critical day length

The period of daylight, specific in length for any given species, that appears to initiate flowering in *long-day plants or inhibit flowering in *short-day plants. In actual fact long-day plants will not flower if the dark period exceeds a certain maximum and conversely short-day plants will not flower unless the dark period exceeds a certain minimum. These periods are termed critical dark periods and must be continuous to have effect (see night-break effect ).