Abstract
HYBRID intermediacy in night requirements for flowering has been reported in a study1 of latitudinal variation of photoperiodic adaptation in cockleburs (Xanthium strumarium L.). The report stated that the data showed that different strains produced F1 hybrids photoperiodically intermediate between the parents and that segregation for critical night lengths was observed in the F2 generation. Ray (personal communication) feels, however, that the hybrid evaluation was possibly affected by facultative apomixis. I have also demonstrated2,3 intermediacy of the photoperiodic response in F1 hybrids in Xanthium but have not found evidence of apomixis in emasculated plants.
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