132 



PHOTOCONTROL OF GROWTH 



Fig. 1. Millet plants received no additional radiant energy, 5 min of far 

 red, or 5 min of far red followed by 5 min of red at the end of each 16-hr 

 photoperiod (left to right). 



versed by shifting the pigment equilibrium back to a predominance of 

 the far-red-absorbing form. This conversion of the red-absorbing to the 

 far-red-absorbing form was accomplished by following the far-red 

 irradiation by a brief exposure to red radiant energy. 



In order to study some of the details of operation of the modulating 

 control of growth, we thought it advantageous to study elongation 

 independent of flowering. Pinto beans were selected as the test plant 

 although other bean varieties or other species of plants would have 

 been equally suitable (Fig. 2). In general, a brief exposure to far red 

 at the beginning of the dark period induced internode elongation. This 

 potential elongation was reversed when the far-red irradiation was fol- 

 lowed by a brief exposure to red radiant energy (Downs et al, 1957; 

 Hendricks et al, 1956). The responsiveness of the plants depended 

 upon the number of hours of darkness during which the pigment re- 

 mained in the red-absorbing form (Fig. 3). This dependence probably 

 does not arise from differences in the amount of pigment present or 

 from differences in the relative sensitivity of the pigment system to red 



