Evening Sun by Pamela Brumbley, submitted to 2016 Maryland DNR Photo Contest.
The human world is so fraught with communication it can feel overwhelming at times. Getting out into nature can be a relief for those of us looking for some peace and quiet. Although the forests, rivers, mountains, or dunes may feel tranquil and devoid of chatter, a barely perceived symphony of plant communication lies just on the edge of human understanding.
The concept that plants may “speak” has been a recurring theme in folklore and mythology throughout human history. In English folklore, willows are said to follow travellers, muttering to themselves as they walk. Herodotus, the ancient Greek historian, wrote in the fifth century BCE of a tree in Dodona that spoke with the voice of Zeus, interpreted by priestesses from the rustling of leaves. Sophocles references Herakles (better known to English-speakers by his Roman name, Hercules) pondering a prophecy from the same sacred oak.
Humans have long used plants and plant symbols as a means to communicate with each other. One of the oldest symbols in written speech is a hieroglyph of the papyrus stem, and any historic overview of ancient Egypt would be incomplete without mentioning the use of papyrus to make the first paper in recorded human history. In fact the word “paper” derives from the plant name!
Outside of the written word, humans have found innumerable ways to have plants speak for us. One of the better known examples is that of Victorian flower language. Floriography, the art of using flowers to communicate, burst into popularity during the 1800s, as Victorian society dictated strict lines around expressing emotion. While plants have always held symbolic meaning in different cultures throughout time, many of the modern meanings of flowers in Western culture were solidified in this era.
With these longstanding historic connections, it may surprise many that the study of plant-to-plant communication is itself relatively new. And much of the modern era of research began with the ephemeral smell of maple and poplar trees in the early 1980s.
Smelling the Flowers by Duane Tucker, DNR Photo Contest 2016
A Yell via Smell, A Wink via Stink
Chemical forms of communication are perhaps the most easily perceived ways plants communicate with each other and with the world around them. While we as humans cannot hear plants (more on that later) or see the ultraviolet colors they employ to signal to pollinators, we can detect and measure the chemistry plants use.
One important element of chemical communication is the use of aroma compounds by plants to attract pollinators and defend against predators. Humans have long enjoyed the scent of flowers and other plants; the history of perfumes would be a terribly short one without them! But even as we find joy in the various smells of plants, many of those scents have a specific message not meant for us.
In 1983, the first study linking scent to plant communication was published. The researchers found that despite no contact via root system or touching, maple and poplar tree saplings would respond to others of their same species receiving damage to their leaves. The conclusion, now supported by decades of research, was that the plants were communicating via air; in short, through smell. Since these early days, 30-some species of plants have been shown to be able to communicate with others of their same species, or even different species, through the release of scent molecules in the air.
Plant VOCs (volatile organic compounds) are chemicals that plants release into the air to communicate danger to each other, attract pollinators, repel herbivores, or even attract third parties with no interest in the plant for its own sake. One commonly used example many of us have experienced is the smell of freshly-mown grass, a plant VOC released in response to leaf damage. Some plants, like petunias, have such a highly attuned ability to recognize specific VOCs that they will reject ones that are identical, but “backwards”: even replicating the VOCs as their mirror image will not confuse the plant. In this regard, petunias have much in common with humans, as seeing a reversed photo of ourselves may come with some degree of confusion and discomfort as well.
In one clever example of plants using VOCs as defense against those that would munch upon them, several species of economically important plants (corn, cotton, and tobacco) were found to release VOCs that attracted parasitic wasps to them. These wasps prey upon one of two closely-related species of caterpillar who would otherwise eat these specific plants, and it was found that the plants seemed to be releasing a distinct scent to let the wasps know their favorite dish was on the menu.
In an exciting turn for those of us interested in plant communication and willing to read studies heavy on chemistry, researchers from Saitama University in Japan have recently been the first to visually document the way plant VOCs are emitted and received. The experiment, done in real time between mustard plants, showed the mechanics of how such communication occurs. Researchers tracked the movement and increase of calcium ions in plant tissue in an undamaged plant following exposure to VOCs released by a damaged one, using high-resolution imaging. This finding suggested that the leaves took in these chemical signals via stomata, tiny openings on the leaf surface used for gas exchange. While we’re far from knowing all the intricacies of these conversations, this marks a major milestone in our understanding.
Mushrooms by Zaphir Shamma, DNR Photo Contest 2020
Root Words
Right on the heels of scent as one of the most discussed methods of plant communication is that of the mycorrhizal network. Mycorrhizae is the term used to describe the intricate networks of fungus and plant roots that stretch beneath the surface of the ground. An estimated 80-90% of terrestrial plants have relationships with one or more types of fungus, some of which are never seen on the surface.
The field of mycorrhiza ecology is divided into general categories based on the kind of plants and fungi involved. They often include: orchid mycorrhiza, ericoid mycorrhiza, ectomycorrhiza, and arbuscular mycorrhiza. Orchid mycorrhiza refers just to the relationships between orchids and fungi (read more on Maryland’s native orchids!) In the same vein, “ericoid” refers solely to the members of the Ericaceae family of plants, which includes heathers, blueberries, and rhododendrons, among others. Ectomycorrhiza includes relationships in which the fungus does not penetrate the cell walls of its symbiotic plant partner. These relationships are largely the ones between fungi and flowering plants (called “angiosperms”) or pines. By far the most common type of mycorrhiza is arbuscular, which accounts for the some 200,000 species of plant and the relationships they form with fungi. Arbuscular mycorrhiza accounts for those between fungi and grasses, herbs, and many trees.
