CONTROL OF STEM GROWTH 



219 



Fig. 2. The response of dark-grown Phaseolus vulgaris seedlings to red 

 and far-red radiation and to gibberellic acid. From left to right: Dark- 

 ness, untreated; darkness, gibberellic acid treated; red light, no gibberellic; 

 red light, gibberellic acid; red followed by far red, no gibberellic acid; 

 red followed by far red, gibberellic acid. Two days after treatment. 



light. The absence of a response to gibberellic acid in dwarf beans 

 grown in darkness is unique among the dwarfs so far examined. This 

 result might be interpreted as suggesting that the beans are dwarfed 

 only in light. The growth of these plants in darkness, then, would be 

 identical with that of the "normals" and limited only by the red light 

 factor. The natural gibberellin in the dwarfs, however, would be more 

 sensitive to "destruction" by light. 



Table I. Effect of Proportion of Time Red-Far Red Pigment Is Present in Far-Red- 

 Absorbing Form on Growth Response of Phaseolus vulgaris, Variety Black Valentine 

 to Gibberellic Acid Treatment (4.0 /Lig/ Plant) (Light treatments repeated daily) 



Height of Plants, cm 



Dark 

 Red 



Red + far red (immediately) 

 Red -f far red (after 6 hr) 

 Red + far red (after 24 hr) 



