Tag Archives: thermonasty

plants falling asleep

White Oxalis purpurea closing up for the evening
White Oxalis purpurea closing up for the evening.
Detail of white Oxalis purpurea thinking about some shut-eye.
Purple-leaved Oxalis purpurea closing up in the late afternoon shade.

A lot of the flowering plants in the garden don’t bother opening their petals until the sun’s up and then shut their flowers as soon as the light begins to fade and temperatures drop in the afternoon. Over the weekend I was noticing this going on with my oxalis plants and, less dramatically, with my arctotis.

There must be a name for this behavior, I thought, and so off I went looking for an answer. Before long up pop three interesting words: photonasty, thermonasty and nyctinasty.

According to one of the sources, the Textbook of Botany by Chhatwal and Singh, photonasty happens when a plant senses light and reacts to it by opening or closing its flowers. Because of this, morning glories open in the…well, morning. Then there’s thermonasty, where flowers react primarily to temperature. Tulips will open with a rise of 2-3 degrees Celsius, while a crocus will zip open when the temperature rises just a half degree.

And then there’s the more complex phenomenon of nyctinasty, which “is influenced by the intensity of light and also temperature differentials, the former stimulus being more powerful and effective. The foliage leaves and also the floral leaves in many species of plants…attain different positions at day time and at night viz during the day, the leaflets remain open or spread up in case of Oxalis, clever beans, alfalfa, etc., while by the onset of darkness they close down. This is also known as sleep movement.”

Yesterday afternoon was pretty bright, but cool. The oxalis barely opened before shutting back up. So it requires both heat and warmth to open fully. So nyctinasty makes sense. The arctotis seemed to open more fully, earlier in the day. My guess is that they respond more simply, mainly to light, which would mean that they exhibit photonasty. (What’s truly going on could be lots more complex than this and really might only be solved by experimentation, a point made in an article, “Flower opening and closure: a review” by Wouter G. van Doorn and Uulke van Meeteren in the Journal of Experimental Botany. Read the interesting text [ here ].)

Next I need to find out what “clever beans” are.

In my web trawl it turns out I’m not the only garden blogger looking at this phenomenon this week. Tilthy Rich took a quick spin around nyctinasty [ here ]. Maybe he has the same plants blooming, making him ask the same questions…

Flowers of Arctotis acaulis 'Big Magenta' beginning to fold up for the night.

Another clone of Artotis acaulis closing up in the afternoon: Photonasty? Thermonasty? Nyctinasty?