REPORT OF THE SECRETARY 121 



dioxide as developed in this laboratory; (6) to determine chlorophyll 

 formation in the different regions of the spectrum; (c) to investigate 

 the wave-length balance associated with optimum plant production 



Many instrumental problems have arisen in connection with this 

 COg-measuring method which have prevented the full use of the ap- 

 paratus in many of the planned experiments. New heat exchangers 

 have been installed for better temperature control and other improve- 

 ments made. After making 67 test runs, each of which required from 

 5 to 6 hours, all but two of the problems have been overcome. 



The Division has recently obtained a suitable spectrophotometer 

 with which to continue its studies on chlorophyll formation. Work in 

 the study of wave-length balance and optimum plant growth has been 

 continued. 



(2) Plant growth and development a^ iniluen/)ed hy light. — The pur- 

 poses of this project are {a) to determine the mechanism of dormancy 

 in light-sensitive seeds, and (5) to study developmental physiology 

 of grass seedlings. 



Role of light in seed germination. — It has long been known that 

 germination of many species of seeds under certain conditions is very 

 markedly stimulated by, or entirely dependent upon, irradiation. 

 About 10 years ago a cooperative investigation carried out in this 

 laboratory (Flint and McAlister) demonstrated that only certain por- 

 tions of the spectrum are stimulatory to germination whereas other 

 regions are inhibitory. The mechanism of these effects of light has 

 remained completely obscure, however. 



Subsequent discoveries by other workers have suggested new ex- 

 perimental approaches to this problem which has been taken up again. 

 These discoveries are (1) that certain chemicals have the ability to 

 evoke germination in darkness and thus appear to act as substitutes 

 for light, and (2) that other chemicals act as germination inhibitors 

 in darkness but that light tends to overcome the inhibitory action. 



A considerable variety of compounds has been tested for ability to 

 promote germination of lettuce seeds in darkness at temperatures 

 which, in the absence of specific stimulations, permit germination only 

 in light. A number of active substances have been found. The tests 

 are being continued in an attempt to correlate the physiological potency 

 with molecular architecture. 



The light-sensitive inhibitory effect produced by coumarin does not 

 appear to be specific, being exhibited also by several other compounds 

 among the many which have been examined. Thus there is little 

 support for the suggestion made by other workers that coumarin, or 

 a chemically closely related substance, is responsible for the natural 

 light-sensitivity of lettuce seed. A report of this work is now being 

 prepared for publication. 



