70 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



within the organism absorb the light and convert it into chemical bond 

 energy and thus initiate a chain of events that produce the observed 

 growth effects. In most instances, relatively little light is necessary. 

 This is in contrast to the photosynthesis of sugars where very high 

 intensities are required for optimal results. There is a similar group 

 of light-controlled reactions in animals which regulate many phases 

 of reproductive behavior. Plans have been prepared for extending 

 the findings and technics developed in the laboratories to this phase of 

 animal physiology. 



The regulatory plant photochemical reactions may be divided into 

 two general groups : (1) Those controlled chiefly by red and far-red 

 light; and (2) those controlled principally by blue light. The 

 respective pigment systems channel the energy into different biochemi- 

 cal pathways and therefore induce entirely different physiological 

 responses. 



Photomorphogenesis. — Included in the photochemical reactions 

 initiated by red light are formative growth processes in seedlings, such 

 as the ability to form normal leaves and the disappearance of the stem 

 hook that often is present on germination. Any one of these responses, 

 among many others, can be used as quantitative bioassays of the photo- 

 reactions. In these laboratories we have developed a technic based on 

 the rate of angular opening of the excised stem hook or arch that 

 appears in seedlings of beans and other dicotyledonous plants that 

 have been grown in complete darkness. If the hook portion of the 

 stem is cut from the seedling and exposed to red light, and then 

 returned to darkness, the hook will open in the following 20 hours to 

 an angle that is proportional to the logarithm of the incident red-light 

 energy. Last year it was determined that the most effective region of 

 the spectrum for producing this response was in the red at 660 nut. 

 If, after an exposure to red light, the hook is treated with far-red 

 energy from 700 to 750 m/*, much of the effectiveness of the initial red 

 treatment is inhibited or "reversed." Dr. W. Klein, Dr. E. B. 

 Withrow, and V. Elstad have completed the action spectrum of this 

 far-red reversal phenomenon! and have found that the maximum 

 reversal occurs at 710 and 730 m/t, with a weak maximum at 640 m/x. The 

 reversal action has been determined at 27 points in the spectrum from 

 365 to 800 m/x in the following incident energy series : 25, 10, 7.5, and 

 5.0 millijoules (mj). The percentage reversal is directly propor- 

 tional to the far- red incident energy, up to values of 10 mj which pro- 

 duces about 85 percent reversal. This linear response is in contrast 

 to logarithmic function of the red-light induction reaction. 



Phototropism. — Another expression of photoregulatory processes 

 in plants is bending toward a light source, or phototropism. Previ- 

 ously, Dr. E. S. Johnston of this laboratory had found the action 



