NO. 20 LETTUCE SEED GERMINATION WEINTRAUB 3 



commenced. Seeds of this age were chosen so that there would be no 

 significant change in germinative energy during the period of some 

 months required for the study. Some of the tests were repeated with 

 variety Black-Seeded Simpson, with similar results. 



Two series of experiments were conducted. The first was designed 

 to determine approximately the effective concentrations of the various 

 substances and to evaluate the possible influence of variations in tem- 

 perature, spectral quality and intensity of the light, duration of the 

 germination period, and other details of technique. The general char- 

 acter of the results was found to be the same at temperatures from 

 16 to 23°, in red light and in white light, and over a hundredfold 

 range of light intensity. 



In the second group of tests a finer series of concentrations was 

 compared, all other conditions being kept uniform. Each treatment 

 was carried out in duplicate (lOO seeds per dish) and the experiments 

 were repeated one or more times at the critical concentrations. The 

 air-dry seeds were distributed on dry blotters in small Petri dishes, 

 amounts of the solutions slightly greater than required to saturate 

 the blotters were added, and the dishes placed, within a few 

 minutes, under the desired condition of light or darkness which was 

 maintained until observations were made 3 or 4 days later. The water- 

 jacketed thermostat employed as a germinator was provided with 

 several independent compartments so that a number of tests could 

 be made concurrently in light and in darkness without interference. 

 The temperature was maintained at 22.55 ±0.05° C. Illumination 

 was furnished by a 20-watt red fluorescent lamp 20 cm. above the 

 seeds with a sheet of window glass interposed. 



Results. — A number of compounds were found to inhibit germina- 

 tion to a greater extent in darkness than in light ; with increasing con- 

 centration the germination was progressively less both in light and in 

 dark, the inhibitory effect of a given concentration being always greater 

 in darkness. Data obtained in the present study suggest that the con- 

 centration-inhibition curves for various compounds may differ greatly 

 in shape. To assess the relative effectiveness of different substances 

 it would be desirable to determine concentration-inhibition curves 

 for each, so that comparison could be made either among the concen- 

 trations required for a given degree of inhibition, or of the effects 

 produced by a selected concentration. A complication arises in ex- 

 periments of this kind in that it is necessary to distinguish between 

 effects upon the initiation of germination and those on subsequent 

 development of the seedling. Several substances were found to permit 



