The eighteenth century knew Mimosa pudica by a variety of names that are no less familiar to us now: it is the “sensitive,” “shy,” “bashful,” and “humble” plant. It is the “touch-me-not,” “shameweed,” “morivivi (dies then lives),” and more. These names are related to one another in that they all refer to the plant’s remarkable ability to contract its leaves in response to being touched, an ability that is also reflected in the Latin species epithet assigned to the plant by Linnæus: pudica means “bashful” or “shrinking.”

Although the sensitive plant’s contraction of its leaves had been known at least since the ancient Greeks, this curious being ignited a particularly intense flame of fascination in the late 1700s. The double sense of “sensitivity,” as social (especially sexual) propriety and physical responsiveness, aided in this wide appeal. Poets and authors were attracted to this plant because it was a fitting metaphor for the age’s ideals of modesty and chastity, while naturalists sought to explain how it moved. Such explanations, though, remained wanting until the nineteenth century; and even then, the mechanisms of this plant’s contractions were not thoroughly understood. Presently, scientists still cannot fully explain its reactions to touch, that is, its seismonastic movements. In fact, current research on this plant only seems to compound its remarkable qualities, so much so that in the last decade writers and scientists have made an addition to the already long list of names: M. pudica is also the “intelligent” plant!

Recent research shows that the M. pudica learns and remembers. The collapse of its leaves prompted by a physical disturbance is thought to be a defensive mechanism that protects the plant against predation. Folding its leaves, however, does not come without a cost. It is estimated that the M. pudica’s photosynthetic rate drops by up to 40% when its leaves are closed, which means that every time it collapses its leaves, it sacrifices opportunities to forage light for energy. In short, if the plant continually collapsed its leaves, then it would end up harming and exhausting itself. But what if it were possible for the M. pudica to protect itself discriminately, that is, to fold its leaves only in reaction to actual threats? Monica Gagliano and her colleagues set out to answer this very question, publishing the results of their research in 2014. They conducted a series of experiments designed to test whether the plant can learn that a threat is innocuous by dropping it 60 times in seven consecutive sessions over the course of one day. They found that within the first session, after just the first four to six drops, the M. pudica already began to reopen its leaves; significantly, by the end of the training, its leaves not only reopened completely but also stopped closing at all. Lest one think that the plant was merely suffering from exhaustion, physically unable to respond, the researchers introduced a novel stimulus—a shake—which induced the plant to fold its leaves with the same vigor as in the beginning of its training. What is perhaps most remarkable is that when researchers returned to these plants 28 days later without having administered any forced stimulus in the interim, they found that the plants had remembered their previous training and did not react to being dropped. They had remembered that this threat was innocuous. Collapsing their leaves just wasn’t worth the effort.

By learning and remembering, M. pudica challenges foregoing conceptions about who or what is capable of such activities. Gagliano et al. underscore this point in the final line of their paper: “The process of remembering may not require the conventional neural networks and pathways of animals; brains and neurons are just one possible, undeniably sophisticated, solution, but they may not be a necessary requirement for learning” (p. 70). This plant was also a challenge to thought in the eighteenth century.

No single work better captures the response to this challenge than Thomas Percival’s short essay from 1785, “Speculations on the Perceptive Power of Vegetables,” in which he “attempts to show ... that plants, like animals, are endued with the powers, both of perception and enjoyment” (p. 4). The sensitive plant figures prominently in the work, in addition to being the topic of a supplement appended to its end. Not all naturalists, however, were ready to ascribe perceptivity or any additional capacities to plants, insisting that the collapse of the leaves could be explained just as one might explain the functions of a machine, that is, through the mechanisms of changes in pressure, the interaction of parts, magnetic force, and so on. Darwin refers to these explanations in his verse. Yet, they were ultimately unsatisfying, leading naturalists to drag nearly every theory then in vogue into the fold. Emerging theories of electricity, oxygen, irritability, and galvanism (animal electricity) were all called upon to explain the seemingly inscrutable sensitive plant.

Naturalists in this period made an interesting connection between the collapse of the leaves in response to touch (that is, the seismonastic movements of the plant) and the drooping of the leaves at night (called the sleep of plants in the eighteenth century and nyctinastic movements now). Though the infamous botanist John Hill is reluctant to use the word “sleep,” thinking it wholly improper, he uses it anyway, since his tract on the phenomenon is written as an open letter to Linnæus who first coined the term. Hill reports a series of experiments that he conducted in order to ascertain the cause of plant sleep. He considers those agents that come into contact with the plant: heat, light, moisture, and air. By process of elimination, he deduces that light must be the cause. In short, he thought that light vibrated corpuscles within the plant, which kept its fibers taut. Once that light was extinguished, so were the vibrations that were keeping the fibers taut, which led to the drooping of the leaves. The sensitive plant, he thought, worked in the same way, except that instead of an absence of light, a violent shock or touch was now the culprit in disrupting the taut fibers of the plant. Hill intended his explanation to be wholly mechanical, one that only relied on the physical properties of plants and nothing more.

Darwin had as much fascination with the sleep of plants as Hill, as evident in the M. pudica footnote, but he took a much different approach. For Darwin, the sleep of plants was important because it functioned as the premise for a much larger claim about the vegetable world. He writes, “The sleep of animals consists in a suspension of voluntary motion, and as vegetables are likewise subject to sleep, there is reason to conclude, that the various actions of opening and closing their petals and foliage may be justly ascribed to a voluntary power: for without the faculty of volition, sleep would not have been necessary to them” (p. 117n). Unlike Hill, for Darwin, plants really sleep. Moreover, since sleep is the temporary abolition of voluntary power, plants must then have voluntary power as well, since sleeping would be entirely superfluous to a being without volition. What does a faculty of volition look like in plants? For Darwin, it is the fact that they close their petals or leaves during inclement weather, in the absence of light, “or from mechanic violence” (p. 124).

As incredulous as Darwin’s claim surely appears, that a plant has volition it relinquishes in sleep, does it not seem just as strange to say that a plant remembers and learns? We, no doubt, are still learning as well, for the M. pudica lives as vividly in the imagination today as it did in the eighteenth century.