Photo above of Mimosa pudica, Sensitive Plant (leaves open), from native habitat in Goa India. Photo by J.M. Garg, from Wikipedia. This plant also displays reaction to touch, thigmonasty.
We don’t know whether sleeping plants dream. That link takes you to an essay I wrote 14 years ago; I commented then that, although plant responses to light and darkness (nyctinasty) had been known for centuries, scientists these days had “mostly ignored” this line of research. That’s no longer true, it seems, so the story needs an update, as some puzzles are slowly being unraveled. Darwin would be proud.
Mimosa pudica, leaves closed, Photo from Wikipedia, by “Bluemoose.”
Darwin was right
Indeed, as with so many topics, Darwin was the original researcher, the observer and experimenter who explored the esoteric. The Power of Movement in Plants, published in 1880, was his next-to-last book, and he despaired at times of ever finishing it!
Darwin to Sir Joseph Hooker, March 25, 1878:
I think we have proved that the sleep of plants is to lessen the injury to the leaves from radiation. This has interested me much, and has cost us great labor, as it has been a problem since the time of Linnaeus. But we have killed or badly injured a multitude of plants.
Darwin to Asa Gray, Oct 24, 1879:
I have written a rather big book—more is the pity—on the movements of plants, and I am now just beginning to go over the MS. for the second time, which is a horrid bore.
Darwin to DeCandolle, May 28, 1880:
My MS. relates to the movements of plants, and I think that I have succeeded in showing that all the more important great classes of movements are due to the modification of a kind of movement common to all parts of all plants from their earliest youth.
I just happen to have a handy copy of The Life and Letters of Charles Darwin, compiled and edited by his son Francis, but Darwin's letters are now also available online where we all can explore at will.
Modern Science Jumps In
Some plants, such as Maranta, are equipped with specialized joints that control their daily movements. These structures, called pulvini (sing. pulvinus), occur where the leaf blade joins the petiole, functionally somewhat like the wrist joint connecting your hand and forearm. Rapid movements should be suspected in plants that have obvious pulvini—e.g., in Spathiphyllum, which wilts dramatically in an attempt to remind you to water it, then recovers with equal alacrity when you do.
Don’t you just love scientific writing? I was going to entertain you with terrific information about phytochrome and potassium fluxes and glucosidase, but I'd rather stick to what I can see and understand (sometimes) and appreciate (always!)... I can offer a picture (you'll have to click to be able to read it):
I’m so out of practice that, even when I understand the individual words and phrases, it can be tough to extract meaning from some passages (try your skill with samples at the end of this post). As Alice said to the Caterpillar:
I’m afraid I can’t put it more clearly, for I can’t understand it myself to begin with.
However, science is like a foreign language, where sometimes you can get the general sense of things without exactly being able to translate it word for word. Here goes with a few gleanings.
- Light hitting leaf blades does nothing, but if it hits the pulvinus, the leaf reacts.
- Ergo, the pulvinus is the photoreceptor and reacts independently of other pulvini.
- Phytochrome is the pigment that keeps leaves from opening when it’s dark.
- Phytochrome controls the direction of potassium movement, which controls water movement and hence cell turgor.
One of these is a leaf-opening factor that “awakens” plant leaves, and the other is a leaf-closing factor that reverses this process such that the plant leaves “sleep”. …significant changes in the concentration of the ratio between leaf-closing and -opening factors in the plant are responsible for leaf movement. And this is a universal mechanism in five nyctinastic plants. …
The motor cells in the pulvini of nyctinastic plants consist of two types: extensors and flexors. Leaflets move upward during closure and downward during opening due to the actions of the extensors located on the upper side of a leaf and the flexors on the lower side. …
So these findings represent an important advance in the bioorganic study of nyctinasty and provide important clues regarding the molecular mechanism of nyctinasty, which has been a historical mystery since the era of Darwin.
To me, the fun thing about this article is that, while the mechanism is universal, the pairs of leaf-opening and leaf-closing factors were discovered to be different chemicals in each of the five species studied! How cool is that? (For extra cool, note that Ueda et al. also address the question of memory in plants, specifically Venus Fly-traps and their mechanism for leaf closure.)
In the case of sleeping leaves that rise, as in Maranta and most others, it seems logical that the sleep position is a tense one, and the leaf relaxes into its daytime posture. If so, then raised leaves are actively holding a position, and wilting must occur through some other process. In Maranta, it does; the leaves relax even more, and leaf margins roll inward as turgor is lost.
So far so good. But these articles tend to use Albizia julibrissin or other leguminous species. What about species whose leaves adopt a drooping posture in sleep (e.g., Oxalis)? Are the "extensors" and "flexors" reversed, or do they respond to different signals? In Oxalis, wilting and sleeping postures may be difficult to distinguish. The upper surfaces are relatively exposed, and covering the lower surfaces may help reduce transpiration, reducing further water loss.
With all that we know about plants, it's nice to know a few mysteries remain. It seems no one has answered the ultimate question: If sleeping is so advantageous, why don't more plants do it?
C. Darwin. The Power of Movement in Plants, John Murray, London (1880).
C. Darwin. Insectivorous Plants, John Murray, London (1875).
Find lots more references by searching nyctinasty at Google Scholar. Here are a few excerpts from just a few samples over the decades to get you started...
Illumination of pinnule tissue alone induced no response, while illumination of an area as narrow as 1 mm, including only the tertiary pulvini and adjacent portions of rachilla and pinnules, was sufficient for a full response. This suggests that the pulvini themselves, the sites of the response, act as photoreceptors. In experiments with various shielding devices, pinnules on the same rachilla responded independently to local illumination, suggesting the absence of any translocatable effects.
Koukkari, Willard L. and William S. Hillman 1968 Pulvini as the Photoreceptors in the Phytochrome Effect on Nyctinasty in Albizzia julibrissin Plant Physiology 43:698-704.
Prolonged irradiation during appropriate parts of the diurnal cycle promotes the opening of Albizzia sic julibrissin leaflets. Leaflets also open without illumination, but such opening starts later and is slower and less complete. Opening in the dark is accompanied by lower potassium efflux from dorsal pulvinule motor cells but equal or greater potassium movement into ventral motor cells than occurs during opening in the light. Far red-absorbing phytochrome inhibits opening in the dark… i.e., a high far red absorbing phytochrome level is associated with low potassium content in ventral motor cells, high potassium content in dorsal motor cells, and a small angle between leaflets.
Satter, Ruth L. and Arthur W. Galston 1971. Phytochrome-controlled Nyctinasty in Albizzia julibrissin: III. Interactions between an Endogenous Rhythm and Phytochrome in Control of Potassium Flux and Leaflet Movement. Plant Physiology 48:740-746.
The nyctinastic leaf movement is induced by a pair of leaf-movement factors, and one of each pair is a glucoside. There are two key proteins that are involved in the control of nyctinasty. One is -glucosidase: a biological clock regulates the activity of -glucosidase, which deactivates the glucoside-type leaf-movement factor, controlling the balance in the concentrations of the leaf-closing and -opening factors. The other is the specific receptor for each leaf-movement factor: the genuine target cell for each leaf-movement factor is confirmed to be a motor cell from leaflet pulvini, and the specific receptors that regulate the turgor of motor cells are localized in the membrane fraction. (Ueda et al., 2007.)
Ueda, Minoru, Yoko Nakamura, and Masahiro Okada. 2007. Endogenous factors involved in the regulation of movement and “memory” in plants. Pure Appl. Chem., Vol. 79, No. 4, pp. 519–527, 2007.