1022 BIOLOGICAL EFFECTS OF RADIATION 



one of these will lead to the correct answer or some other explanation 

 of their role will be found will in all probability depend upon further 

 investigation of their chemical composition and their physical and 

 chemical properties. 



As in the case of chlorophyll, no simple, definite statement can be 

 made concerning the influence of light on the formation of these pigments. 

 The extensive older literature has been well reviewed by Kohl (54) and 

 the more recent methods of estimation by Deleano and Dick (18). The 

 fundamental difficulty in the problem is that until recently there have 

 not been reliable methods for the separation of the various leaf pigments 

 and their accurate determination. Even at the present time, with modern 

 spectrometric apparatus, the task is associated with difficulties and pit- 

 falls which reveal themselves only after considerable experience. If the 

 pigment has not been isolated in pure form, the depth of color of a 

 solution can very rarely be taken as a measure of its concentration. 



The presence of xanthophyll in etiolated seedlings has been estab- 

 lished by Euler and Hellstrom (27) and by Sjoberg (108). From their 

 results it is also apparent that the amount of this pigment is increased 

 through the action of light. In etiolated barley Euler and Hellstrom (27) 

 could not detect any carotene with the exception of a few cases in which 

 traces of chlorophyll were also found. On illumination, the carotene 

 was found to increase appreciably. On the other hand, Sjoberg found 

 distinct yellow coloration, due to carotene and xanthophyll, in the 

 etiolated tips of the leaves of oats, though the latter pigment was present 

 in relatively much larger quantity than the former. The amounts were 

 decidedly increased by exposure to light. He also was able to detect 

 traces of carotene in the tips of etiolated seedlings of cress. It is just 

 possible that differences of temperature at which the seeds are sprouted 

 may account for these variable results. Sjoberg found that the etiolated 

 shoots of plants with storage organs contain both carotene and xantho- 

 phyll and that the amounts they contain are increased by exposure to 

 light. As was the case in the experiments of Guthrie (36) with chloro- 

 phyll, long periods of illumination decreased the amounts of carotenoids 

 in both soy beans and tomatoes. When plants were placed in the dark the 

 chlorophyll content decreased, but the carotenoids did not show a parallel 

 decrease. On the basis of the similarity in composition of carotene and 

 phytol the possibility of a genetic relationship between carotene and 

 chlorophyll was considered by Willstatter and Mieg (141) and by Smith 

 (109), and this has received some experimental support by Rudolph 

 (95). 



THE OPTICAL PROPERTIES OF LEAVES 



The light which falls upon a leaf is in part reflected by it, in part 

 absorbed by the materials of the leaf, and is in part transmitted through 