Mycorrhizal relationships provide a great many functions to both the plant and fungi involved, including transport of nutrients, increased plant productivity, protection from disease and drought, and nourishment of seedlings. The mechanics are complicated, to say the least, but researchers have found that the results of the mycorrhizal networks below forests encourage biodiversity. Beyond that, or perhaps as part of it, is the fact that the mycorrhizal network passes along communication from plant to plant.
“Common mycorrhizal networks” are localized fungi networks that connect plants of both the same and different species. Signals relating to disease or herbivore attacks on the connected plants have shown to be communicated via these networks, allowing plants in the area a heads-up so they may strengthen their immune defenses against potential attack themselves. Unlike human news networks, these mycorrhizal communications happen without the use of helicopter footage of local traffic, which is perhaps an area for improvement for the future.
Sound the Alarm
As quiet as the plant kingdom may seem to humans, our limited hearing filters out a positive cacophony of sound! The field of studying the sounds produced by plants is known as “plant bioacoustics.” Researchers in this field must use instruments able to pick up sounds that are typically of exceptionally high frequency and thus unheard by the human ear.
The field of plant bioacoustics is actually a century old, with documentation of the more audible noises made by plant sap in capillary tubes (a “click”) first made in 1914. The 1960s saw several studies of “audible acoustic emissions” of plants, and the first recordings thereof.
A recent study showed that tomato and tobacco plants make a popping sound when stressed by damage to their leaves and dehydration. While the sound itself was noted to be as loud as human conversation in volume, the frequency is far above what we can hear as humans. The researchers also recorded a handful of other plant species, including grapes and wheat, and found that they too emitted stress sounds.
Milkweed Munching Monarch by Annette Conniff, DNR Photo Contest 2015
So if plants can produce sound, does that mean they can “hear” sound as well? In a sense, yes! While plants do not have organs dedicated to the vibration of airwaves that we call sound, they do respond to vibrations that travel through the air. In 2014, researchers at the University of Missouri found that plants responded to the sound of caterpillars eating by mounting chemical defences. Not only did the study confirm the plants responding, but also that the plants seemed to retain the memory of the sound; the plants were played the noise, then later exposed to the caterpillars. The plants behaved as if they had been warned of the possibility- mounting a defense by producing mustard oils once the caterpillars arrived. The same study found that the plants did not respond similarly to the sounds of other, non-predatory insects, or of the wind, meaning the plants were able to distinguish between sounds that implied danger and sounds that did not.
Similarly, a species of evening primrose native to Mexico and the southeastern United States has been shown to respond to vibration recordings that mimic bee and moth wingbeats. The primrose was found to have produced sweeter nectar when it perceived a pollinator was nearby. In this case, the flower of the plant seemed to act as the “ear,” because when the flower was covered, no change in sugar content was found.
Studies of sound and its impact on vegetation have shown discovery after discovery. Sound travels well underground, and corn roots have been shown to grow towards sources of vibrations, while peas have been shown to use vibrations in the soil to locate water. Further, certain frequencies have shown to delay the ripening of tomatoes, while others have shown the ability to stimulate growth in plants. While we ourselves can’t hear all the sounds of the plant kingdom, we would still encourage them to rock on!
Roadside Flowers by Lori R. Bramble, DNR Photo Contest 2013
Showing Their True Colors
By now, it is probably not surprising to learn that, like plants producing sounds we can’t hear and communicating with scents that we don’t understand, they also display colors we cannot see. (It’s always something new with these guys, isn’t it?)
In fact, many flowers display colors outside of our normal range of light vision. Pollinators (and herbivores, and many other animals besides) see a greater range of colors on the ultraviolet end of the spectrum of light, making some of the flowers they visit vibrantly bright. Many flowers display “landing strips” or highlight exactly where to find nectar (and coincidentally, pollen) so bees and butterflies, and some birds and bats, find exactly what they’re looking for while also pollinating the flower in question.
“False color” techniques have been developed to assist us with visualizing just how differently bees see color. What these, and other imaging techniques, including UV light, have shown is just how much plants are saying with colors that we can’t see.
Hold My Calls
When you’re in your garden this spring, or simply enjoying a mild day in the great outdoors, consider the chatter going on all around you in a vast unknown conversation. Even more methods of communication are utilized by plants than what we have covered here, from electrical signaling to fluorescence and beyond; some researchers are even attempting to communicate with plants in their own language.
As we learn more about the world we can barely perceive with human senses, it throws into stark relief how little we really know about the ecosystems we inhabit. Even as science develops, our lantern of knowledge held forward to light the way, we find more delights and puzzles emerging from the darkness. A vast world of beautiful secrets surrounds us- how are we to find our way?
Aldo Leopold wrote, “to keep every cog and wheel is the first precaution of intelligent tinkering,” and what better advice could we have for an ecosystem we are just now beginning to see as a full and complicated network. And how can we save what we have before we damage it, or lose it altogether?
When you’re digging your hands into the warming earth, consider what you’re planting there. Native plants, besides being easier to care for in our climate, provide habitat to all the beneficial insects we love to watch flit through our gardens. The mycorrhizal fungi in the soil seek plants they know to form partnerships with, and the bees and butterflies seek colors only they can see on flowers they evolved alongside for thousands of years. Sounds we can’t hear travel through the earth, reaching ears we don’t understand, and if we want to save that symphony for a time when we may hear it, then please consider using your space to make a wildlife oasis in an increasingly cacophonous world.
And cool it with the pesticides, y’all.
For more on how to introduce native plants to your yard or garden, see our Wild Acres site.
If you’d like to throw off the shackles of manicured grass and embrace the beauty of natural lawns, see our explainer on lawn alternatives.
by Katy Gorsuch
