402 THE PIGMENT SYSTEM CHAP. 15 



and Diatomeae, absorb some green light transmitted by chlorophyll, and 

 thus change the pure green color of the latter into a dull olive or brown. 



The colors of Rhodophyceae and Cyanophyceae are brought about by 

 mixtures of chlorophyll with phycohilins, so called because of their 

 similarity to the bile pigments (e. g., bilirubin). They have absorption 

 maxima in the middle of the visible spectrum — in green and yellow — 

 between the two main absorption bands of chlorophyll. This explains 

 why many of these organisms show a vivid red color {Florideae), while 

 others are purple or blue {Cyanophyceae). 



Purple and green bacteria contain green pigments closely related to 

 chlorophyll (bacteriochlorophyll and bacterioviridin) and a large assort- 

 ment of carotenoids, similar to, but not identical with, the carotenoids 

 of the higher plants and algae. 



Of the four types of pigment mentioned above, two — the chlorophylls 

 and the phycobilins — are strongly fluorescent in extracts, and also 

 fluoresce (although much more weakly) in the living cells. The carote- 

 noids and the flavones, on the other hand are usually described as non- 

 fluorescent (c/., however. Vol. II, Chapter 23). 



B. The Chlorophylls* 

 1. Chlorophylls a and h and their Ratio 



As early as 1832, Pelletier and Caventou suspected that chlorophyll 

 was a mixture. This hypothesis was proved 32 years later, in 1864, 

 when the physicist Stokes, while investigating the phenomenon of 

 fluorescence, beat the plant chemists to the discovery that both the green 

 and the yellow leaf pigment are mixtures of at least two major con- 

 stituents. Another 42 years later, Tswett, a botanist, showed chemists 

 how to separate pigment mixtures efficiently. He devised analysis by 

 chromatography and at once put it to practical use, achieving the first 

 separation of the two chlorophyll components of green leaves. Tswett 

 gave to the two green pigments the names of chlorophyll a and jS, which 

 later became plain a and h. Chlorophyll a gives blue-green solutions; 

 those of chlorophyll h are yellow-green. 



Zscheile (1934, 1935) thought that he had found, in the chromatograms from leaf 

 extracts, a third component, "chlorophyll c," with properties intermediate between 

 those of a and h. His conclusions were, however, criticized by Winterstein and Schon 

 (1934) and Mackinney (1938); and Zscheile himself later (1941) agreed that the adsorp- 

 tion layer attributed to chlorophyll h was due to pheophytin (c/. Chapter 21, Vol. II). 



Willstatter and Stoll (1913) proclaimed the identity of chlorophylls a 

 and h in all green plants, in contrast to earlier authors who believed that 



* BibUography, page 432. 



