Why Do Flowers Close at Night?

Written by rebecca miller
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Why Do Flowers Close at Night?
Pistil, or stigma, of a flower (Image by Flickr.com, courtesy of Olivier)

Science has not been able to prove definitively why flowers close at night. There are, however, two main theories, both of which have to do with the preservation of reproductive organs and protection from the weather.

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Mechanism

Certain types of plants contain a mechanism, called nyctinasty, that allows them to open in the day and close at night. Scientists know how this occurs, but they are not yet entirely sure why.

Features

Motor organs and genes enable nyctinasty, which is affected by light intensity, humidity and temperature in some species of flowers.

Why Do Flowers Close at Night?
California poppy opens for the day (Image by Flickr.com, courtesy of Cliff Hutson)

Internal Clock

Flowers and plants have their own circadian rhythm (internal clock) that triggers the opening and closing. Centuries ago, the first experiments were conducted where plants were placed in total darkness but still opened and closed with no change of light, temperature or other stimulus.

Theory One

According to Ric Rupnik, a scientist from the Newton division of the Argonne National Laboratory, flowers close at night to conserve energy for pollination during the day, when insects are most active.

Why Do Flowers Close at Night?
Poppy closing for the night (Image by Flickr.com, courtesy of Powi) (Per Ola Wiberg)

Theory Two

A flower closes its petals to protect its pollen from getting wet from dew. Dry, sticky pollen is more easily transferred to and by insects, improving the plant's reproduction prospects.

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