14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



DISCUSSION 



The use of the plant photometer in determining the sensitivity of 

 seedHngs to light has in its favor the elimination of the operator's 

 judgment at many points during the experiment. The plant itself is 

 used as a null point instrument. After a time interval the plant has 

 grown toward or away from the standard light. There is no need for 

 the operator to estimate the angle of curvature or the exact time at 

 which bending begins. Repeated experiments demonstrate that by 

 moving the standard lamp 0.5 cm toward or away from the plant 

 when located at a balance distance of approximately 25 cm, the curva- 

 ture of seedlings can be changed from one direction to the opposite. 

 It is interesting to note that repetition of balance points seldom 

 differed from each other by more than 5 percent. Very rarely was 

 an unorthodox seedling or an apparently nonsensitive seedling found. 



One possible objection to this method might be raised. Each point 

 on the phototropic curve is not strictly comparable to the others. This 

 arises from the fact that the plant was at a fixed distance from the 

 monochromator. The intensity of the various wave lengths used was 

 different. The intensity of the standard light was changed to balance 

 that of the monochromator light. A better method perhaps would be 

 to maintain the standard light at a fixed intensity with respect to the 

 plant and change the monochromator light to balance the standard 

 light. 



It is of interest to note that the maximum phototropic response 

 occurs at wave length 4400 A. This point lies midway between the 

 greatest absorption maxima of chlorophyll a and chlorophyll b re- 

 cently measured by Zscheile (1934) by an improved method. It is 

 also the position of one of the maxima found by Hoover (1934, data 

 unpublished) for carbon dioxide absorption by young wheat plants. 

 Since phototropic response is an index of growth retardation it would 

 at first appear that photosynthesis progresses best at a point in the spec- 

 trum where growth is least. Such is not the case, however, when the 

 other and somewhat greater maximum of carbon dioxide absorption 

 is considered. This occurs in the region of 6400 A. Here there is 

 no phototropic response and no retardation in growth. 



The absence of any phototropic effect in the red and infrared, as 

 shown in these experiments as well as by those of other investigators, 

 and the sharp rise in the curve from about 5000 A into the blue, is 

 typical of an electronic photochemical reaction. The photochemical 

 nature of at least some of the underlying processes involved in photo- 

 tropism is also suggested by the part played by auxins. 



