Electric flowers and platform plants

…a hidden world of communication

Many plants produce flowers as a way of attracting animal visitors that can pick up pollen and move it to another flower. Flowers come in an array of colors—all the wavelengths we can see plus UV (which most humans cannot see). A yellow flower with a UV pattern is readily distinguishable from other yellow flowers—to the many kinds of animals (including insects and hummingbirds) that can see UV. That’s been known for many years.

However, flowers actually have (at least) two ways of enhancing their distinctiveness that humans generally cannot detect without special equipment. More recent research has found that plants can exploit another sensory system of animals. 

There is a natural electric gradient from the ground (negative) up into the atmosphere (positive). And around every plant and its flowers there is a weak electrical field. Flowers have a negative charge (like the earth they come from), but they bloom in positively charged air, creating a little electrical field. Around the flowers that electrical field is strengthened: the electric effect is best developed around edges, such as the rims of petals and the inner (sexual) parts of the flower. So the size and shape of the flower is emphasized and made even more distinctive.

Bumblebees and other insects can detect the presence and shape of the floral electrical fields and use the information to decide which flowers to visit. Bees detect the electrical fields with their fuzzy hairs. The floral electrical fields are weak, but they are strong enough to deflect the hairs (just a wee fraction of a degree) and set off neural signals that the bees can interpret. Bees’ antennae can detect the fields too but no neural signal is sent on. Experiments with artificial flowers (identical in color, shape, and size) with and without nectar rewards and with and without electrical fields around them showed that bees quickly learn to choose the rewarding, electrical flowers.

Further research revealed that hoverflies, which are also important pollinators, can also do this well. Their body hairs are deflected by small electrical fields and a neural signal is sent. The flies learn to read the signal and their efficiency and speed of finding floral nectar rewards increases.

This hoverfly can perceive electrical fields around the edges of the petals, the big white stigma, and the stamens of this fireweed flower. Photo by Bob Armstrong

Electricity also helps move pollen from floral anthers to insects, because insects have a positive charge and the flower a negative one. So loose pollen can actually jump small distances from anther to insect, even before the bug lands. If the plant is lucky, the insect carries the pollen to another flower. If not, a bee may groom the pollen off its body and packs it away in its pollen baskets to feed bee larvae.

Plants are commonly used as platforms for transmitting vibrational signals, usually among themselves. For example, treehoppers suck plant sap, often gathering in considerable numbers on plants. The treehoppers contract their abdominal muscles very fast, creating surface vibrations that move over the plant. We can’t hear them without the aid of special devices, but the vibes are picked up by the legs of other treehoppers. Those vibrations are like songs, varying in pitch and tempo, and are clearly interpreted by the receivers.

Some songs are used for courtship, drawing male and female closer together; a second male can jam the first male’s songs and decrease a female’s response, thus interfering with the courtship. Baby treehoppers (called nymphs) emit cries of alarm when danger is perceived; this elicits defensive behavior of the mother, who signals after evicting a predator, calming thenymphs. 

Lots of other insects (and spiders) can communicate with each other using plant vibrations. Many female insects use variation in male vibrations to choose the right species and the best male to mate with. It goes the other way too: males can use female vibrations to discriminate among females. Insects such as katydids make sounds we can hear, but they also make vibrational signals that indicate body size, and females prefer larger males. Leaf-cutter ants create vibrations when chewing into leaves; if it’s a really good leaf for growing fungi to feed the colony, the distinctive vibrations can recruit other foraging ants to exploit the good resource. The larvae of some insects use vibrations to attract others of the same kind or to keep competitors away.

A typical range of transmission for most of these vibrational signals is up to about two meters, although it can be longer for large insects and spiders. All vibrational calling is energetically expensive, and some studies have shown that an insect that calls a lot is not likely to live as long as one that calls infrequently. The host plants that provide the platform have different vibrational properties, so they differ in their signal transmission capacity. 

Some plant-based vibrations are not meant as communication among members of the same species. Vibrations produced by feeding, for instance, can be risky if it attracts predators—and lots of potential predators can track such vibrations. For example, a feeding caterpillar inadvertently gives vibrational cues to a predatory stinkbug and perhaps to parasitic wasps. In contrast, some butterfly caterpillars vibrate along with chemical signals to call in mutualistic ants to provide protection from predators. And there’s a spider that mimics the vibrational signals of the males of other species, to lure females of those species into hunting range for the mimicker.

