PHOTOTROPIC SENSITIVITY IN RELATION TO 

 WAVE LENGTH 



By earl S. JOHNSTON 

 Dk'ision of Radiation and Or(/anisnis, Smithsonian Institution 



(With Two Plates) 



INTRODUCTION 



Asymmetric growth resulting from unilateral stimulus has heen des- 

 ignated tropism. Growth curvatures following unilateral illumination 

 are usually classified under the term phototropism. Different plants 

 respond in different degrees to light, but perhaps those most fre- 

 quently used in phototropic experiments are the sporangiophores of 

 Phycomyces and the coleoptiles of Az'ena. In such studies the in- 

 tensity, the wave length, and the duration of exposure to light each 

 acts as a contributing factor toward the final result. Just as there 

 appears to be a threshold of intensity for a given duration of light 

 exposure, so there are wave lengths which seem to exert no influence 

 on these growth responses, but with exposures to other wave lengths 

 the plants show distinct degrees of sensitivity. Not only do different 

 plants vary in their sensitivity, but separate portions of the same plant 

 respond differently. Recent work on growth substances indicates the 

 presence of factors other than light in this complex plant-response. 



In the present paper the subject is limited, in the main, to the influ- 

 ence of radiation of different wave lengths on phototropism as shown 

 by the response of the coleoptiles of Avena sativa. The variety used 

 is Culberson, C.I. no. 272,, for which the author wishes to thank 

 Mr. T. Ray Stanton, of the United States Department of Agriculture. 

 All the light intensity measurements were made by Dr. E. D. 

 McAlister, to whom credit for that part of this work is given. 



HISTORICAL SURVEY 



Many of the early experiments on phototropism have been reviewed 

 by Parr (1918) and the data classified under four general theories: 

 I. The " intensity " theory originating with De CandoUe in 1832 and 

 adhered to in a more or less modified form by Wiesner, Darwin, 

 Engleman, Oltmanns, Yerkes, Loeb, and Davenport. 2. The ray- 



Smithsonian Miscellaneous Collections, Vol. 92, No. 11 



