Biochemical and Biophysical Phenomena 759 



melanin pigment granules move centripetally and concentrate in a small 

 area in the center of the cell. When that fish becomes darker, the 

 melanin pigments move centrifugally and fill the branches of each cell 

 to increase the blackness of the skin. This transition from one shade to 

 another may occur in a few minutes. The other chromatophores un- 

 dergo similar phenomena. Stimuli which produce changes as described 

 above usually include chemical, mechanical, thermal, or photic. In 

 fishes the melanophores and xanthophores are controlled by the termi- 

 nation of different peripheral nerves of the autonomic nervous system 

 so as to function independently. Hormones in the secretions from duct- 

 less glands of the fish supplement this nervous control. In amphibia the 

 chromatophores (Fig. 209) are controlled by the secretions in the blood 

 produced by the hypophysis (ductless gland). In reptiles the chroma- 

 tophores arc controlled directly by the nervous system supplemented by 

 the secretion of the medulla of the adrenal gland (ductless gland). 

 When a fish is placed on a yellow background, it assumes a quite dif- 

 ferent color from a similar fish placed on a gray background. The wave 

 lengths of the light reflected from these backgrounds are influential fac- 

 tors in this behavior. This light-reflecting capacity of the surroundings, 

 regardless of the degree of illumination, is the important factor. For 

 example, a dull black surface placed in direct sunlight causes a fish to 

 be black, while a white surface in diflfuse light causes it to be pale. 

 Recent research also suggests that fishes have a somewhat limited color 

 vision. Many of the colors of plants and plant pigments arc considered 

 in detail in another chapter. 



Coloration in plants and animals may be due not only to the absorp- 

 tion of certain waves of light by pigments or other substances, but also 

 to the interference of light. A certain type of interference known as 

 refraction (L. re, back; frango, to bend) is caused by the bending of 

 rays of light as they come to our eyes with the result that a variety of 

 colors is produced. Another type of interference known as diffraction 

 (L. dis, apart; frango, to break or bend) is caused by the separation of 

 light into parts which produces a variety of color sensations on our eyes. 

 The metallic blue color of the tropical butterfly (Morpho sp.) is due to 

 the interference of light. The bright colors on the neck of a humming- 

 bird are due to interference, while those on the body are due to absorp- 

 tion of fight. Colorations due primarily to absorption are those of cer- 

 tain moths and butterflies, the skin pigments of vertebrates, the feathers 

 of many birds, and the hemoglobin of the red blood corpuscles. 



