When scientists announced that gently rubbing plant leaves made them less susceptible to disease, the mediasphere erupted in predictable delight. The Prince of Wales may want to consider caressing his plants as well as talking to them, suggested
The Sunday Telegraph in an observation kinder than many.
The subject of plant intelligence is a one to send many a biologist reaching for a proverbial blunt instrument. The situation has not been approved by it having attracted the attention of popular publications ranging from Maclean’s, through New Scientist to Penthouse.
So when Floriane L’Haridon and colleagues reported in a paper in the open access journal BMC Plant Biology that gently rubbing the leaves of thale cress plants (Arabidopsis thaliana) between thumb and forefinger activates an innate defense mechanism, it was bound to do more than raise eyebrows. Within minutes, the researchers reported, biochemical changes occur, causing the plant to become more resistant to Botrytis cinerea, the notorious fungus that causes grey mould of plants.
But can plants really think?
As the late Arthur C. Clarke once observed, the only way to discover the limits of the possible is to go beyond them into the impossible.
Scientists have been questioning the impossible for well over a century. The great Charles Darwin (1809-92), hardly anybody’s fool, proposed that spreading plant roots could act in a similar manner to a brain, he suggested in The Power of Movement in Plants (1881). But do they really have minds of their own? “The little beggars are doing just what I don't want them to,” a frustrated Darwin once observed of plants in his Kent garden.
Many a gardener has felt the same way. Why would a plant of exactly the same species and source flourish for one neighbour only to turn up its toes and succumb for another gardener close by? Why does a stem cutting taken from one plant take root and grow while another from a similar plant simply sulk and refuse propagation?
In the 1920s, brilliant physicist Jagadish Chandra Bose demonstrated that electrical currents flow through plants but in the world dislocations that followed his discoveries went largely ignored. Fifty years later, the era of flower children had gone to seed. Certain practitioners, desperately seeking rejuvenation, decided to connect plants with a polygraph, or lie detector. The results were startling. When a tomato plant had a nail thrust into one of its fruit, the polygraph needle went wild. Unfortunately later experiments involving houseplants confused dracaena and philodendron which did nothing to promote acceptance, and then claims made for plants detecting their owner’s emotions when separated by the width of the continent put a spike in the project.
Nevertheless, not all were deterred. The 1980s saw the emergence of the ‘talking trees’ phenomenon with plant biologists demonstrating that trees communicate with each other. Controversial in its day, it is now widely recognized that plants release chemical volatiles into the air as well as through their roots. In the latter case microbes in and around their roots perhaps aid them in this communication.
Even more astonishing came the news that plants that throw off plantlets via runners or ‘stolons,’ remain in contact with their progeny. Using white clover, Josef Stuefer at Radboud University in Nijmegen, the Netherlands sought to prove this, publishing his study in the journal
Evolutionary Biology.
But do plants recognize their relatives, communicate and actually care for them? In 2007, Susan Dudley at McMaster University in Ontario, published research indicating that sea rocket (Cukile edentula) from the shores of the Great Lakes does just that. However, other studies from elsewhere seem to indicate that not all plants are so altruistic.
More recently Dudley and her team at McMaster have gone on to disentangle this mare’s nest of contradictory research, publishing their results in the 2011 in the journal Proceedings of the
Royal Society B. It turns out that like everything in the natural world, things are not that simple. In short, different plants, different communications.
Earlier in 2004, Stefano Mancuso, professor of horticulture at the University of Florence had gone out on a limb and formed the
International Laboratory of Plant Neurobiology and an associated journal to promote the same research. The following year the first symposium was held in Florence, Italy, attracting some 100 scientists. The name though caused some furor and, bowing to pressure, Mancuso renamed the venture the International Society for Plant Signaling and Behavior. Earlier this year a similar meeting was held in Vancouver at the University of British Columbia.
They may lack a central nervous system let alone anything else the animal kingdom uses to communicate. But despite protests from some well-entrenched quarters, it appears plants are far from unthinking, photosynthetic organisms, merely sitting and soaking up the sun. As Lynne Dicks explained in her 2007 article in
New Scientist magazine:
It is becoming clear that plants have much richer social relationships than anyone imagined. Many are in contact with one another by direct line or mysterious underground signals. Others recognize and tolerate neighbours. Some are even generous and self-sacrificing towards their kin.
Which brings us to return to Arthur C. Clarke and conclude that his First Law might have something in it: “When a distinguished but elderly scientist states that something is possible he is almost certainly right. When he states that something is impossible, he is very probably wrong.”
