PHOTOCONTROL OF VEGETATIVE GROWTH 131 



for some reason the second operation of the regulator, the photo- 

 periodic on-off switch, is less sharply defined in control of growth of 

 herbaceous plants than it is in woody ones. 



Flowering of a rosette type plant typically involves two phenomena: 

 the production of the flower primordia and flowers and the promotion 

 of stem and branch development. The two phenomena are so inter- 

 woven and interdependent that we seldom think of them as separable. 

 Although they are usually not separable, they can, to a degree, be 

 independently controlled by light. Thus, one is able to influence stem 

 elongation by appropriate light treatment and thereby indirectly influ- 

 ence flowering. 



Flowering of millet {Set aria italica (L.) Beauv.), which occurs 

 slowly on 16-hr photoperiods, is accelerated by use of incandescent 

 supplemental light rather than fluorescent (Table III). This promotion 



Table III. Effect of Photoperiod Length and Kind of Supplemental Light on 



Stem and Spike Lengths of Millet 



Photoperiod," 12 hr Photoperiod," 16 hr 



Incandescent Fluorescent Incandescent Fluorescent 



Stem length, mm 420 308 222 82 



Spike length, ''mm 21.6 16.9 1.4 0.6 



° Eight hours of natural light plus the duration of supplemental light necessary to obtain 

 these photoperiods. 



^ A measure of the rate of flowering. 



is apparently due to an increased rate of stem elongation resulting 

 from the plant's entering the dark period with the pigment system 

 predominantly in the red-absorbing form. If the red-radiant energy 

 from the incandescent lamp is prevented from reaching the plant so 

 that only relatively pure far red is available, a more efficient method 

 is obtained for converting the pigment system into the red-absorbing 

 form. As little as a 5-min exposure to far-red radiant energy at the 

 close of each 1 6-hr period of high-intensity light was adequate to in- 

 crease the rate of stem elongation and consequently the rate of flower- 

 ing of millet (Fig. 1 ). The high-intensity light period was obtained by 

 means of fluorescent lamps, and the control plants, therefore, entered 

 the dark period with the pigment predominantly in the far-red-absorb- 

 ing form. The potential effect of the far-red irradiation could be re- 



