SECRETARY'S REPORT 89 



/xw/cm^. The growth response also has been found to vanish for 

 sporangiophores adapted at intensities greater than 1,300 /xw/cm^. 

 As this intensity is approached, the growth rate becomes 30-50 percent 

 higher tlian tliat observed for sporangiophores adapted in the normal 

 range of intensities, and this increased rate is maintained for long 

 periods of time (3 or more hours). However, the mechanism control- 

 ling the level of light sensitivity (the range adjustment mechanism) 

 appears to function at any intensity and with the same time constant 

 of about 4.0 minutes as in the normal range. 



The bending rate in the normal range is about 5-7 degrees/minute 

 for continuous unilateral stimuli given at 90° to the long axis of tlie 

 sporangiophore. As the intensity approaches 1300 /iw/cm", the bend- 

 ing rate decreases rapidly to zero. Apparently, the gradient across 

 the cylindrical growing zone disappears, just as found previously 

 for immersion oils with an index of refraction near 1.205. Whether 

 this loss of a gradient is due to saturation of the light sensitive system 

 or to bleaching of the photoreceptors is not yet known. 



Preliminary action spectra, at 20 m/x intervals, for tliis disappear- 

 ance of the tropic response have been completed and found to have 

 very nearly the same wavelength dependence as observed previously 

 in the normal range tropic response. 



The dimensions of the cytoplasmic layer within the growing zone 

 and adjacent regions of the sporangiophore have been measured imder 

 oil immersion. On the average, the cytoplasm occupies about 40 per- 

 cent of the diameter from the sporangium to 1 mm below. In the 

 growing zone itself it is about 25 percent of the diameter, and below 

 the growing zone decreases to about 15 percent. The cytoplasm is con- 

 tinuously streaming while being observed, and its thickness fluctuates 

 as much as 10 percent within a few minutes at any one point. 



The growth promoting effect of cobalt in etiolated leaf tissue is in- 

 dependent of growth inhibition by 2,4-dinitrophenol, which uncouples 

 oxidative phosphorylation. Cobalt does not raise the adenosine tri- 

 phosphate (ATP) concentration of leaf tissue, but does prevent a 

 decrease in ATP content in the presence of 2,4-dinitrophenol. In 

 order to elucidate the role of cobalt, experiments were performed with 

 isolated mitochondria, the subcellular organelles which are the sites 

 of oxidative phosphorylation. The results showed that cobalt alone 

 had no significant effect on respiration or phosphorylation. However, 

 when mitochondria were exposed to both cobalt and 2.4-dinitrophenol, 

 phosphorylative activity increased about 10 percent over that of the 

 2,4-dinitrophenol control. Mitochondria contain not only enzymes 

 that synthesize ATP, but also enzymes that decompose ATP. Since 

 the influence of cobalt on the S}mthesis of ATP was relatively small, 

 the possibility of cobalt's influencing the enzymatic decomposition was 



