Don't look now, but that tree may be watching you. Several lines of recent research suggest that plants are capable of vision—and may even possess something akin to an eye, albeit a very simple one.
The idea that plants may have “eyes” is, in a way, nothing new. In 1907 Francis Darwin, Charles's son, hypothesized that leaves have organs that are a combination of lens-like cells and light-sensitive cells. Experiments in the early 20th century seemed to confirm that such structures, now called ocelli, exist, but the concept of a “seeing plant” fell by the wayside—only to reemerge in the past few years.
In a recent issue of Trends in Plant Science, František Baluška, a plant cell biologist at the University of Bonn in Germany, and Stefano Mancuso, a plant physiologist at the University of Florence in Italy, lay out new evidence for visually aware vegetation. To make their case, the researchers first point to the 2016 discovery that Synechocystis cyanobacteria, single-celled organisms capable of photosynthesis, act like ocelli. “These cyanobacteria use the entire cell body as a lens to focus an image of the light source at the cell membrane, as in the retina of an animal eye,” says University of London microbiologist Conrad Mullineaux, who helped to make the discovery. Although researchers are not sure what the purpose of this mechanism is, its existence suggests that a similar one could have evolved in higher plants. “If something like this is already present at the lower level of evolution, it is most likely kept,” Baluška says.
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Recent work also shows that some plants, such as the cabbage and mustard relative Arabidopsis, make proteins that are involved in the development and functioning of eyespots—the ultrabasic eyes found in some single-celled organisms such as green algae. These proteins specifically show up in structures called plastoglobuli, which are famed for giving autumn leaves their red and orange hues. “This discovery suggests that plastoglobuli in plants may act as eyespots,” Baluška says.
Other observational research reveals plants have visual capabilities we just do not understand yet. For instance, as reported in 2014 in Current Biology, the climbing wood vine Boquila trifoliolata can modify its leaves to mimic the colors and shapes of its host plant.
Although the evidence for eyelike structures in higher plants remains limited, it is growing. “I had never heard about plant vision, and I would have dismissed it as unlikely until my own discovery of cyanobacteria acting as a camera eye,” says biotechnologist Nils Schuergers, co-author of the 2016 study on Synechocystis. The next challenge is to confirm the early 20th-century experiments showing that plant cells themselves can act like lenses—and researchers still need to figure out all the ends to which plants put their rudimentary sight.