Ask Mr. Smarty Plants

Mr. Smarty Plants would very much like to give you an exact number, a ball-park estimate, or even a swing-in-the-dark guess, but we really don't know of the information you're requesting having ever been collected and compiled anywhere. Are you up to the challenge? If we had to characterize its commonness, we would say that the number of species exhibiting nyctinastic flower response (closing at night) is a small percentage of the total number of flowering plant species, but an impressive number nevertheless.

Nastic movements are relatively rapid movements of plant parts (most often leaves and flower parts) in response to an external stimulus, but independent of direction. Nastic movements are distinguished from tropic movements (phototropism, gravitropism, etc.) which cause a directional response either toward or away from the stimulus.

Some examples of nastic movements include nyctinasty, which is night- or circadian-induced, thigmonasty or haptonasty, which is touch-induced, hydronasty which is humidity-induced, seismonasty, which is shaking- or vibration-induced, and thermonasty, which is temperature-induced. By the way, Mr. Smarty Plants proposes the term hydronasty be changed to hygronasty since the Greek root, hygro- specifically refers to humidity.

Much is known about the mechanism of nastic movements (how plants actually go about moving their plant parts) and about the causal stimuli. What really remains a mystery is why certain plants exhibit these responses at all. Theories abound, but it seems that all of the theories have shortcomings and there probably isn't a one-size-fits-all rationale for the various responses observed. Many plants exhibit nastic responses to an entire suite of stimuli.

While it is natural to assume that nyctinastic responses are immediate reactions to darkness, experiments have shown that plants will continue making nyctinastic movements at about the same time each day in continuous light or continuous dark conditions. Thus, the response is circadian in nature. After several days of either 24-hour light or 24-hour darkness conditions the effect will diminish. Nyctinastic movements often have a temperature-related (thermonastic) component.