244 Plant Biology 



are not definitely established. The blue-green algae (phylum Cyano- 

 phyta) contain a blue pigment, phycocyanin, in addition to the green 

 chlorophyll and yellow pigments. The red algae (phylum, Rhodophyta) 

 contain a red pigment, phycoerythrin, in addition to the green chloro- 

 phyll. The brown algae (phylum Phaeophyta) contain a brown pigment, 

 fucoxanthin, in addition to the green chlorophyll. The green algae 

 (phylum Chlorophyta) contain green chlorophyll which predominates 

 over the carotene and xanthophyll pigments. The diatoms (phylum 

 Chrysophyta) have a yellowish-brown pigment in addition to their green 

 chlorophyll. A brief summary of the pigments of higher plants is given 

 in the accompanying table. 



Green pigments, such as chlorophyll, may occur in any part of a plant 

 which is exposed to light, although they also occur without light in such 

 tissues as lemon and melon seeds, in embryos and endosperm, certain 

 fruits, and in the wood of many Rosaceae. Chlorophyll A in alcoholic 

 solution appears blue-green by transmitted light and blood red by re- 

 flected light and has a blood red fluorescence. Chlorophyll B in alcoholic 

 solution appears yellow-green by transmitted light and has a brownish- 

 red fluorescence. The formation of chlorophyll is dependent on ( 1 ) iron, 

 which is necessary to form chlorophyll but is not a part of the pigment; 

 (2) at least a minimum of light to develop chlorophyll from the unstable 

 pigment chlorophyllogen, although certain algae, young ferns, and the 

 seedlings of certain conifers become green in darkness; (3) moderate tem- 

 perature for an optimum formation of chlorophyll, because there is no 

 greening at very low or very high temperatures; (4) an excess of oxygen 

 which seems necessary for greening; (5) the proper quantity and quality 

 of carbohydrates; (6) certain mineral salts, especially magnesium, which 

 is an important constituent of chlorophyll. 



Yellow pigments may occur in any part of a plant and their presence 

 is not related to the presence of light. One important yellow pigment is 

 xanthophyll (Or. xanthos, yellow; phyllon, leaf), which is common in 

 the leaves of elms, birches, and poplars. Xanthophyll is also found in 

 animals in ^^^ yolk and yellow feathers. Xanthophyll is one of several 

 pigments known as carotinoids which form about 0.5 per cent of the 

 weight of fresh leaves. In the fall, as chlorophyll decomposes, the caro- 

 tinoids become visible, often together with the red anthocyanins in 

 leaves. From its chemical formula xanthophyll appears to be merely an 

 oxidation product of carotene which is another carotinoid pigment. 



Another carotinoid pigment is carotene (Or. karotin, carrot yellow), 

 which is almost insoluble in alcohol (cold) and which forms flat rhombic 



