Science fiction worlds provide a laboratory in which to explore all the many and varied ways in which life can differ from that on Earth. Science fiction aliens range from the very familiar to the very strange. Amongst these it’s perhaps inevitable that we find a number of sentient members of the vegetable kingdom - plant people! [1]

Perilous Plants

Amongst the earliest representations of sentient plants in science fiction, many were monstrous. Based on the endless fascination of carnivorous plants here on Earth, they represented their subjects as also hungry for human life. An influential early example can be found in the short story "Proxima Centauri", published by Murray Leinster in Astounding Stories in 1935. This tells the story of the first human interstellar spaceship which, after seven years of travel (in a form of very early generation ship), approaches the eponymous star system. Unfortunately for the passengers and crew, they encounter a species of sentient and civilised plant people who are instinctually ravenous for animal flesh and have entirely denuded their home solar system of animal life. The Centaurians have developed plant biology in place of metallurgy and grow their buildings and spaceships. The creatures themselves are described by an incredulous officer:

"They're not our idea of intelligent beings at all. We haven't any word for what they are. In one sense they're plants, apparently. That is, their bodies seem to be composed of cellulose fibres where ours are made of muscle fibres. But they are intelligent, fiendishly intelligent.
The nearest we have to them on Earth are certain carnivorous plants, like pitcher plants and the like. But they're as far above a pitcher plant as a man is above a sea anemone, which is just as much an animal as a man is."

With an entire, space-faring species of carnivorous plants for whom their flesh is more precious than gold at risk of tracking them back to Earth, the ship is not fated to plant a successful colony, and the chances of survival for any of its personnel are slim.

While Leinster's plants were rational and sentient, other monstrous plant-people are completely incomprehensible in their communications or motivation. An example can be found in the film The Thing from Another World (1951, dir. Nyby, based on the short story "Who Goes There?", John W Campbell, 1938). One of a number of near-contemporary science fiction monster movies, this film focused on a remote Arctic scientific base which was threatened when a spaceship crashed nearby. Initially frozen in ice, a recovered body proves to be a still-viable form of humanoid plant life, which feeds on mammalian blood. Attempts to communicate fail, and inevitably, the Thing begins to feed off the base personnel. The film is a classic isolated-group-under-threat story, and the alien protagonist is leant a sense of mystery and difference by its vegetable origins.

John Wyndham’s well known novel The Day of the Triffids was first published in 1951, and has been dramatised a number of times on television, film and radio. The story follows the story of a young man and woman, Bill Mason and Josella Playton, who are amongst the few individuals to remain fully sighted after most of the human race is struck blind overnight. Over the subsequent weeks, months and years, most humans die off, unable to survive the changed conditions, but many more are killed by Triffids - a species of plants that were farmed for their oil. These plants are able to walk on three short stems and communicate between themselves through drumming sounds, and - as the story elapses - show clear evidence for intelligence and cooperative behaviour.

Crucially for the plot, they are also equipped with a sting which is capable of killing animal life in the vicinity of each plant. Unlike the Thing from Another World, Triffids are not even close to humanoid, but have been envisaged on screen as similar in form to some carnivorous pitcher plants and show a relentless, literally-inhuman perseverance in their pursuit of prey.

The Quatermass Experiment (TV serial, 1953, written by Nigel Kneale for the BBC) goes a step further than these earlier examples, raising the stakes in terms of body horror. Not satisfied with a plant person attacking humans, The Quatermass Experiment shows a human astronaut, Victor Caroon, gradually becoming subsumed within an alien life form (the same theme reemerges in Doctor Who’s “The Seeds of Doom”, mentioned later. This shows definite plant (as well as fungus and animal) features, and appears to be able to control the actions of the astronaut, showing evidence for some kind of consciousness, although its objectives remain unknown and many of the hybrid-creatures may simply be instinctual.

A slightly more light-hearted, but still ominous, note is struck in one of the more overtly science fictional episodes of the television series The Avengers (1961-1969). In the tradition of The Thing from Another World and Quatermass, “The Man-Eater of Surrey Green” imagined an alien life form that landed on Earth after collision with a derelict human space capsule. In this case, the plant has not landed in the remote depths of the ice sheet, or been carried by a contaminated astronaut. Instead it lands comfortably in the leafy London suburbs and begins telepathically kidnapping botanists in order to carry out its dastardly plans for propagation here on Earth. Amongst these, the lead individual has a fascination with carnivorous plants, feeding his venus fly traps by hand. Perhaps inevitably, it soon becomes clear that the alien plant has both brain cells and a requirement for an enzyme that can only be extracted from human blood. Since there is no evidence that this creature can move under its own power, beyond extending and contracting tendrils, it is debatable whether the Man Eater qualifies as a plant person, but it certainly shows the requisite intellect. Unfortunately the aggressive and alien nature of this plant require protagonists Steed and Mrs Peel to destroy it with herbicide [2].

