96 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



red occurs about 30 minutes after induction. Preliminary measure- 

 ments of the spectral sensitivity of leaf disk expansion indicate that 

 at 546 and 677 uifi, the promotive effect is as great for equal quantum 

 flux as at 660 mfi. 



For leaves stimulated by red energy, an additional growth stimu- 

 lation of expansion is exerted by cobalt ions, which also promote 

 expansion in the dark. The maximum growth promotion due to co- 

 balt was found to be 3 X 10-*M. and was not found to be affected by 

 2,4-dinitrophenol (DNP) which uncouples oxidative phosphorylation. 

 Adenosine triphosphate (ATP) levels in the leaf tissue were not af- 

 fected by cobalt alone. However, complete deletion of ATP by 

 DNP did not occur if cobalt ion was added simultaneously. It ap- 

 pears that this effect is not due to the formation of a complex between 

 DNP and cobalt ion which is inactive in the oxidative phosphoryla- 

 tion process. Experiments with isolated mitochondria indicate that 

 the cobalt ion inhibits the activity of adenosine triphosphatase, 

 thereby increasing the net gain of ATP from oxidative phosphoryla- 

 tion. 



The yield of chromosome aberrations induced by a given dose of 

 X-rays is increased by supplemental far-red radiation. Sinc« far-red 

 energy is effective as either a pretreatment or posttreatment, it is 

 apparently the rejoining mechanism rather than breakage per se 

 that is affected. Alternatively, the increase of X-ray-induced aber- 

 rations may result from mitotic delay induced by far-red. These 

 possibilities are not necessarily mutually exclusive since some particu- 

 lar phase of the mitotic cycle, e.g., that portion of mitotic interphase 

 during which DNA synthesis takes place, may be preferentially 

 affected. 



Studies were conducted of cell population kinetics of root systems 

 of broad bean, Vicia faha, using flash labeling with tritiated thymi- 

 dine assayed by autoradiographs of squashed preparations. The rel- 

 ative frequency of labeled nuclei in each of the various stages of 

 mitosis was determined for dark-grown and far-red treated material. 

 The average duration of the mitotic cycle in Vicia faha was found to 

 be 19.1 hours. Cell division required 1.8 hours, and 17.3 hours were 

 spent in mitotic interphase. During this latter portion of the cycle, 

 DNA synthesis occupied 9.0 hours, while presynthetic and postsyn- 

 thetic interphase averaged 5.1 and 3.2 hours, respectively. There was 

 no evidence of mitotic delay in far-red treated material. Mitotic 

 indices, which averaged 9.2 for the far-red and 8.8 for the control 

 series, were comparable throughout. 



The responses of sporangiophores of Phycomyces hiakesleeanus to 

 diverging unilateral blue-light stimuli given in air were determined. 

 It was found that 3-minute stimuli given through a thin cylindrical 

 lens (approximately the same diameter as the sporangiophore) placed 



