Short-day plants are really counting the night
- The idea
- Predict
- See it
- Why
- Check
The idea
“A short-day plant flowers once the days get short enough.”
In nature, short days always come with long nights, so the day looks like the signal. The names short-day and long-day were given before anyone tested the night.
The short version
The mistake: A short-day plant flowers once the days get short enough.
What's actually true: Short-day and long-day plants time flowering by their longest unbroken stretch of darkness.
- Phytochrome, a light-sensing pigment, switches form in light and slowly switches back in the dark. That helps the plant time the night.
- A short-day plant flowers when the night is longer than a critical length. A long-day plant flowers when the night is shorter.
- A flash in the night restarts the count. A break of darkness in the day changes nothing.
On the exam: Find the longest unbroken stretch of dark. That's the number the plant reads.