Lumbering around on his barky limbs, sprouting flowers and even dancing in a pot, one of the stars of the film "Guardians of the Galaxy" bizarrely blends the plant and animal kingdoms. "Groot," a walking, talking tree, seems to defy nature — but how outlandish is the idea of a plant-animal hybrid?
Plants that can smell and animals that regenerate show that animal and vegetable kingdoms may not be as far apart as they first appear. Some scientists even say Earth's biology suggests the possibility of "thinking plants" somewhere in the universe.
Here, experts tell how Groot-like blending could occur, and some reasons it couldn't. [Science Fact or Fiction? The Plausibility of 10 Sci-Fi Concepts]
Plant sight, plant hearing
In the film, Groot clearly hears, sees, feels — and talks (albeit, only three words, "I am Groot"). While one would be hard-pressed to find a talking vegetable on Earth, the idea of communicating and sensing plants is not at all outlandish, Danny Chamovitz, director of the Manna Center for Plant Biosciences at Tel Aviv University and author of "What a Plant Knows" (Scientific American, 2012), told Live Science.
In fact, plants have a much richer, more dynamic life than most people give the leafy beings credit for, Chamovitz said. "We think of plants as un-living, because they're unmoving," Chamovitz said. "The strong scientific evidence is that plants have every sense familiar in animals, except hearing."
They respond to chemicals, with lock-and-key mechanisms that resemble how animals smell. Plants have specific photoreceptors, which are proteins that respond to different wavelengths of light. They "know" when they're being touched, Simon Gilroy, a professor of botany at the University of Wisconsin-Madison, told Live Science.
Plants also have proprioception, or a sense of their location in space, Chamovitz said, which is why they can tell when they're planted upside-down.
Some plants can even "hear," able to distinguish the vibration patterns made by different chewing caterpillars, according to a study detailed this summer in the journal Oecologia (opens in new tab), Gilroy said. (Decades-old claims that plants can "hear music," however, have little to no scientific support, he added.)
This plant sensing may not seem evident — after all, plants don't scream in pain or comment on Van Gogh's use of color. But senses serve the same function in plants as they do in animals: Plants take in information, which travels through the plant body and causes some sort of response, Chamovitz said. [In Photos: Animals That Mimic Plants]
The familiar phenomenon of houseplants growing toward the window, for example, shows how plants sense and respond to light. When part of a plant gets eaten, that causes distress chemicals to propagate through the plant, which responds with chemical changes to make itself less tasty to the predator, Gilroy said.
Not only can plants send signals within their own "bodies," but those signals can also influence other plants. In other words, they communicate, Chamovitz said. A disease or pest infestation in a neighboring plant sends out chemical signals that cause nearby plants to respond. "They can smell when neighbors are sick," Chamovitz said. "It gives off a chemical, so the plant defends itself."
Researchers have also shown corn seedlings lean toward sounds with a 220-Hertz frequency, the same tune emitted by the plants' roots, and chili seedlings grow quicker when they sense a fennel plant is growing nearby.
Moving … but slowly
Groot does more than just sense and communicate, though. He also moves around. This woody best friend to a talking raccoon shifts around on mobile stumps and lifts alien bullies up by their nostrils.
"Thinking about it, the feature that really isn't plantlike about Groot is the speed of the movement," Gilroy said.
Plants, for the most part, simply can't move that fast. Animals have "squishy cells" that can move past one another, making muscles feasible, Gilroy said. But a hard wall encloses each plant cell, and neighboring cells are glued together into a rigid grid.
"Things like muscles are impossible with how plants are set up," he said.
Moreover, a plant's entire body-plan centers upon staying put, so they can maximize photosynthesis with networks of branches and roots. Chamovitz calls "rootedness" the primary principle of plant biology.
Plants do make movements, however — they just do it by growing, Gilroy said. For example, they "move" toward the sunlight by elongating cells on the dark side of the stem. "Plants grow through the environment," Gilroy said.
Due to some clever mechanisms, a few notable species buck the rule of the slow-moving plant. The famous Venus flytrap shuts its jaws rapidly by essentially "growing them shut," Gilroy said. Acid released at the flytrap's hinge softens cell walls and makes them expand quickly. [Image Gallery: Carnivorous Plants]
Finally, plants have to stay put because movement burns so much energy —
photosynthesis simply can't power animal-style activity, which is why animals eat plants and other animals. To move around like Groot does on screen, such a tree creature would have to eat other things, too, Gilroy said.
Stem cells and Groot
Despite his (relatively) quick movements, Groot still maintains some plantlike characteristics. Capable of unlimited growth and regeneration, he grabs an out-of-reach battery by simply growing taller. His lopped-off arms return, and there's even a bit in the movie about replanting a Groot "cutting."
Clearly, the movie speeds up such vegetable growth, but real-life plants can indeed keep growing in ways animals can't, Ed Rayburn, an extension specialist and forage agronomist for West Virginia University, told Live Science. Those plant species capable of "unlimited growth" can keep going and going "for what, to humans, seems like forever," he said. And plants commonly regenerate lost limbs.
Those abilities derive from the way plants are structured and their access to stem cells.
Arranged in modular body plans, plants can grow limbs in different directions and add secondary limbs in a fractal pattern. Animals, by contrast, grow to a predetermined size and shape, and much of that happens in the womb, Gilroy said.
"We cram all our development into as short a time as possible," Gilroy said. But since plants can't run, and so will inevitably suffer damage, they have to keep growing throughout life, he said.
Plant stem cells are found in meristems, "small groups of cells that stay perpetually embryonic," Gilroy said. Located at stem and root tips, these plant stem cells are pluripotent, meaning they can develop into any type of plant cell. After injury, any plant cell can revert to similar pluripotency.
Animals, by contrast, lack meristems, and stem cells are much harder to come by, as shown by the difficulties faced by cloning efforts, Rayburn said. Some animals do approach plantlike regenerative growth, but usually those with simple body plans, like the planarian worm, Gilroy said.
Plant communication even has some researchers in the new field of plant neurobiology considering the potential for leafy intelligence. Alien worlds, some scientists say, could theoretically birth sentient plants.
But, Chamovitz said, such green thinkers would have to change the core trait of plants — their rootedness. Animals developed thought because of their "searching" strategy for finding food, he told Forbes. Alien tree-thinkers would have to incorporate movement, perhaps with "some type of root system that can push itself out of the ground, take three steps forward and then re-root," he told Forbes.
As for creating plant-animal hybrids here on Earth, that's most likely to happen in a geneticist's lab. It's theoretically possible, given the right gene transfers, to give people a coating of green, photosynthetic skin. To actually make much use of such a feature, however, people would have to grow a canopy of leaves.