GENERAL COMPOSITION 401 



In recent years, attention has been increasingly drawn to the part 

 played in photosynthetic production by marine plants, particularly the 

 microscopic algae of the plankton. The majority of algae are yellow, 

 brown, olive, red, or blue, but not green. On land, too, a few plant 

 species have yellow or red, instead of green, leaves. However, our belief 

 in the importance of the green pigment for photosynthesis is not shaken 

 by these facts because, whenever the pigment system of a "colored" 

 (that is, nongreen) photosynthesizing organism has been analyzed, it has 

 been found to contain chlorophyll. No case of "photosynthesis without 

 chlorophyll" has as yet come to light. Even though light absorbed by 

 other pigments may also be utilized in photosynthesis (c/. Vol, II, 

 Chapter 30), this utilization seems to be impossible without the co- 

 operation of chlorophyll. 



Yellow, orange, red, or blue "accessory pigments," combined with 

 green chlorophyll, determine the appearance of leaves and algae. Even 

 in green leaves, chlorophyll is regularly accompanied by several yellow 

 carotenoids, whose presence remains concealed because their absorption 

 bands, situated in the blue and violet part of the spectrum, are blotted 

 out by the near-by absorption bands of the more abundant chlorophyll. 

 The presence of these yellow pigments (for which the name "xantho- 

 phyll," from x(xvtos, yellow, was suggested by Berzelius in 1837) is 

 revealed in autumn when chlorophyll undergoes decomposition into 

 colorless products. The color of the leaves of the "aurea" varieties of 

 certain trees and bushes, which are poor in chlorophyll, is also caused 

 by the carotenoids. 



A second group of yellow pigments present in green leaves are the 

 water-soluble fiavones, contained mainly in the vacuoles (while the 

 carotenoids are associated with chlorophyll in the plastids). In some 

 species, or in certain periods of development, the fiavones are supple- 

 mented by their oxidation products, the red anthocyanins. This brings 

 about the transient red coloration of some young or decaying leaves, as 

 well as the permanent red color of the leaves of the "purpurea" varieties. 

 In contrast to "aurea" leaves, "purpurea" leaves are not necessarily de- 

 ficient in chlorophyll (addition of a green pigment to a red one does not 

 change the color as strongly as does its addition to a yellow pigment). 



Yellow and red leaves form only a few bright spots on the green 

 cover which vegetation spreads in summer over the surface of land. 

 In the sea, olive, brown, and red algae predominate among the vegetation. 

 Their colors are caused by the mixture of chlorophyll with accessory 

 pigments of two types: carotenoids and phycobilins. Chemically, the 

 carotenoids of the algae are not very different from those of green leaves ; 

 but some, particularly the fucoxanthol of the Phaeophyceae (brown algae) 



