CONTROL OF LEAF GROWTH 167 



Table II. Cobalt and Light Versus Expansion 



red. We have no ready explanation for this phenomenon but are 

 investigating it further. There is no really definitive explanation for 

 the action of cobalt in a great many biological responses which it 

 affects, although it has been suggested that it may act as an anti- 

 peroxidative agent (Galston and Siegel, 1954). Certainly the action 

 of cobalt appears to be rather closely linked with the light action 

 mechanism. 



In connection with our studies we desired to determine the nature 

 of chemical substances which could overcome, replace, or in some way 

 alter the effect of light in its control of leaf expansion. Among the 

 compounds which we wished to examine was kinetin, which had been 

 demonstrated to be active in cell division (Miller et ai, 1955). When 

 it became generally available in the latter part of 1955, some of this 

 compound, along with a number of similar compounds, was obtained 

 for our studies through the courtesy of Dr. William Shive and Dr. 

 C. G. Skinner of the Biochemical Institute, University of Texas, 

 Austin, Texas (Skinner and Shive, 1955; Ham et al., 1956; Skinner, 

 Shive, et al, 1956; Skinner, Ham, et al, 1956). All compounds of 

 the first group examined, among which was kinetin, showed a pro- 

 motive effect in darkness, and this promotion was simply additive to 

 that obtained by red light. Although the red-light effect could be 

 reversed by far red light, the promotive effect by kinetin could not. A 

 second group of these compounds, on the contrary, gave an inhibition 

 in red light as compared to its effect in darkness, i.e., the growth in red 

 light was always less than that in darkness. Thus it appeared that red 

 light exerted an inhibitory effect, not a promotive one, if given while 

 these compounds were present. 



Up to this time we looked upon these substances as simply foreign 

 chemicals not really related to the action of light, until we tried the 

 compound 6-thiopurine-2-succinic acid (Skinner, Shive, et al, 1956). 



