166 CHEMICAL AGENTS AND GROWTH 



a growth inhibitor, caused a very marked promotion of growth of discs 

 from green leaves of Chenopodium album, although this has not yet 

 been confirmed. Others have tried unsuccessfully to promote leaf 

 growth with many different compounds (de Ropp, 1945, 1947; Juhren 

 and Went, 1949). 



INTERACTION OF LIGHT AND CHEMICALS 



In 1951 Miller (1951b) pubHshed experiments aimed at eluci- 

 dating the biochemical nature of the light-induced expansion of etio- 

 lated leaves, by using a system almost identical with the one we now 

 use. Miller tried a number of chemical compounds, including some 

 that Bonner et al. (Bonner et al., 1939; Bonner and Haagen-Smit, 

 1939; Bonner, 1940) had tried, and observed that they were, in 

 general, without effect. He made the interesting discovery, not yet 

 completely understood, that the cobaltous ion produced a rather pro- 

 nounced expansion even in darkness and that this promotion was in 

 addition to that caused by exposure to light. This effect was not 

 specific, since the nickelous and the manganous ions were also active 

 (Miller, 1951b). It has since been observed that cobalt is active in 

 other fight-controlled responses. It produces the same effect as red 

 light in Xanthium flowering (Salisbury, 1957) and in the growth of 

 oat coleoptiles (Thimann, 1956; Liverman and Bonner, 1953), in the 

 unhooking of the hypocotyl of bean (Miller, 1951b), and in other 

 responses. 



We have repeated Miller's experiments on cobalt in detail and have 

 found, in addition, that the growth due to cobalt is finear with time 

 under all our conditions of light — red, red-far red, and far red — and 

 in darkness. We made the additional observation that the amount of 

 growth obtained in response to cobalt is almost linearly related to the 

 length of time the cobalt is left in contact with the leaves. The effect 

 of light is superimposed upon the effect of cobalt, and it appears to 

 make no difference whether the light is given before or after the cobalt 

 is supplied. Another of our observations, the meaning of which is not 

 yet clear, is shown in Table II. The importance of these data is in 

 showing that a concentration of cobalt which gives maximum growth 

 in far-red light alone gives the minimum growth in red followed by far 



