PHOTOTROPISM: A SPECIFIC GROWTH 

 RESPONSE TO LIGHT 



By Eakl S. Johnston 

 Division of Radiation and Organisms, Smithsonian Institution 



[With 2 plates] 



Psychologists and physiologists have shown that the human eye 

 is most sensitive to yellow and becomes less and less sensitive to light 

 of longer and shorter wave lengths as one passes toward the red and 

 the blue, respectively, of the visible spectrum. In other words, when 

 all the colors of the spectrum are of equal intensity, yellow looks 

 the brightest to the human eye. Of the entire range of radiant energy, 

 only that falling between the approximate limits 4,000 and 7,600 

 angstrom units is visible to the average human eye. However, longer 

 radiation in the infrared can be detected by a sensation of warmth on 

 the skin, and if the skin is exposed to certain short radiations in the 

 ultraviolet, a temporary burning results. Experiments with animals 

 such as the honeybee and a number of unicellular organisms have 

 shown other ranges of sensitivity than that found in man. The 

 question arises: To what wave lengths of radiant energy are plants 

 sensitive ? 



There are a number of structures in plants which distinctly indicate 

 a mechanism that responds to light. In certain dimly lighted caverns 

 there is found a small moss, Schistostega osmundacea^ whose proto- 

 nema consists of a layer of lens-shaped cells so constructed that light 

 gathered over a relatively large area is concentrated onto a few chloro- 

 plasts located in the bottoms of the cells. In the leaf cells of other 

 plants the chloroplasts arrange themselves differently under different 

 light intensities. The chloroplasts are the small bodies in green plants 

 that bear the chlorophyll, the substance essential to life. When the 

 light is weak, these chloroplasts spread out at right angles to the rays 

 of light and are thus able to intercept more light. If, however, the 

 light is very strong, these same chloroplasts arrange themselves along 

 the cell walls that are parallel to the light rays, thus intercepting a 

 small fraction of the light. Another light-response reaction with 



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