Clearly, plants do far more than most of us ever imagined! That’s just a sample of sensory worlds that humans cannot experience directly. We miss a lot!

The stories of electric flowers and singing tree hoppers came mainly from a fascinating book about the sensory world of animals (An Immense World, by Ed Yong).


Communication among plants

sessile green birds

As a young professional, I was interested in animals—how they make a living, how they raise offspring, how they behave. Plants were just things that birds perched on, used as nest material, and produced bird-food such as edible fruits and seeds. Of course, they also used the sun’s light to photosynthesize carbohydrates, generating oxygen in the process. But beyond that? They just sat there.

Quite belatedly, I came to realize that plants could be viewed as sessile green birds, even though (being sessile) they can’t fly or walk, and even though they can do some things birds can’t do (photosynthesis). Nevertheless, they really can do many of the things that birds and other animals do: As plant ecologists have long known, they defend themselves (with spines or chemical compounds), they compete for light and soil nutrients, they sometimes even produce chemical compounds that suppress other plants that try to grow competitively close. More interesting was the revelation that they compete for mates, by developing more attractive flowers to draw in more pollinators, for example, and in some cases they can even choose their mates, by rejecting pollen from certain pollen donors. That provided a direct parallel to what many animals do, and it was an eye-opener for me.

Later, reports began to surface about plants communicating with each other. No, they don’t cry or yelp or speak as many animals do, but they use other means to ‘talk’.

Back in the 1980s, two research papers elicited a strong backlash of disbelief, scorn, and derision. One paper focused on poplar and maple trees, the other on Sitka willow, a western tree that grows here in Juneau. Both papers reported that if leaves of one individual tree were damaged by chewing insects, that individual tree ramped up not only its own chemical defenses but also, surprisingly, the neighboring trees did likewise! Later papers showed that the neighbors don’t even have to be of the same species: damaged sagebrush could cause close-by wild tobacco plants to bolster their chemical defenses. Airborne chemicals (pheromones) from the damaged plant were perceived by neighbors, which then mounted their own defensive response.

As the years passed, the experiments continued and became more convincing, so the scoffers grew less vociferous. However, as far as I have determined so far, there are still some big questions: is this really some form of signal, which implies an intended receiver? Does the initial damaged plant benefit from emitting volatile warnings of potential danger from leaf munchers? If the neighboring plants are genetic relatives, there might be a benefit to them all. Or are the neighbors merely eavesdropping on a signal that the initial damaged plant emitted, sending a quick message to the rest of its own leaves? The story continues to unfold as research continues.

Much less controversial is the underground network that links most of the land plants in a given area. This network is made up of filaments of fungus; some fungal connections extend for tens and even hundreds of meters. Many kinds of fungi contribute to this network: some link only certain kinds of plants, while other have more general associations with a variety of species. The fungi in question here are called mycorrhizae (fungus-root), because they are closely associated and in some cases even penetrate the roots of land plants.

The classical view of mycorrhizal systems has been that carbohydrates from green, photosynthetic plants are transported to the fungus, and the fungi garner mineral nutrients from the soil, transporting them to the green plant. A nice simple symbiosis, in which both partners benefit.

But it turns out to be far more complex than that. Mycorrhizae constitute a system of conduits through which pass not only carbohydrates and nutrients, but also molecular signals of stress resulting from damage and sometimes other molecules as well. Some studies have shown that stress signals from a damaged plant can induce defenses in the receivers in just a few hours, by turning on the genes that control the production of defensive compounds.

Research has shown that the fungal network can connect many different plants, even of different species. A big green conifer tree may supply carbohydrates or defensive chemicals to numerous other plants, not only near neighbors but those that are some distance away. A mycorrhizal fungal network might draw soil nutrients from a fertile area to plants that are trying to grow in less fertile ground. On the other side of the coin, so to speak, it is also possible for biological warfare to occur via the network: one plant can send damaging chemicals to its connected neighbors, suppressing their growth and thus reducing competition from them. The mechanisms by which all these interactions take place are still a subject of study.

