• Question: why do the flowers close up at night time

    Asked by farrell to Aime, Akshat, Diana, Gemma, Judith on 14 Jun 2011.
    • Photo: Judith McCann

      Judith McCann answered on 14 Jun 2011:

      Not all plants do! like night blooming jasmine…

      anyway, there seems to be lots of reasons why it would so I’ll try and order them a bit 🙂

      It is most likely to protect the pollen in the flowers. At night time the cooler air means that water vapour condenses on most sufaces, leaving dew, which would stop the pollen from travelling as far or as effectively(if its a fine powder it will stick to the damp surfaces)

      This seems to have stuck up a debate in my office (so good question!) but I’ll put the other theories in here too.. it could be due to the plant protecting itself from water loss-if it closes up, there will be less surface for water to evaporate from.

      And the other theory it that it could be the flower protecting itself, since most pollen-carrying insect do their business in the day time, its stop moths and nighttime bugs bumping into the flowers and loosing the pollen. Maybe on of the other scientists can clear it up better…

    • Photo: Gemma Sharp

      Gemma Sharp answered on 14 Jun 2011:

      Really good question! As far as I know scientists haven’t been able to prove why this happens, but there are a few theories. The behaviour must have evolved for a good reason (else it wouldn’t exist!).

      I don’t think the flower would close to prevent water loss through evaporation, because water loss is more likely in the day when temperatures are higher. Night time bugs can spread the pollen too, so I don’t think many flowers would evolve to close to avoid them… I think the most likely reason is probably that flowers close up to protect their pollen.

      The posh scientific term for flowers closing up at night is ‘nyctinasty’ (good word!), and it’s an example of a ‘circadian rhythm’ which is something plants, animals, etc do every day at the same time.

      Flowers will still close up at night time even when they’re kept in total darkness for 24 hours, but they might do it a couple of hours or so early or late. That’s because they need to be able to detect the light/dark to be able to time it perfectly.

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