CHAPTER 9 



Phototropism 



F. W. Went 



California Institute of Technology, Pasadena 



Phototropism is the response shown by plants when they bend within 

 a light gradient. This response may be either a bending toward the light, 

 which is positive phototropism, or a bending away from the light, which 

 is negative phototropism. There are also plant organs, such as leaves, 

 which place themselves at some definite angle to the incident rays. This 

 kmd of response is called "diaphototropism." Extensive reviews of pho- 

 totropism can be found in du Buy and Nuernbergk (1932), Johnston 

 (1936), van Overbeek (1939), and Galston (1950). 



In most cases the phototropic reaction is a growth movement; conse- 

 quently only the younger parts of the plant are involved. In a few cases, 

 such as in the leaves of Malva (Yin, 1938), the movement is due to turgor 

 change; this response can continue as long as the reacting tissue remains 

 alive. Turgor movements do not always lead to phototropic but usually 

 to photonastic responses (in which the direction of the movement is 

 determined by the structure of the organ and not by the direction of 

 the light), such as occur in most leguminous leaves. The mechanism of 

 these movements has been investigated in detail by M. Brauner (1932) 

 and L. Brauner (1948). 



During the last part of the nineteenth century and the beginning of 

 this century, a great deal of work was done on phototropism and its 

 theoretical explanation. Since our knowledge of the phototropic response 

 of the Avena coleoptile is rather complete as compared with what is known 

 of other organs and species of plants, it seems logical to analyze this case 

 in detail and then compare phototropic response in other plants with that 

 found in Avena. 



Growing Avena coleoptiles in weak red light for the first 30 hr after 

 germination represses the growth of the mesocotyl or first internode. 

 At that time the coleoptiles are strongly negatively geotropic and are 

 growing straight upward. When they have a length of approximately 

 30 mm, they are highly sensitive to light; they respond within 1 hr, 

 curving toward the light, when they have received as little as 1 m-c for 



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