

 progressed. In 1923, Blegvad^^ ^nd in 1924, Bloch^* showed that vitamin A 

 deficiency results in xerophthalmia, a sclerotic inflammation of the eyes. In 

 1925, Fridericia and Holm'^^ found that night blindness (hemeralopia) is a 

 direct consequence of vitamin A deficiency which must be related to the 

 incapacity of forming visual purple in the retinal rods. 



After the relationships between certain processes in the eye and vitamin A 

 or the carotenoids had thus been demonstrated, several investigations were 

 begun with the view to isolating the pigments concerned from the eye and to 

 identifying them. However, this task proved to be a very difficult one. The eyes of 

 animals only contain very small quantities of pigments and, furthermore, these 

 substances are unstable and to some extent sensitive to light. The first success 

 was achieved by von Euler and Adler*'^, who isolated compounds of a 

 carotenoid nature from the pigmented epithehal layer of bull and fish eyes. 

 Shortly afterwards, Wald*'' proved the presence of vitamin A in the retina 

 of bulls and frogs. Later, Wald*^ sfiowed that the retina of frogs contained 

 xanthophyll ester and recently Wald and Zussman'*^ found strongly coloured 

 oily discs in the pupils of many birds and reptiles. In the chicken these discs 

 are red, golden and yellow-green and from them three carotenoids can be 

 isolated, one of which appears to be identical with esterified astacene, while the 

 other two are of as yet unknown constitution. Honigmann^" has reported to 

 have found a photolabile pigment in the retina of young chicken. This pigment 

 appears to be of a carotenoid nature and to be similar to, but not identical 

 with, rhodopsin and porphyropsin (cf. below). 



The investigations just described show that carotenoids or very similar 

 pigments occur in the eyes of numerous animals. Their function has not yet 

 been clarified, but it seems possible that they act as light filters which ensure 

 that only rays of certain wavelengths enter the inner part of the eye and reach 

 the photolabile substance. The question then arises as to the nature of the 

 photolabile substance. Wald^^ showed that after a brief illumination of the 

 eyes of frogs or mammals, a new pigment with different spectral properties 

 is formed from the rhodopsin (visual purple). He suggested the name of 

 References p. ly-ig. 