Pacificistic Plants

Another sessile (the opposite of mobile) but still intelligent plant can be found in Stanley Weinbaum’s short story “The Lotus Eaters” (text). Published in Astounding Stories in 1935, it was based on the then-current (since disproved) theory that Venus was a tidally-locked planet, always showing one face to the Sun, such that it had a light side and a dark side. On an expedition to the dark side, a human expedition encounters a speaking, and clearly extremely intelligent, vegetable life form that they dub Oscar. Surprised at its passivity in the face of an animal threat, it takes them some time to realise and articulate the fundamental difference between plants and animals (at least in the author’s view):

"There is a difference. An animal has will, a plant hasn't. Do you see now? Oscar has all the magnificent intelligence of a god, but he hasn't the will of a worm. He has reactions, but no desire. When the wind is warm he comes out and feeds; when it's cold he crawls back into the cave melted by his body heat, but that isn't will, it's just a reaction. He has no desires!"...

"All they do, all they can do, is sit before their caves and think. Probably they think godlike thoughts, but they can't summon even a mouse-like will. That's what a vegetable intelligence is; that's what it has to be!"

Returning to more mobile vegetation, and venturing beyond our Solar System, most long running science fiction television series have also encountered plant people. In Star Trek, the first animate plants encountered were the Phylosians, who appeared in “The Infinite Vulcan”, an episode of the 1973 Animated Series. The crew of the Enterprise were startled to encounter first an ambulatory ankle-height Retlaw plant, and then - after Sulu fell foul of its sting - larger, humanoid plant people. These multi-limbed and tentacled creatures prove to be the diminished survivors of an originally larger people, with a peaceful ethos and great skill in medicine. These were originally intent on conquering the galaxy in order to impose peace, although they apparently happy to subordinate themselves to a lone human - and the Vulcan he opts to designate his successor.

By contrast, in "Savage Jungle", a 1968 episode of the long-running science fiction television drama Voyage to the Bottom of the Sea, it is, rather surprisingly, a submarine crew who encounter alien plant life. In this case the humanoid and sentient plants from another planet are actually determined to xenoform Earth (modify its environment to better suit themselves) by blanketing the land surface with jungle and increasing the carbon dioxide content of the atmosphere.

Similarly Doctor Who has encountered a number of alien organisms descended from plant life. Amongst these are the monstrous alien Krynoids who engulf animal life in their unrestrained growth (“The Seeds of Doom”, 1976), and who are released after a frozen seed pod is discovered buried in the permafrost by a small polar expedition - a knowing nod to The Thing from Another World. We also encounter the enslaved Vervoids who hate all animal life - understandably as they are intended to replace expensive robots and work as labourers for sunlight and water alone (“Terror of the Vervoids”, 1986), and a sole-survivor-of-his-race villain whose species descended from a vegetable cactus analogue but can transform to energy and from there to humanoid form (“Meglos”, 1980, which also features some non-sentient but motile carnivorous plants). The solution to the rebellion of the bioengineered Vervoids is actually rooted in the biology of the vegetable organisms - they were flooded with carbon dioxide and light to accelerate their lives and deaths, while the alien Krynoid was tackled with strong military defoliants and high explosives. More recent incarnations of Doctor Who have also offered examples of benevolent and self-aware plant life. These include the sentient and humanoid Trees of Cheem seen in “The End of the World” (2005) - including Jabe, an attractive feminine figure - who evolved from Earth rainforests over the course of five billion years [3]. Another, although less humanoid example can be found in the sentient trees of Androzani, who create giant wooden avatars to seek assistance from a human mind in order to escape from a commercial harvest (“The Doctor, The Widow and the Wardrobe”, 2011).

It’s relatively rare for a plant person to not only appear in a single story of a science fiction serial but to be a recurring main character. An exception appears in the television SF drama Farscape (1999). This series follows a stray human astronaut called John Crichton stranded in a distant part of deep space, who becomes part of a misfit crew aboard a living space ship. One of these, a Delvian priest Zotoh Zhaan, appears to be an attractive blue skinned woman, and it is only after some time that Crichton learns that she photosynthesises and is in fact a plant rather than animal in her biology. In fact, like many plants on Earth, the blood-analogue of her species has powerful pharmaceutical uses, while her pollen (released under stress) is highly allergenic and toxic. Zhaan’s species are ruled by priests and are generally shown as empathetic and pacifist rather than aggressive - an interesting nod to human assumptions regarding the usually passive role of plant life - although Zhaan is shown to give in to violent impulses from time to time.

