THE LIGHT FACTOR IN PHOTOSYNTHESIS 1021 



are subject to the uncertainties of determining the concentration of the 

 two components by spectroscopic examination of the hving leaves.^ 



When we turn to the question of the chlorophyll content of plants 

 under natural conditions, a much more complicated condition is encoun- 

 tered. Willstatter and StoU (143) found that the content of chlorophyll 

 and the ratio of the two components did not change during long periods 

 of photosynthesis even under conditions of high activity. This, however, 

 was under laboratory conditions, and they themselves point out (143, 

 page 40) that it is conceivable that under conditions of excessive illumina- 

 tion chlorophyll may be decomposed. As a matter of fact, fluctuations 

 in the chlorophyll content of plants growing under natural conditions 

 have been observed. Variations as high as 30 per cent in 24 hr. have 

 been reported by Henrici (42, 43) occurring on sunny days in Bechuana- 

 land grasses. The decreases occur from early morning to midday and 

 the increases during the ensuing night, both depending upon the complex 

 of meteorological conditions. The chlorophyll content also varies during 

 the course of the year, being high in the young leaves, decreasing with the 

 intensity of the drought periods, and increasing after the rains. There 

 are also differences from year to year. Similarly, Stalfelt (113) has 

 reported decided fluctuations during the course of the year, in the chloro- 

 phyll content of Pinus silvestris and Picea excelsa growing near Stock- 

 holm, though no correlations could be established between these fluctua- 

 tions and light intensities or temperatures. On the other hand, in the 

 experiments of Sjoberg (108), also carried out near Stockholm under 

 natural conditions, but with Vicia Faha and Tropaeolum majus, the varia- 

 tions of chlorophyll content were such that the high values coincided in 

 general with periods of high light intensity and vice versa. It is evident 

 that a great deal more investigation is required in which light intensities 

 (both total and of different spectral regions) are measured and also other 

 factors, such as leaf temperatures, must be considered in order to arrive 

 at conclusions of more general validity. 



Carotenoids 



The carotenoid pigments, carotene and xanthophyll, have been found 

 always to accompany chlorophyll in organs capable of photosynthesis. 

 The role of these orange-yellow pigments in the photosynthetic process 

 is as yet not known ; in fact, it is not certain that they play a direct part 

 and opinions on this are still at variance. In general, there have been 

 two schools concerning the possible function of the carotenoid pigments 

 in photosynthesis: the one ascribes a purely chemical role to them, the 

 other places chief emphasis on their optical properties. Whether either 



' It may not be amiss to state here that the nomenclature of the chlorophyll 

 components, as nearly as they are comparable, is as follows: Chlorophyll a (Will- 

 statter) = neochlorophyll (Marchlewski) = chlorophyll a (Tswett); and chlorophyll 

 b (Willstatter) = allochlorophyll (Marchlewski) = cMorophyll ^ (Tswett). (Cf. 46.) 



