For most short-day plants in the wild, the length of the night will first reach its threshold value in autumn or winter, and typically at the most polar latitudes at which that species is found. Nights will continue to get longer as the year proceeds, and therefore the onset of flowering will sweep through the latitudes inhabited by that plant like a wave moving towards the equator. A short-day plant will not flower if the night's darkness is interrupted by even a brief flash of light. In other words, the plant does not flower in response to the actual length of time from sunset to sunrise, but rather to the longest period of unbroken darkness that it experiences. The receptor for photoperiodism is in the leaves.
Some plant species integrate other environmental parameters into the decision to flower.
For instance, winter wheat is a long-day plant which must first experience a long period of cold temperatures before it can respond to short nights by flowering. Each plant species has several similar kinds of phytochrome, each one composed of a protein homodimer combined with a non-proteinaceous pigment or 'chromophore' that absorbs light. The chromophore molecule of phytochrome can exist in two different 3-dimensional conformations. Phytochrome is synthesized in the P r conformation but is rapidly converted to P fr if exposed to red light. Under sustained sunlight it remains in that conformation.