94 PHOTOCONTROL OF GROWTH 



Table II. Development of Dormancy in Grand Rapids Lettuce 

 Seeds Fully Promoted and Then Held at High Temperatures" 



Per Cent Seeds Germinating at 20°C 

 After Period at 



35°C 



94 

 85 



55 



20 



6 



95 



° After being imbibed 1 hr at 20°C in darkness, seeds irradiated 4 min 

 in red and then transferred to 30° or 35°; after various periods at the 

 high temperatures, seeds transferred to 20° in darkness for 2 days for 

 germination. 



'' Seeds given an additional 4 min of red at time of transfer to 20°C. 



The interaction of temperature and light response is evident in 

 Lepidium in a different way (Toole et al., 1955a), At constant 

 temperatures of 15° or 25 °C, seeds of Lepidium virginicum show a 

 definite response to light, but maximum germination is below 50%. 

 However, if after imbibition for 48 hr at 15°, followed by a saturating 

 exposure to red light, the seeds are transferred to 25°, the germination 

 is markedly increased (Table IV). The photoreaction had been the 

 same in all sets of tests, but germination of many of the seeds was 

 blocked when they were maintained at a constant temperature. This 



Table III. Germination at 20°C with and without 



Irradiation of Great Lakes Lettuce Seeds Previously 



Held Imbibed at 35° for 4 Davs" 



Period of Irradiation, min 



Seeds Germinating 



Red Far Red at 20°C, % 



11 

 10 95 



1 1 95 

 1 32 22 



" Germination 95',^ at 20° in darkness when not ex- 

 posed to 35°. 



