CONTROL OF LEAF GROWTH 165 



and that after 20 hr very little of the potential growth can be reversed. 

 These results are in agreement with those of Downs (1955) on intact 

 bean plants. Thus, for experimental purposes this system responds in 

 the same way as the lettuce seed system, and we may assume that the 

 same general type of light action is involved. 



CHEMICAL CONTROL OF LEAF GROWTH 



A number of workers have studied chemical control of leaf growth 

 entirely apart from its control by light. Among the early workers in 

 this field were Professor Went and his colleagues at the California 

 Institute of Technology (Bonner et al., 1939; Bonner and Haagen- 

 Smit, 1939), who observed that a diffusate from peas stimulated the 

 growth of radish and other leaves. In 1939 Bonner and Haagen-Smit 

 reported that certain crystalline substances appeared to affect the 

 growth of green leaves of radish and cosmos. Since these were green 

 leaves, it is possible that the growth-limiting factors are different from 

 those for etiolated leaves. In etiolated leaves certain growth-limiting 

 requirements can be satisfied by red light. Among the active crystalline 

 substances tried were amino acids, which promoted a 10 to 25% 

 increase in growth as measured by wet weight when supplied at a 

 concentration of 500 mg/l. It is not immediately obvious that this 

 measurement reflects the same net result as a measurement of the 

 increase in diameter. Their results indicate that the stimulation may 

 differ for leaves from different plants; for example, arginine most 

 actively satisfied the requirement in Nicotiana sylvestris, whereas 

 proline and asparagine were most active for radish. None of these 

 amino acids was as active as the diffusate from peas. Substances found 

 to be inactive include vitamins and their derivatives, as well as a 

 number of pyrimidines. Bonner and Haagen-Smit did observe that 

 adenine, xanthine, and other purines gave about an 18% increase in 

 wet weight. Only adenine was active at 10 mg/l, whereas higher con- 

 centrations of the other purines were necessary. In comparative tests 

 the pea diffusate in every case showed more activity than adenine. 

 Miller confirmed reports of Bonner ( 1940) that KNO3 was also very 

 active in promoting growth of leaves. 



Miller (1951a) found that coumarin, normally considered to be 



