CONCLUSION 



In birds, some work has been carried out in identifying precursors 

 of one or two characteristically avian carotenoids, but of this problem 

 too, only the surface has been scratched. 



The state of knowledge is a little different in plants, for they have 

 the ability to produce carotenoids de novo. In the case of phanerogams, 

 environments which produce healthy plants will also result in high 

 yields of carotenoids. Apart possibly from some work on tomato 

 fruit, there is no evidence which compels assent to any hypothesis 

 concerning the role of micronutrients or of light in carotenoid pro- 

 duction. The evidence concerning possible carotenoid precursors in 

 phanerogams is flimsy ; it is equally so in the cryptograms, even when 

 one considers the fungi in which the absence of chlorophyll must 

 simplify the problem. Work on these organisms is now under way in 

 a number of laboratories. 



With respect to the functions of carotenoids, it will be noted that 

 the ability to absorb visible light is the common factor in all well- 

 established effects. In plants, carotenoids possibly function in photo- 

 synthesis and probably do in photokinesis. 



In fish they are constituents of the xanthophores and thus play a 

 vital part in phototropism ; in many invertebrates the photoreceptor 

 substance in the eyes is almost certainly astaxanthin. Apart from these 

 two fields, knowledge of the participation of carotenoids in the biological 

 processes of the animal kingdom is slight. The mediation of carotenoids 

 in these two functions may be more widespread in the animal world 

 than is usually appreciated, yet there are numerous species, especially 

 marine, which are well provided with carotenoids, but in whose life 

 light plays a very minor role. We have no inkling of any function 

 which can be ascribed to the carotenoids in these animals. 



The argument that the presence of a carotenoid necessarily points 

 to a function is repugnant to many ; but when an animal absorbs 

 ingested carotenoids, alters them in a very specific manner, and stores 

 them preferentially rather than excretes them unchanged, it is surely 

 not irrational to proceed on the assumption that, until a function is 

 apparent, knowledge is incomplete. The author hopes that there will 

 emerge a comprehensive theory of carotenoid function embracing the 

 whole animal and plant kingdoms. The absence of carotenoids from 

 some species (especially mammals) should not be taken to indicate that 

 such a general hypothesis is unattainable. The absence of carotenoids 

 from a number of species would be linked up with some metabolic 

 idiosyncrasy of the species and the apparently fortuitous distribution 

 of carotenoids would no longer appear so, but would fall into the 

 general pattern. 



287 