Further studies have shown that at least some plants can identify the genetic relatedness of their neighbors, distinguishing kin from non-kin by means of exudates from the roots, probably carried by the connecting fungi. That makes possible discriminatory behavior of such plants…they provide more carbohydrates to kin than to non-relatives, for example.

So what we see as individual, independent plants above ground is really an association of interdependent plants that are connected underground, interacting in a variety of ways. In short, no land plant really stands alone.

Plant behavior

communication, competition, and subtle movements

Long ago and far away, when I was a kid, I took plants for granted. They stood there, being green, and that was that. Animals were way more interesting; they ran or climbed or flew, they barked or snorted or sang, they DID things.

Considerably (embarrassingly) later, I began to understand that plants and animals lead parallel lives in many ways. I’m not referring here to the obvious facts that both plants and animals grow, reproduce, and die. I’m thinking in terms of behavior—whole books and thousands of scholarly papers have been written about animal behavior, but plant behavior has been largely neglected, except by Darwin and a few others.

Plant BEHAVIOR?????? Well, why not! Consider the following parallel: Each species of animal has particular behaviors involved in competition for mates, courtship, and mate choice. Depending on the animal species, maybe the males dance up and down, or chirp or sing, or wave their antennae or legs in a certain way. Our male humming birds whir in great swinging loops, male goldeneye ducks cock their heads far back and throw out their chests, male sooty grouse hoot from conifer trees, and so on. The visible and audible displays of animals are all regulated by hormones. Plants cannot dance or sing, or fly, or show off fancy plumage, but their behavior is regulated by hormones, just as it is in animals.

Plants certainly can and do compete for mates. All those varied and lovely flowers are a plant’s way of being attractive to visiting animals that will pick up pollen and carry it to another flower of the same kind. The equivalent of courtship takes place subtly in plants; it’s conducted largely by chemistry between a pollen grain and the female parts of the flower that it landed on. The plant receiving the pollen can even exert some choice of which pollen grains are acceptable, depending on the chemistry of the interaction. The details of many of the male/female interactions are still being worked out, but there is no doubt that they occur.

Plants are generally stationary; once they get established in a spot, they typically stay there until they die. Most animals, in contrast, can leave a place if it becomes unsatisfactory. However, even though the plants themselves can’t move (except in Hobbitland and similar countries), parts of plants can move in self-generated, hormonally regulated ways. Thus, we can observe in some orchids, for example, that once a pollinator has removed pollen, certain portions of the flower move to expose the receptive female surfaces, ready to receive pollen from the next visitor. In a number of flowering plants, if no pollen has been received from another flower, the male parts of the flower will bend until the flower’s own pollen is deposited on the female surfaces, effecting self-pollination. The little woodland plant called ‘shy maiden’ has one flower, which demurely faces down, until it is pollinated and the fruit starts to ripen; then the flower turns its face upward and becomes a ‘brazen hussy’.

Some plants can signal to their potential pollinators by color changes in the flower. A good example from the local flora is lupine, which is pollinated by bees. The simple version of the story goes like this: The upper (‘banner’) petal of each flower is usually white. But when the flower has been pollinated, that petal turns pink or purple. This tells other bees that the flower has been visited and its nectar removed, and therefore it would be more profitable for those bees to visit other flowers, ones that still have white banner petals.

The leaves of some plants ‘go to sleep’ at night, folding up and reducing water loss; other fold up when touched, perhaps moving away from an inquisitive herbivore’s mouth. Tendrils of climbing plants twist and curl until they touch a support, and then they twine around that structure. When an insect contacts a sticky hair on sundew plant leaves, the sticky hair bends; this draws the insect into contact with more sticky hairs, so the insect is more securely trapped, and the sundew is ensured of a good meal. Darwin, a very close observer, wrote a book on plant movements.

Plants can even communicate with each other in some cases. Some do it through root grafts, a physical connection among individuals, even those of different species, passing nutrients and hormones back and forth. More surprising, perhaps, is that when certain kinds of plants are attacked by herbivores, they send out aerial chemical signals to which nearby plants can respond by ramping up their own anti-herbivore defenses. And, of course, each plant communicates regularly with itself (just as animals do), using hormones to control growth, reproduction, wound-healing, and other activities.

So plant behavior is a real thing. It’s much less obvious than much (but not all!) of animal behavior and therefore commonly requires very close observation. But don’t give it short shrift, just because it may be hard to see!