XO. 20 LETTUCE SEED GERMINATION — -WEINTRAUB 5 



emergence and some elongation of the root and shoot but then to exert 

 secondary toxic effects upon one or the other organ. A similar ob- 

 servation has been made by Audus and Quastel (1947) in the case of 

 coumarin. As the criterion of germination used in this study is elon- 

 gation of the seedling axis, a secondary suppression of such elongation, 

 if sufficiently great, renders impossible the accurate determination 

 of the germination percentage. 



For this reason it was thought preferable to compare the lowest 

 concentrations (within a twofold range) that showed a significant dif- 

 ference between light and dark. At such concentrations there was no 



Table 2. — Effect of several less active compounds on germination of Grand 

 Rapids lettuce in light and in dark 



Molar 

 Compound concentration 



Sucrose o.i 



Maltose i 



Lactose i 



Glucose I 



Fructose i 



Galactose i 



Mannose i 



Arabinose 15 



Xylose 15 



Sorbitol I 



Mannitol i 



Ascorbic acid i 



a-Alanine i 



/9- Alanine i 



uncertainty in classifying a seed as germinated or not: a portion of 

 the population showed no macroscopic development at all, while the 

 remainder had radicles at least 4 to 5 mm. long. 



In table i are listed, in descending order of activity, some com- 

 pounds which, at approximately the same order of magnitude of con- 

 centration, act similarly to coumarin. In table 2 are listed several 

 substances which are much less toxic than the foregoing, but which, 

 in sufficiently high concentration,^ also cause greater suppression of 

 germination in darkness than in light. 



2 Whether the high osmotic pressure of these solutions is entirely responsible 

 for their inhibitory effect is not known. Stout and Tolman (1941) found that 

 comparable inhibition of New York No. 12 variety of lettuce required concen- 

 trations of sodium chloride or sucrose equivalent to 5 or 6 atmospheres. 



