My Campbell's Biology textbook discusses photoperiodism of plants and introduces us to short-day and long-day plants. What I don't understand is why short day plants will not flower if they are exposed to a very brief flash of light during the night, even if that brief period + the actual day's length is still less than their critical day length? Or, put more concisely:

Length of Flash + Length of Day < Critical Period <-- Why does the short-day plant not flower?

The following diagram is from Campbell's and makes no sense to me, because it also seems like a long-day plant which is exposed to a brief period of light during the dark will flower even if that period of brief light + its actual day's length is not greater than its critical period (i.e. it never fulfilled the pre-requisite for flowering).

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1 Answer 1


I think you are looking at other the wrong way around. There is no critical light period, there is a critical dark period. That long-day flower is also a short-night flower. As long as there is never any period of darkness that exceeds the critical dark period, it will flower.

In that second picture at the bottom, exactly that is the case. The dark period is not long enough to prevent the flowering.

And in the bottom example of the first picture, there is no uninterrupted dark period longer than the critical dark period, so the plant never flowers.

  • $\begingroup$ What doesn't make sense for me is this: "As long as there is never any period of darkness that exceeds the critical dark period, it will flower." Take a look at part (b) of the diagram and the very bottom scenario. The period of darkness is longer than the critical night period, but the plant is a short-night flower and thus requires a night period that is shorter--not longer--than its critical period. Even with the added flash of light, the yellow bar does not exceed the black bar. Could you please explain that part? That's mostly what confuses me. $\endgroup$ Commented Dec 7, 2015 at 15:43
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    $\begingroup$ The thing to remember is to not add them up. It only counts if it's in one "piece". For those flowers it's not like our sleep, where being awake for 5 minutes between two 4 hour periods still feels like a good night's sleep. For the long day plants, it's only important that no continuous dark period is longer than x hours. Does that help? $\endgroup$
    – YviDe
    Commented Dec 7, 2015 at 18:15

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