92 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
Dr. Alice P. Withrow and Dr. Walter Shropshire, Jr., measured 
the growth of tomato plants given various regimes of light sequences 
which involved supplementing a 17-hour period of high-intensity 
fluorescent light with 7 hours of red and/or far-red radiant energy. 
It was found that neither red nor infrared caused any increase in 
height as compared to those that received a 7-hour dark period. 
However, a far-red exposure given during the 7-hour period, either 
continuously or in a sequence of 15 minutes on and 15 minutes off, 
caused the stems to almost double in length. If, in the 7-hour period, 
the far-red were followed by red, only a slight growth was noted. If 
this procedure were in effect for 14 complete cycles on each of 15 con- 
secutive days, totaling 210 cycles, more than 90 percent reversal of 
the far-red growth potentiation was secured. Undoubtedly, the far- 
red acts as a reversing agent of the red-inhibited stem growth initiated 
during the 17-hour high-intensity period. Cycling the far-red with 
red prevents the far-red action. It also was found that the chloro- 
phyll content of the youngest mature leaves was reduced by 20 per- 
cent if they were exposed to 7 hours of far-red. There was little 
change if red or infrared were used during the 7-hour period. 
It has previously been reported that far-red radiant energy in the 
spectral region from 710 to 820 mp, when used as a supplement to 
X-rays, significantly increases the frequency of chromosomal aberra- 
tions. This year, Dr. Robert B. Withrow and Dr. Carl C. Moh ex- 
tended the study to determine the region of far-red which gives the 
maximum effectiveness in increasing the X-ray effect on chromosomal 
breakage. 
Root tips of broad bean (Vicia faba) were pretreated with a 3-hour 
exposure of far-red radiant energy of a specific wavelength and then 
irradiated with 100 roentgens of X-rays. Using an irradiance of 
1500 pw/cm?, wavelengths at 730, 760, 780, 800, and 820 mu were 
tested. It was found that roots irradiated at 760 mp yielded 67 per- 
cent more chromosomal aberrations than the control (X-rays only), 
and those with 780 mp, 54 percent, both increases statistically signifi- 
cant. Pretreatment with radiant energy at 800 mp increased the ab- 
errations by 20 percent, but 730 and 820 mp gave no increase. It may 
be concluded that far-red radiant energy from 760 to 780 mp is most 
effective in increasing the frequency of chromosomal aberrations in- 
duced by X-rays. In the past year it also was found that red radiant 
energy from 620 to 680 mp, when applied subsequent to or simul- 
taneous with the far-red treatment, can nullify the far-red potentiat- 
ing effect. On the other hand, blue energy was ineffective in re- 
versing the potentiation induced by the far-red. This result implies 
that the red versus far-red reversible system found at the chromosomal 
level is analogous to that occurring in a wide variety of photo- 
