CORRELATION OF Xanthium AND LETTUCE SEED 307 



exposed for 8 hr each day to light intensities of 2000 or of 5000 ft-c 

 (at the tops of the plants). Seeds implanted in Xanthium plants ex- 

 posed for 8 hr per day to 5000 ft-c light (total of 40.000 ft-c hr/day) 

 responded like seeds in cockleburs exposed to 8 hr of 2000 ft-c light 

 (total, 16,000 ft-c hr/day) and not like similar seed implanted in 



Table IV. Effect of Day Length on Germination of 

 (Dark) Implanted Grand Rapids Lettuce Seed 



plants which were exposed to 20 hr of light daily of 2000 ft-c intensity 

 (total, 40,000 ft-c hr day). Thus, within the experimental limits de- 

 scribed, the lettuce seed germination response is related to the length 

 of the photoperiod to which the host plants are exposed, and not to 

 the total light energy which these plants received. 



Investigations were also made to determine whether treatments, 

 other than variation in day length, which interfere with photoperiodic 

 induction affect the lettuce seed germination response. Petioles of a 

 number of Xanthium plants were implanted with Grand Rapids lettuce 

 seed and enclosed in light-tight wrappers. The leaf blades of one lot of 

 plants was sprayed with an aqueous solution of 500 mg per liter of 

 indoleacetic acid (lAA) before each of the six 16-hr dark periods to 

 which the plants were subjected. This treatment is known to interfere 

 with the flowering of Xanthium exposed to photoperiods which are 

 normally inductive (Bonner and Thurlow, 1949). Only about 2% of 

 the seeds which were implanted in the sprayed plants germinated; the 

 plants which were sprayed with the lAA solution failed to flower when 

 they were subsequently maintained on a regime of 20 hr of light daily. 

 These data again support the conclusion that the lettuce seed germina- 

 tion response is related to metabolic changes associated with photo- 

 periodic induction of the host cocklebur plants. One disquieting note 

 has been the failure to get consistent support for this conclusion from 

 a few experiments in which interruption of the critical dark period by 



