SECRETARY'S REPORT 49 



tors. Of the latter, light is one of the most important and determines 

 both the rate of growth and the ultimate form that the plant assumes. 

 The light factor exhibits its influence through various types of photo- 

 chemical reactions. One of these is photomorphogenesis, the process 

 by which light exerts its formative effect on higher green plants. In 

 the absence of light but with adequate food reserves, the young plant 

 fails to develop normal leaves and stems. The leaves are usually very 

 small or rudimentary and the first formed stem nodes are much elon- 

 gated. At germination, most dicotyledonous seedlings such as bean, 

 tomato, and many others have an apical stem hook which disappears on 

 exposure to weak light. In darkness, this hook never completely dis- 

 appears. These formative effects are brought about by several photo- 

 reactions which are activated chiefly by the red end of the visible spec- 

 trum. The blue and green portions are relatively ineffective. 



This division has found that the apical stem hook is an excellent and 

 reproducible test object for measuring the effect of various radiant- 

 energy and chemical factors on the photomorphogenic response. If 

 the hook is removed from the plant and placed in a petri dish on moist 

 filter paper in complete darkness, it does not open for the first 20 to 24 

 hours at 25° C. In the presence of very low energies of red light, 

 however, the hook begins to open within a few hours. The rate of 

 opening in degrees of angle is directly proportional to the logaritlim 

 of the incident radiant energy. 



We have reported previously that the opening of the stem hook of 

 6-day old Black Valentine beans is due to a differential growth reaction 

 which can be mediated by the native auxin, indoleacetic acid (lAA) 

 which tends to inhibit the effect of the photoreaction. The antiauxin, 

 triiodobenzoic acid (TIBA) , which opposes the effect of auxin in many 

 other plant responses, produces an effect similar to that of the photo- 

 reaction. It causes the hook to open partially in complete darkness, 

 although it does not appear to be able to reproduce completely the ef- 

 fect of the photoreaction. Studies are now in progress to determine 

 the kinetical relationships between auxin concentrations and red 

 irradiance. 



Histological studies of sections from dark-treated bean hooks show 

 that the hooking is caused by the retarded growth of a group of cells 

 on the inside of the hook which are about 60 percent shorter than those 

 on the outside. When the hooks are irradiated with red energy, these 

 cells increase very rapidly in length, while those on the outside elongate 

 relatively slowly. This differential growth causes the hook to dis- 

 appear and the stem to straigthen. 



The effect of the red energy on initiating the early opening of the 

 bean hook can be inhibited by a short exposure to far-red energy 

 beyond 700 m/x. If the excised hooks are exposed to approximately 10 

 milli joules of red energy (625 to 700 ni/x) and then placed in complete 



