308 CONTROL OF REPRODUCTION 



light has been used to interfere with floral induction. This point re- 

 quires considerable further study. 



Finally, the effects of a few possible inhibitors of lettuce seed germi- 

 nation were examined. The effects of xanthinin, coumarin, and lAA 

 were investigated. Crystalline xanthinin was prepared according to the 

 method of Geissman et al. (1954). This compound had no effect on 

 the germination of lettuce seed in vitro at any concentration tried, in- 

 cluding 1000 mg per liter. Earlier reports on the inhibitory effect of 

 coumarin on the germination of lettuce seed and the reversal of this 

 inhibition by light (Evenari, 1957) were confirmed. The reversal by 

 light appears to be inconsistent with the nature of the inhibition of 

 germination of seed implanted in plants exposed to long photoperiods 

 (e.g., Table I). Thus, inhibitors of the coumarin type seem to act 

 differently from the inhibitor or inhibitors which are active in the im- 

 plantation experiments. 



The germination in vitro of the Grand Rapids lettuce seed used in 

 these experiments was little affected by lAA in concentrations up to 1 

 mg per liter. Fifty percent of the seed germinated in the presence of 10 

 mg per liter lAA. At concentrations of lAA which did not affect seed 

 germination, a swelling of the seedling radicle was observed. At con- 

 centrations at which the percentage of germination was decreased, the 

 cotyledons emerged but the radicles did not. By contrast, seedlings pro- 

 duced from those seeds which had germinated in the petioles of 

 Xanthiiim plants exposed to long-day conditions appeared normal. 

 Thus, it appears that the pattern of inhibition of germination by lAA 

 in vitro does not correspond to that observed in the implantation ex- 

 periments. The active material in cocklebur plants is being sought. 



Summary 



The germination of lettuce seed implanted in petioles of cockelbur 

 plants shows an inverse correlation with the duration of the daily light 

 period to which the hosts are exposed. The evidence thus far obtained 

 strongly suggests that the germination response is a function of meta- 

 bolic changes associated with floral induction of the Xanthiiun plant. 

 Additional evidence along this line is being sought. The germination 

 response can be interpreted as an inhibition of germination of seeds 

 implanted in petioles of Xanthium plants exposed to long-day condi- 