Another plant person with pacifist tendencies can be found in the Guardians of the Galaxy series of comics and blockbuster films. Here Groot is an animate tree, of the species Flora colossus. Famously, while he is clearly intelligent, his speech is limited to the phrase “I am groot”. He is able to endure severance of limbs when forced into conflict, and he retains the ability to regrow from a cutting of his original form. The origin and nature of his species is discussed in more detail in his comic series than in the better known films in which he also appears.

Of course, not all plant people appearing in science fiction can be taken entirely seriously. The spoof monster-movie film Attack of the Killer Tomatoes (1978) showed how humans react to the development of consciousness in garden tomato plants as a result of genetic engineering. Its spin off animated cartoon series (1990) expanded on this premise, presenting numerous conscious tomato characters - both hostile and a few friendly exceptions including a young woman who turns into a tomato when touched by salt. Another television cartoon, Jayce and the Wheeled Warriors (1985) also features plants as the primary antagonists. In this more traditionally science fictional premise, a biochemist’s experimentation to develop a food plant is corrupted by radiation from a nearby supernova, creating the Monster Minds - sentient vine creatures who fight a constant battle with a small group of humans led by the biochemist’s son, Jayce. Again, a few friendly plant people exist, including a female protagonist, Flora, who was created in an earlier experiment.

While these cartoons make the best use of the animated medium to show truly alien plant people, with a range of forms and organic properties, some other television series lack such freedom. Amongst the most lamentable attempts to portray plant people on screen can be found in an episode of the classic television series Lost in Space (1965-1968), which follows the adventures of the Robinson family, their pilot, their robot and an unwelcome stowaway, as they journey between planets. The episode “The Great Vegetable Rebellion” describes their discovery of a planet in which all dominant life is vegetable, and antagonistic to the animal world. Unfortunately, it is rather notorious for the poor quality of the costume design - the antagonist is literally a giant walking carrot with a human face. In fact, so comical was the production that two of the regular cast were famously suspended without pay for two episodes after ruining takes with laughter. The premise of the episodes is as unlikely as its costumes - suggesting that the crew will themselves turn into vegetables if they linger too long in the influence of the world, and never really explaining the motivation of the villain Tybo or whether he is a stray mutation or one of a more populous species.

Pervasive Plants

So why do so many plant people occur in science fiction? The pragmatic answer - which likely applies to at least some extent - is that the writers are human beings who look to their surroundings for inspiration. Science fiction aliens include cat people, dog people, bird people, fish people, even rhinoceros people. For writers struggling to come up with yet another alien species at short notice, and looking around for new ideas, plant people were probably an inevitability. In the DVD extras for Doctor Who’s “Meglos”, for example, the writers John Flanagan and Andrew McCulloch acknowledged being inspired by an old cactus sitting in the house where they were writing the script. John Wyndham’s Day of the Triffids - before the terror really starts - also addresses the inevitabliliy of the idea:

“People had felt the same about novelties of other days - about kangaroos, giant lizards, black swans. And, when you came to think of it, were triffids all that much queerer than mudfish, ostriches, tadpoles, and a hundred other things? The bat was an animal that had learned to fly: well, here was a plant that had learned to walk - what of that?” (pg 42).

Perhaps in recognition of both this inevitability and its potential to surprise an audience, “Star Healer” in James White’s Sector General series of medical science fiction mentions the sentient vegetable AACP life form as an anomaly in the four-letter classification system used in the series - the possibility of such aliens should have been anticipated but hadn’t been, and so was not allowed for in the system. Unfortunately, we don't see actual examples of these species.

However plant aliens also play on deeply ingrained reactions in the readers or viewers. It’s interesting to note that such emotional responses can work in two contrasting and conflicting directions. Where a plant person is portrayed as monstrous or aggressive, it is often formed of vines, jungle plants or carnivorous plants, with a humanoid form distorted by trailing tendrils providing a ragged outline. Such tentacled plant people can be found, for example, in “The Seeds of Doom”, “The Man-eater of Surrey Green” and Jayce and the Wheeled Warriors. Here the imagery is of jungles, dense forests and untamed nature - the threateningly alien environment that cost so many European lives as explorers encroached into territories they were unprepared for during the age of empire. It invokes a sense of otherness for an audience accustomed primarily to urban or tamed environments.

