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CONTROL OF REPRODUCTION 



Fig. 1. Lettuce seeds implanted in the petioles of Xanthium plants and 

 covered with aluminum foil. 



darkness at 20-2 1°C. After 4 to 6 days the leaves with implanted 

 petioles were removed and the number of germinated seeds was de- 

 termined; all the Xanthium plants were then returned to the green- 

 house where they were maintained on a 20-hr photoperiod and ob- 

 served for appearance of flowers. Seeds of Grand Rapids Tipburn Re- 

 sistant No. 1 lettuce were used in all but a few experiments in which 

 seeds of White Paris Self-Folding Cos lettuce, a variety which is not 

 photosensitive, were used. Several different lots of seed of each variety 

 were used in the course of the experiments, and the same trends were 

 observed with the various lots, although absolute values varied. Gen- 

 erally, at least three plants (nine implanted petioles) implanted with 

 a total of 150 to 400 seeds were used in each treatment. 



Figure 3 (upper) shows a dissected petiole from a cocklebur plant 



Fig. 2. Lettuce seeds implanted in Xanthium petioles and covered with 

 Saran Wrap. 



