HOW, I HAVE often wondered, did people first discover the highly specific applications of particular plants and herbs? That ginseng improves energy, say, or that ginger alleviates nausea, or that horsetail, which contains silica, might help hair to grow? One theory, possibly apocryphal and certainly much maligned by modern medicine, is that the physical characteristics of plants themselves provided clues as to how they might be used. This notion, known among scholars of ethnobotany and practitioners of herbal medicine as the doctrine of signatures, holds that plants have a “signature” — color, texture, shape, scent, even the environment in which they grow — that resembles the body parts and diseases they heal. Thus bloodroot, or Sanguinaria canadensis, whose roots and rhizomes secrete a red sap when cut, was once thought to heal blood disorders and hasten wound healing. And eyebright, or euphrasia, whose flowers resemble the human eye (or rather, with its yellow dots and purple stripes, a jaundiced, bloodshot one), has for centuries been used to treat ocular ailments, like conjunctivitis. (In German, eyebright is called Augentrost, or “consolation of the eyes.”) Signatures, in other words, made it easy to divine a plant’s medicinal properties. Form reveals function; function echoes form.

Credit…Photo by Esther Choi. Styled by Beth Pakradooni

It’s difficult to say when and where the doctrine of signatures originated; the concept is an ancient one and has been observed across numerous cultures and healing traditions. It is a hallmark of traditional Chinese medicine and Native American herbalism, and appears in Indian Ayurveda and African herbalism, too. It’s first mentioned in the writings of classical antiquity, including those of the Roman naturalist Pliny the Elder. According to the 16th-century Italian scholar Giambattista della Porta, Dioscorides, the first-century A.D. Greek physician, wrote in his famous five-volume pharmacopoeia of plants and their medicines, “De Materia Medica”: “The Herb Scorpius resembleth the tail of the Scorpion, and is good against his bitings.” In her 2012 book, “The Language of Plants: A Guide to the Doctrine of Signatures,” the herbalist and naturopathic practitioner Julia Graves notes, “Each culture has imbued this art with its own flavor, and each epoch added its own twist.”

The idea gained traction in medieval Europe, particularly among Christians, who gave it theological underpinnings. Paracelsus, the 16th-century Swiss physician, alchemist and philosopher, was one of its biggest advocates, writing, as though about a divinely orchestrated scavenger hunt, “God does not want things to stay hidden, which He created for mankind’s benefit and which He gave man as his property into his hand. … And even though He Himself hid it, so did He mark upon it outer, visible signs, that are special marks.” In the 17th century, Jakob Böhme, a cobbler turned Christian mystic, popularized the doctrine when he published a book-length treatise on it, “The Signature of All Things” (1621), while in England, the herbalist and physician Nicholas Culpeper and the botanist William Cole soon wrote seminal books of their own. Together, these works helped codify the anthropocentric idea that God had given humans hints about nature’s therapeutic gifts, and it was up to us to find and use them. So ubiquitous did this notion become in the West that it even found its way into literature: In Milton’s “Paradise Lost” (1667), the Archangel Michael purges Adam’s eye (his “visual nerve”) with euphrasia to cure it of the “filme” caused by eating the “false fruit” of temptation.

Credit…The Morgan Library & Museum/Art Resource, N.Y.
Credit…Via Wellcome Collection

WITH THE ADVENT of modern medicine, the doctrine eventually fell out of favor, and nowadays is viewed mostly as pseudoscience. Detractors point out the obvious: It can be dangerous to medicate oneself this way — bloodroot’s efficacy is strongly contested, for example, and although it is sometimes used by herbal types to treat cancer, it can be toxic in large doses. There’s also the inherent subjectivity of the enterprise. Your heart-shaped leaf may be, to my eye, a kidney-shaped one. And of course almost all herbs have myriad uses, not merely those that correspond to their primary signatures. Horsetail not only promotes hair health, as its long, coarse, taillike stalks might suggest, but is also used for bone healing. Purslane, which the Cherokee used as a vermifuge because its scarlet stalks looked vaguely wormlike, is also a powerful antioxidant.

Credit…In the collection of the Louvre. Scala/Art Resource, N.Y.

Yet herbalists today still subscribe to the concept, in part because many of the plants are effective in precisely the way their signatures indicate. Lungwort, with its spotty leaves that look like lung tissue, is often used for respiratory issues; dandelion, thought to help jaundice and other hepatic ailments because of its bright yellow color, really does protect the liver; and medicinal mushrooms that bear a resemblance to tumors have been shown in studies to slow their growth. In his oft-cited 2007 paper, “Doctrine of Signatures: An Explanation of Medicinal Plant Discovery or Dissemination of Knowledge,” the ethnobotanist Bradley C. Bennett writes that the doctrine might not have been entirely baseless. Although he is deeply skeptical of the notion that the doctrine was used to discover remedies, particularly with respect to a plant’s visual or superficial aspects, he writes that signatures can also encompass “olfactory or gustatory clues,” and that “potent odors and strong tastes” are fairly reliable indicators that a plant will have bioactive compounds. He also argues that rather than leading to the discovery of medicinal properties, plant signatures were instead used to remember those that had already been uncovered — the signature as a mnemonic device. Such a practice would have been especially useful in nonliterate societies where knowledge was transmitted orally. “Plants that are both efficacious and easy to remember,” the ethnobotanist and medical anthropologist Glenn H. Shepard Jr. has written, were more likely to endure in a culture over time.

But those who work with plants find even this idea — the signature as bookmark, if you will — too reductive. A plant is more than the sum of its parts, and certainly more than the sum of its parts that resemble the human body. Indeed, traditional cultures have long revered plants and herbs as teachers and guides, and even today, not only herbalists and naturopaths but also a new generation of florists have been inspired by the principles of the doctrine, viewing it as a means of hearing what nature has to say, of decoding her secrets. This notion feels particularly relevant in this strange, claustrophobic moment when many of us find that our only respite is the outdoors. As anyone who has ever taken psilocybin mushrooms knows, plants are mysterious beings with an intelligence of their own: They do have messages for humans, but they also don’t exist purely for our use. The Brooklyn-based floral artist Joshua Werber tells me that he incorporates these notions visually and metaphorically in his work, and that his plants, which he grows in his backyard garden in Brooklyn, let him know what they want. “We’re in dialogue,” he explains, “I’m listening to them.” The doctrine of signatures, those who believe in it might argue, is one way that plants make themselves heard.