Flowers in Action

by Carl Strang

Plants don’t move much, at least not at a speed that fits our normal use of that verb. It’s possible to forget for a moment that they are alive and not simply passive objects. But in recent days while on my rounds at Mayslake Forest Preserve I have noticed that trout lily flowers, once they have opened, do not stay that way all the time.

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My first thought was that this active opening and closing conserves pollen for times when pollinators are active. The low flowers point downward, after all, and if they were bumped by a passing animal or shaken by a strong wind, perhaps some pollen would be lost. Is their pollen a little less sticky than usual for an insect-pollinated plant?

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Another possibility, given the frequent rains we have been getting and the low stature of these plants, is that the flowers close in response to cues that there may be a danger of raindrops splashing up into flowers and removing pollen that way.

I did some literature searching, and found that many species of trout lilies occur around the northern hemisphere, in Europe and Asia (including Japan) as well as North America. Where they have been studied in some detail, their most common pollinators are bumblebees (which would have to be queens this early in the season), followed in importance by honeybees and local native species of bees. The low, inverted flowers require these pollinators to cling to them while taking nectar and pollen. They produce large amounts of pollen but get few pollinator visits so early in the season, so they are efficient: only 2-3 pollinator visits are enough to fertilize all of a flower’s ovules. By the way, a study of the Japanese species found the average life expectancy of these plants to be an impressive 40 years.

There was some mention of the flowers closing, in the form of general observations (unsupported by quantitative data) that they close at night or on cloudy days. One speculative comment was that the flowers close to facilitate self fertilization, but more persuasive are the general comments on flower closing by the venerable pioneer plant ecologist Frederic Clements. In his text Plant Physiology and Ecology, he emphasizes that flower closing has several advantages. It conserves pollen for times when pollinators are active. It prevents “injury” to pollen by rain and dew, by which Clements means interference with pollen removal by pollinators, and also the premature stimulation of pollen germination. Finally, flower closing conserves nectar. He mentions that temperature rather than light per se is the usual physical stimulus.

This seems like it could be a good study for students in upper elementary to high school grades, and in some future spring I want to collect some data. It’s too late to do much with this in the 2009 season, but in the future I want to accumulate some records of times and conditions (cloud cover, temperature, relative humidity) when the flowers are open and closed, to try and get a handle on what these active plants are up to.

Some of the authors I encountered, in addition to Clements: P. Bernhardt, Yukio Ishikawa and Kazushige Honda, Shoichi Kawano, Alexander F. Matten.

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