By contrast, where a plant character is benign, the imagery tends to be of flowers and familiar, broad-leaved plants, smooth-skinned perfect fruit or alternatively the solid familiarity of hardwood trees. Here the emotional memories being invoked are those of the tamed and familiar aspects of nature. An interesting exception is Zhaan in Farscape, who initially appears to show no sign of her plant origin - however her smooth, hairless skin distances her from the animal origins of her shipmates, while her pacifist philosophy echoes the peaceful passivity associated with garden plants. Similarly, the Phylosians of Star Trek are also not clearly attractive but also show the same pacifist philosophy (albeit one they are prepared to impose by force).

A subset of plant people also invoke the apparent inversion of the natural order associated with carnivorous plant species. In most food chains plants harvest energy from sunlight to chemical storage, while animals release that chemical energy to power their metabolism, ultimately converting it to movement, sound and heat as well as organic tissue. By contrast carnivorous plants such as the pitcher plants (e.g. sarracenia) or venus fly traps (e.g. dionaea) instead reprocess nutrients from decaying animal life - often life trapped and killed by motile elements of the plant itself. This abnormality is explicitly invoked in The Avengers’ “The Man-Eater of Surrey Green” and in the stings and behaviour of John Wyndham’s Triffids (Wyndham’s narrator character describes carnivorous plants as “in some way slightly indecent, or at least improper”), as well as in the appearance of Doctor Who’s Vervoids. Also invoking a sense of the unnatural are the fictions that warn against the artificial mutation of plants - either due to human pollution, deliberate breeding or genetic engineering. Again, the Triffids hint at this, although Wyndham leaves their origin deliberately vague. However this is certainly the trope invoked by the Vervoids in Doctor Who (who destroy their agronomist creator, Professor Lasky [4]) and in the cartoons Jayce and the Wheeled Warriors and Attack of the Killer Tomatoes - in all of which the plant antagonists result from the ill-advised work of human creators.

Plausible Plants

Setting aside the motivation and choices of science fiction writers, just how realistic is the prospect of intelligent plant life either on Earth or elsewhere? Biologically, the structure of the plant tissues and animal tissues are similar but with key differences. The thick cellulose cell wall of plant cells restricts the flow of material compared to the more permeable membrane surrounding animal cells, leading to a slower metabolism and transport of chemical compounds such as sugars or oxygen supply.

However, here on Earth, the primary argument against plant intelligence is one of energy supply and demand. Active animals must constantly consume the products of days or weeks of photosynthetic energy collection, whether directly or processed through the food chain. Photosynthesis itself, under Earth conditions, does not provide enough energy for sustained movement - while some plants do move occasionally, this is rare and those plants may become quickly exhausted by rapidly repeated stimulus and die. Supporting a large, active brain of any kind we understand would be similarly energy intensive. Environments such as the dark side of Venus in “The Lotus Eaters” would seem extremely unlikely to be a viable home of intelligence of any sort without external supplies or chemical energy sources. Of course, this might not preclude the possibility of photosynthetically-supported intelligence in environments with a richer supply of light energy, although regions that are too energetic might lead to damage of DNA or other hereditary mechanisms, or to a break down of the chemical reactions of photosynthesis itself. As a result, the range of conditions able to support such plant life might be very narrow.

On the other hand, a throwaway line encountered fairly often in science fiction is that an alien organism has both animal and plant features or ancestry. This is entirely possible. Here on Earth, the differentiation between plant and animal cells happened early in life’s evolutionary tree. Cells typically fall into one type or the other, with very little in between (although even here fungi fit neither category). Even so, some animals have reached a symbiosis, embedding algal cells in their own tissues (or vice versa) to make use of their chemical products. In an alien evolution of life, the differentiation between robust plant cells and flexible animal cells might well be replicated, with each serving analogue roles to those here. However it is entirely possible that alien life would result in creatures that have neither animal nor plant metabolisms, and whose tissues show elements of both including both warm blooded respiration and photosynthesis. Organisms that humanity might classify at first sight as plants (or animals) might well prove to be an entirely new class. A charitable reading of Lost In Space’s "Great Vegetable Rebellion" might put its antagonist in this category, while it would also likely describe Delvians such as Zhaan in Farscape. The photosynthetic and cellulose-rich cells of these individuals may have as much in common with our own animal biology as with the plants they resemble.

Perilous and peculiar as they are, plant people provide a perfect perturbation to our parochial paradigm. Perhaps perceptive plant people are predictable speculations, perhaps they are probable entries in the panoply of populations, and perhaps they represent peculiar possibilities for which no life on our planet leaves us prepared. Nonetheless, people will probably persist in playing with the potentialities on planets near and far, probing our preconceptions and peeking at these possible plant personalities.