Long-day plants can be defined as those which flower when the length of the days is longer than the usual critical day length, i.e the photoperiod is longer.
On the contrary, the short-day plants are those which flower when the length of the days is shorter than the usual critical day-length, i.e. photoperiod is shorter.
Chrysanthemums are short-day plants while spinach is a long-day plant. These two plants can never flower at the same place at the same time because of the differences in their required photoperiods.
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