6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I07 



DISCUSSION 



Consideration of the studies of lettuce germination reported in the 

 literature together with results of as yet unpublished experiments of 

 the writer leads to the following view : 



Dormancy and germinability of lettuce seed appear to be controlled 

 by a delicately balanced mechanism which is highly sensitive to a di- 

 versity of factors. The response of a given lot is influenced both 

 by the integrants of the environment at the time of testing and by the 

 internal condition of the seed, which is determined in part by its his- 

 tory from the time of harvest or even earlier. The precise effect of 

 each of the several operative external and internal elements is depen- 

 dent upon the concurrent status of all the others. 



Light is not essential for germination of lettuce. Depending upon 

 conditions it may or may not be favorable. Germination of all the 

 viable individuals of a given population can occur in darkness provided 

 that the constellation of influential internal and external factors is 

 optimal. When this is not the case, as in freshly harvested seed or at 

 elevated temperature, the suppression of germination which is found 

 in darkness can be overcome to some degree by illumination. 



Similarly, light tends to counteract the adverse influence of exog- 

 enous chemical germination inhibitors. The foregoing results demon- 

 strate that this behavior is independent of the specific inhibitor. In 

 other words, stimulation by light, which obviously can be manifested 

 only under conditions which are suboptimal for germination, is a gen- 

 eral response to such conditions. 



The finding that chemical induction of light-sensitivity is not a spe- 

 cific effect of coumarin, or even of compovmds possessing similar mo- 

 lecular groupings, would appear to weaken materially, although not 

 necessarily to disprove, the hypothesis that coumarin plays a role in 

 the natural dormancy of lettuce seed. 



SUMMARY 



It has been proposed that coumarin may be the endogenous germi- 

 nation inhibitor responsible for light-sensitivity of lettuce seed, on the 

 basis of the finding that the inhibitory action of this substance is greater 

 in darkness than in light. In the present paper it is shown that the 

 effect is not specific for coumarin. Other compounds demonstrated 

 to act similarly are parasorbic acid, melilotin, y-pyrone, sorbic acid, 

 hydrocinnamic acid, sodium fran.y-cinnamate, sucrose, maltose, lactose, 

 glucose, fructose, galactose, mannose, arabinose, xylose, sorbitol, man- 

 nitol, ascorbic acid, a-alanine, and /?-alanine. 



