The Private Life of Plants

If human beings have learned anything about nature during my lifetime, it is that we know almost nothing about nature. And the more we learn, the more we realize how little we know.

Consider the nature of plants. Aristotle divided the living world into two kingdoms: plants, which don't move, and animals, which do. Most of us probably still think that way. Pushed a little harder, we would probably say that many animals are conscious. They think, feel, understand, communicate and interact. Plants don't.

But nothing in nature is as simple as our mental models of nature. Some microscopic organisms are not clearly animal or plant; they have characteristics of both. And plants interact in much more sophisticated ways that we ever imagined.

A decade ago, for instance, I was fascinated by the research of Dr. Suzanne Simard, then a biologist with the provincial Ministry of Forests in Kamloops, BC, and now a professor at the University of BC. Dr. Simard was researching the interactions between trees and the various species of fungi which sheathe the tree’s roots and – like a set of roots upon the roots – send out tiny threads to forage in the soil for nutrients. The fungi convey these nutrients to the tree. The tree in turn reaches up into the sunlight to capture carbon from the air through photosynthesis – and it conveys carbon to the fungi in the form of sugars.

This is a classical symbiotic relationship. The fungi and the tree depend on each other. Simard and her colleagues, however, showed that there was another whole level to these interactions, in that the fungi connect the trees not only to the earth, but also to other trees. Furthermore, the fungi can transfer carbon from one tree to another. Seedlings blocked from sunlight by the forest canopy are nurtured by their elders – and even by trees of different species.

Among the trees which overshadow young Douglas fir, for instance, are fast-growing paper birches, whose roots can be connected with the roots of the fir by as many as 10 different species of fungi. By “feeding” different radioactive carbon isotopes to different trees, and then noting where the isotopes wound up, Simard’s group was able to show that the birch was actually providing carbon through the fungi to the fir.

Simard’s research changes the whole definition of a forest. Above the surface, the trees look like isolated individuals — but almost half their biomass is underground, where they interact to form what one biologist calls “a superorganism.” Or, one could say, a social organization. That's what a forest really is. (And that's why a “managed forest” of identical trees is not a forest at all.)

More recently, researchers at McMaster University have demonstrated that a humble beach weed called the Great Lakes sea rocket is able to tell whether another sea rocket is related to it – and to react accordingly. If a neighbouring sea rocket is not related, the plant aggressively puts out roots, grabbing nutrients in a highly competitive way. If the other sea rocket is kin, however, the first one restrains itself.

No plant has ever been known to do this. But the discovery is consistent with a whole range of new discoveries about plant interactions. Some plants can determine whether nearby plants are potential competitors by detecting the wavelengths of sunlight that the other plant absorbs or reflects. And plants send electrical signals around within their bodies, just as your nervous system does, though nobody yet knows what information is being transmitted, or why.

We now know that a parasitic plant called “dodder” locates a potential host by sniffing the air. Unlike almost all other plants, dodder can't grow its own roots or draw sustenance from photosynthesis. Instead, it grows on and into other plants. But how does it find those particular plants?

Scientists set up time-lapse movies to see how dodder seedlings located their victims. As they watched, the seedlings sent out tiny sprouts, which circled around sampling the air like a dog. When they detected traces of certain airborne chemicals, they recognized their victims, and grew immediately and rapidly in that direction. One member of the study team described the process, as seen in the time-lapse film, as “like a little worm moving towards this other plant.”

These findings are tremendously controversial, because they show that plant behaviour is far more complex and sentient than anyone had imagined. Furthermore, since plants don't have obvious organs for thought or perception, it must be the case that these functions can be accomplished in some other way. The dodder, for example, “smells” without a nose, which seems to imply a whole alternative physiology of sensation and perception.

Fascinating. Wondrous. The more we learn about nature, the more we realize how little we know.

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