426 THE PIGMENT SYSTEM CHAP. 15 



mechanism for two different purposes. Sargent (1934), on the other 

 hand, denied that the color of Ught has any importance at all and asserted 

 that algae respond in the same way to changes in intensity of light of 

 any color. Gloeocapsa montana, for example, becomes blue in weak 

 light and yellow (or light green) in strong light — whether this light be 

 blue or green. 



Despite Sargent's experiments, it appears illogical to deny the exist- 

 ence of any other but intensity adaptation. The development of red 

 pigments in algae living in the blue-green light filtered through thick 

 layers of sea water is so obviously a reaction to the color rather than to 

 the low intensity of illumination that the existence of chromatic adapta- 

 tion seems proved beyond doubt by this fact alone. The recent con- 

 firmation of the availability for photosynthesis of the light energy ab- 

 sorbed by the phycobilins (c/. Vol. II, Chapter 30) removes the last 

 objection which could be raised to Engelmann's concept — the doubt con- 

 cerning the practical usefulness of chromatic adaptation for the photo- 

 synthesis of algae. 



On page 430, we shall find indications that the f or matioji of chlorophyll 

 and of the carotenoids proceeds most rapidly in the light absorbed by 

 these pigments themselves. The photochemical decomposition of each 

 pigment also will occur most efiiciently in the light absorbed by it. The 

 combination of photochemically autocatalyzed pigment synthesis with 

 photochemical decomposition may be the basis of both chromatic and 

 intensity adaptation; but Ave can expect a more detailed understanding 

 of the mechanism of these phenomena only from a quantitative study, 

 using truly monochromatic (instead of filtered) light and calculating 

 (as precisely as possible) the absorption of energy by each pigment 

 (cf. Vol. II, Chapter 22). 



To sum up, both chromatic adaptation and intensity adaptation 

 appear to be real phenomena, even though the superposition of both, 

 together with the influence of heredity, lead to a confusing variety of 

 phenomena. Although Engelmann's far-reaching conclusions were based 

 on an apparently insufficient experimental evidence, his intuition has 

 proved correct. The intensity adaptation of Oltmanns and Lubimenko 

 provides a corollary, but not an alternative, to Engelmann's chromatic 

 adaptation. 



The advantages of chromatic adaptation for plants will be discussed 

 in volume II, chapter 22, in terms of additional energy available to the 

 plants because of their content of chlorophyll b, carotenoids, and phyco- 

 bilins, and in volume II, chapter 30, in terms of the contribution to 

 photosynthesis of the light absorbed by these pigments. 



It has been asked whether nature's choice of chlorophyll as the 

 main sensitizing pigment in photosynthesis itself is a chromatic adapta- 



