• Question: Why do flowers turn and follow the sun? Why do flowers close when the sun has gone?

    Asked by Zealousy to Chris, Josh, Rebecca, Rob, Susan on 15 Jun 2015.
    • Photo: Susan Cartwright

      Susan Cartwright answered on 15 Jun 2015:

      – Health warning: I am not a biologist or a botanist. Treat this answer with caution. –

      This is actually a good question, and I’m not sure the answer is really well established. Plant *leaves* follow the Sun because sunlight is needed for photosynthesis, so if the leaves are turned to maximise exposure to the Sun they will maximise the growth potential of the plant. Flowers don’t photosynthesise, so this is not the reason that they follow the Sun (and not all flowers do).

      It is possible that turning to follow the Sun makes the flower more attractive to insect pollinators: perhaps it looks brighter in full sunlight, or perhaps the added warmth is an attraction for insects, which need heat to function effectively.

      It is also possible that turning to follow the Sun after pollination makes the pollen grow better or assists the development of the young seeds (because the warmth helps the relevant chemical reactions).

      As for why some flowers close at night, this is probably to protect the delicate interior of the flower – the bits that are needed to form the seeds – during a time when the flower’s insect pollinators are not active. Some flowers are pollinated by night-flying insects, and some of these do the reverse, opening in the evening and closing during the day.

    • Photo: Josh Meyers

      Josh Meyers answered on 15 Jun 2015:

      The difference is in the plant stem. A plant hormone called auxin causes the stem to elongate. If there are equal amounts of auxin on each side of the stem, the plant will grow directly upwards.

      However, UV light degrades auxin! So there is always less auxin on the sunny side of the plant. This means the other side elongates faster causing the plant to bend towards the light! Clever right?

      The variation in plants is tremendous! So there might be many reasons a flower will close. Usually it is not to do with the sun, but to do with pollination! If a flower has been pollinated, the flower will close in order to produce seeds.

      However there are cases where a plant will close in order to retain water inside the plant. Plants respire at night and this produces water, plants close to avoid losing the water they produce.

      Here’s a cool video of a dandelion being pollinated and producing seeds:

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