306 CONTROL OF REPRODUCTION 



mination data obtained in two series of experiments performed at dif- 

 ferent times and using different lots of seed. These results again suggest 

 that the effects observed are probably largely or entirely a consequence 

 of an inhibition of germination of seed implanted in petioles of cockle- 

 bur plants exposed to long days. These experiments also show that 

 photosensitive seeds are not required in order to obtain the germination 

 response. Thus, the initial objective, to determine whether a common 

 active material is produced as a consequence of the photoreactions in 

 the lettuce seed germination and photoperiodic induction systems, was 

 not achieved. Instead, lettuce seed germination appears to serve as a 

 bioassay for some metabolic change or changes associated with photo- 

 periodic induction of flowering in X ant Mum. 



What evidence is there that the responses are reflections of photo- 

 periodic induction of Xanthium and not, for example, responses to 

 duration or intensity of photosynthesis? In one set of experiments 

 Xanthium plants, the petioles of which were implanted with Grand 

 Rapids lettuce seed, were sprayed with a 5% sucrose solution daily 

 during the photoperiodic treatment. This application of sugar failed to 

 result in any appreciable alteration in the response of the implanted 

 seed (Table III). Another series of experiments was performed to 



Table III. Effect of Sucrose Applications to Xanthium on the 

 Germination of (Dark) Implanted Grand Rapids Lettuce Seed 



Germination, % 

 Photoperiod, hr 



{Xanthium) With Sugar Without Sugar 



8 29.4 28.8 



20 17.9 15.3 



determine the relation between the percentage of germination of the 

 implanted lettuce seed and photoperiodic treatment of the host. Table 

 IV contains the results of experiments using Grand Rapids lettuce 

 which showed about 80% germination in vitro in darkness at 20°C in 

 48 hr. It is interesting, though perhaps coincidental, that the break in 

 the germination response curve falls somewhere between 16 and 20 

 hr of light daily and that the critical day length for these Xanthium 

 plants is very close to 16 hr. Furthermore, the lettuce seed germina- 

 tion response was about the same whether the cocklebur plants were 



