758 General and Applied Biology 



those containing nitrogen, to form proteins. When sugars are stored, 

 they are changed by enzymes through a process of dehydration (loss of 

 water) to form starch. For example, glucose (G6H12O6), a sugar, 

 through the process of dehydration (loss of H2O) is changed to starch 

 (C6Hio05)x, where x is a very large number. The chloroplasts con- 

 taining the green chlorophyll are especially concentrated in the chlor- 

 enchyma cells of the palisade layer of plant leaves where a maximum of 

 light is available (Fig. 60). Iron is not a constituent of chlorophyll, 

 but it must be present in the presence of light in order for chlorophyll 

 to be formed. The visible green color of chlorophyll is due to the ab- 

 sorption of certain light rays (Fig. 368) in the red, blue, and violet 

 regions, with the transmission to our eyes of the remaining rays which 

 give us the characteristic leaf-green color. The absorption of these 

 specific rays by the chlorophyll explains the ability of green plants to 

 manufacture foods. Certain lower plants as the blue-green, the brown, 

 and the red algae, as well as such higher plants as the coleus and red 

 cabbage, possess other pigments which may mask the chlorophyll, which 

 is nevertheless present in them. These phenomena are considered in 

 greater detail in other chapters. 



Plant and Animal Colorations 



The ability quickly to change color and shades in animals in response 

 to external stimuli is usually confined to reptiles, amphibia, fishes, Crus- 

 tacea, and cephalopod Mollusca. Pigments are produced by cells known 

 as chromatophores, which may function as single cells or as groups of 

 cells (Fig. 209). In fishes the chromatophores are rather large and star 

 shaped, and with a small central disk with repeatedly subdividing 

 branches which radiate from the center. The four types of chromoto- 

 phores which have been most extensively studied are: (1) Melanophores 

 (Or. melan, black; phoros, to bear), containing the black pigment 

 melanin. The granules of melanin can be dispersed in cells by ether or 

 dilute solutions of sodium chloride and can be aggregated by adrenalin 

 or potassium chloride. (2) Xanthophores (Gr. xanthos, yellow; phoros, 

 to bear) containing the yellow carotinoid pigment xanthophyll. The 

 amount of xanthophyll in certain fishes, at least, depends upon the type 

 of plant food. (3) Erythrophores (Gr. erythros, red; phoros, to bear), 

 containing the red carotinoid pioment. (4) Guanophores, containing 

 white crystals of guanin. 



In each type of chromatophore an external stimulus causes a move- 

 ment of the pigment within the cell. When a fish becomes paler, the 



