304 CONTROL OF REPRODUCTION 



promotive effect of illumination on the germination of the seeds im- 

 planted in petioles of plants maintained on 8 hr of light daily (Fig. 4, 

 upper) is apparent, but there was no comparable influence on seeds 

 implanted in petioles of plants subjected to 20-hr photoperiods daily 

 (Fig. 4, lower). 



The first questions raised by these observations were: Is the response 

 of implanted Grand Rapids lettuce seed a result of stimulation of 

 germination when the Xanthium host plants are subjected to several 

 8-hr photoperiods; are these results a consequence of inhibition of 

 germination of seed implanted in Xanthium plants maintained on 20 

 hr of light daily; or, does a combination of these influences lead to 

 these results? These questions are not easily answered by germinating 

 the lettuce seeds in darkness on moist filter paper in petri dishes (desig- 

 nated hereafter as "germination in vitro") because germination in vitro 

 is not entirely comparable to germination in Xanthium petioles. There 

 is no readily available independent control situation, so one must com- 

 pare the results when the Xanthium hosts are subjected to different 

 experimental conditions. The question of inhibition vs. stimulation of 

 germination was approached experimentally in two ways. 



First, two groups of seeds of Grand Rapids lettuce were treated in 

 the following manner. "High dark germination seeds" were prepared 

 by exposing imbibed seed to red light (660 m^^x-G-A-B. Interference 

 filter) and then drying them. "Low dark germination seeds" were 

 prepared by germinating lettuce seeds in petri dishes on moist filter 

 paper for 48 hr in darkness at 20°C and then drying all the seeds in 

 darkness. The seeds of the latter group which had germinated were 

 then removed and discarded. 



Table I shows the results obtained in implantation experiments when 

 either high dark germination seed (80% germination in vitro at 20°C 

 in darkness in 48 hr) or low dark germination seed (less than 2% 

 germination in vitro). The results with low dark germination seed im- 

 planted in Xanthium petioles and covered with aluminum foil support 

 the conclusion that there is little or no stimulation of lettuce seed 

 germination when the hosts are maintained under 8-hr photoperiods. 

 On the other hand, the results with the high dark germination seeds 

 indicate a marked inhibition of germination when the host plants are 

 exposed to 20 hr of light daily. These results suggest, too, the possibil- 



