256 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 30 



end of the spectrum have the greatest retardino; effect so far as the 

 rate of stem elongation is concerned. Thus, plants grow and develop 

 differently in accordance with the intensity of light, its duration, 

 and its color. 



The three light variables just mentioned, which influence the gen- 

 eral character of growth, likewise should be considered in the specific 

 case of phototropism. In many of the early experiments dealing 

 with the bending of plant stems toward light sources, the lights used 

 were such that it was impossible to determine accurately whether it 

 was the color or the intensity of the light that was causing the bend- 

 ing. To be sure, both these factors of light influence this one-side 

 growth response, but the relative influence of the different colors, or 

 wave lengths of light, could not be determined because of the different 

 intensities of the colors used. In order to determine which colors are 

 most effective in phototropic bending it is necessary to use wave 

 lengths of known intensity. With the object of determining in a 

 quantitative way some of the fundamental growth responses of 

 plants to light, the Division of Kadiation and Organisms of the 

 Smithsonian Institution has initiated a series of experiments which 

 will eventually clarify this problem. Before describing the methods 

 used in eliminating the intensity from the wave-length effect of light 

 on growth, it may be well to examine briefly the mechanics of 

 phototropism. 



About 100 years ago De Candolle, the Swiss physician and botanist, 

 thought that positively phototropic bending was due to the retarding 

 effect of light on growth. The side of the stem most brightly 

 illuminated would grow slower and bring about a bending toward 

 the light source. The enormous elongation of potato sprouts in a 

 darkened cellar would seem to substantiate the view that plant stems 

 grow longer in the dark than in light. Thus the shaded side of the 

 stem of the geranium plant growing by the side of a window will 

 elongate more than the side more brightly illuminated. The result 

 is obvious. The stem and the petioles bend toward the window. 

 Mature plant tissues that have almost completed their growth cycle 

 do not show this bending nearly so much as young tissues. Only 

 the tissues that contain cells still capable of dividing or of enlarging 

 are the ones concerned in this phenomenon. 



Objections were made to this early view of phototropism. Such 

 distinguished men as Darwin and Pfeffer, the noted German plant 

 physiologist, were led to believe that there existed a region in the 

 plant capable of receiving a stimulus and that such a region was 

 more or less localized. In one experiment the apex of a young sprout 

 was exposed to light while the bending occurred at the base which 

 was not illuminated. It thus appeared that in addition to a region of 

 perception, there was a region of response. If this were true, then 



