Krynoid

Krynoids were large, carnivorous alien plants that could be thought of as galactic weeds. On planets where the Krynoid took root, all animal life would become extinct, as Krynoids desired that all plants should win against the animals that ate them.

Biology
Krynoids travelled through space as tendrils in small pods, always in pairs. They were very hardy and could easily withstand the extreme cold of space, (TV: The Seeds of Doom) although snow and ice could still slow them down and render their pods inert. (PROSE: An Apple a Day)

A Krynoid itself was bulky, covered in tendrils and vines, some of which ended in red flower-like structures. They had an undefined anatomy with no front or back. Though they had great tolerance for cold, high temperatures could damage them. They required animal proteins while young, but could survive on normal photosynthesis later in life, and gathered nitrogen directly from the air without a need for roots. Over time, Krynoids were also able to channel their power into other plants, at least within a mile radius; they were also able to psychically possess other humans, though this may have been due to some humans' sympathy for plants.

While they were unaffected by gunfire and lasers, they were affected by being cut as well as fighter plane missiles, (TV: The Seeds of Doom) and could be killed with special rockets. (COMIC: Don't Step on the Grass) One strain of Krynoids were affected by high-pitched noises, allowing the original consciousness of their host to reassert itself temporarily. (AUDIO: Hothouse)

Life cycle
Krynoids started life as small pods about twenty centimetres in diameter. Possibly due to the volatile nature of the Krynoids' home planet, many of these pods were launched into space, always in pairs. These pods could survive long trips in space, hibernating for thousands of years.

Once a pod reached a planet and was exposed to sunlight and more tolerable conditions, it grew. When a creature came too close to the pod, the pod would release a long shoot which grabbed and infected the victim, slowly turning it into a Krynoid. The transformation began almost immediately: first the skin turned plant-like in colour and texture, then it looked covered in green tendrils. Finally all but the basic shape of the lifeform was lost under a mass of green, vegetable growths. The body temperature and the pulse rate dropped severely and plant bacteria — schizophytes — appeared in the bloodstream.

The Krynoid took over its host's mind first. By the time the body had completely changed into vegetation, the host's mind had given way to the Krynoid sentience. Initially, the shock of this transformation kept the hybrid sedate, but once the Krynoid had taken over fully, it became mobile again. It seemed to have great strength. During this early period, the animal-Krynoid hybrid was desperate for warmth and protein, which it absorbed as it converted its host. As it ate, it grew larger. The Fourth Doctor theorised that amputating the area first infected might slow the rate of infection, but this was never put to the test.

They could grow incredibly quickly, easily outgrowing a house in a few hours, and within a day the Krynoid would reach its full size, becoming as large as St Paul's Cathedral. Finally, the Krynoid entered its germination period where it would release hundreds of pods, reproducing a thousand-fold until Krynoids covered the entire planet. (TV: The Seeds of Doom)

Intelligence
The Krynoid ideology was directed towards the revolution of all plant life against the animals that ate them and depended on them to live.

A Krynoid seen on Earth in the 20th century could speak English and attempted to bargain with humans for their lives. It spoke of its host as being part of the Krynoid, referring to "us" and "we". (TV: The Seeds of Doom) Hosts being consumed by a Krynoid would hallucinate about being in a jungle or rainforest and would begin to feel a part of it. (AUDIO: Hothouse)

History
The Krynoids were well known throughout the galaxy, though difficult to study because of their violent nature.

The Antarctic pods
Two Krynoid pods arrived on Earth in Antarctica approximately twenty thousand years before the 20th century. They lay dormant until the 1980s, when one of them was discovered by a scientific expedition.

The pod hatched after being thawed out by an ultraviolet lamp. It infected Charles Winlett, one of the human scientists who had found it. The Fourth Doctor, having been alerted to the situation by the World Ecology Bureau, went to the base and investigated. He found the second pod. The second pod was stolen by Scorby, who destroyed the base and the Krynoid with a bomb and took the pod to Chase Mansion, botanist Harrison Chase's home. Chase allowed the pod to infect one of his scientists, Arnold Keeler. This second Krynoid grew to gigantic size and developed a psychic link to Chase, who agreed with it that plants should be dominant over animals. The Krynoid was destroyed when UNIT bombed the mansion in an aerial attack. (TV: The Seeds of Doom) UNIT kept information on these Krynoids in the Black Archives (COMIC: Don't Step on the Grass) and pods were kept in the Vault. (AUDIO: Tales from the Vault)

Alex Marlowe's experiments
In the early 21st century, former musician Alex Marlowe led a project attempting to genetically engineer a new Krynoid strain that would cause the physiological changes induced by the Krynoid infection to still take place, including granting the power to control plants, while keeping the victim's original consciousness intact. The project was a failure. It produced a type of Krynoid with a far higher metabolism. It completed its life-cycle in a single hour, not the day normal Krynoids took. Fortunately, the new Krynoid strain had an unexpected weakness in that high-pitched sounds, rather than just causing it pain, allowed the victim's consciousness to re-assert itself for a brief period. The Eighth Doctor was able to exploit this weakness and destroyed Marlowe and his new Krynoids. (AUDIO: Hothouse)

On Trenzalore
A pair of Krynoid pods landed on the planet Trenzalore. The Eleventh Doctor speculated that "someone" had sent them to Trenzalore. One of them lay inert in the snow, but another fell into a climate-controlled orchard inside a greenhouse, and subsequently hatched and infected the farmer Pieter. The Krynoid grew to the size of a barn and fed on live animals in its aim to colonise Trenzalore and consume all animal life. The Doctor and his companion Theol Willoughby drew the Krynoid to the Clock Tower in Christmas, where one of Christmas' townspeople, Jerl Tompkinson, used a rubber hose to fire water at the Krynoid, freezing it. With the Doctor's help, the townspeople pulled at a rope to ring the Clock Tower. The reverberations from this noise shattered the Krynoid's frozen body. The Krynoid's—and Pieter's—remains were burnt on a makeshift funeral pyre, along with the unhatched pod. Theol carved the Doctor a walking stick out of a piece of Krynoid for the Doctor to remember Pieter by. (PROSE: An Apple a Day)

Other references
When Handrel was trying to give himself unlimited regenerations, he used a Time Scoop to capture a Krynoid for research. (PROSE: The Time Lord's Story)

After suffering from memory loss, Sarah Jane Smith could not tell the difference between Krynoids and Pescatons. (PROSE: Interference - Book Two)

The Tenth Doctor compared the swamp monsters to stage trees, comet flowers and Krynoids. (PROSE: Wetworld) He once thought an occurrence of hostile plants was caused by a Krynoid. He was proven wrong. (COMIC: Don't Step on the Grass) He later worked out that the Spores, which turned out to be Rutans, were not Krynoids. (PROSE: The Taking of Chelsea 426)

A Virgoan space probe had an encounter with a Krynoid on Melandra IV, which resulted in it bringing the Krynoid to 21st century Earth. (PROSE: Stop the Pigeon)

When the telepathic Emily Fairfax read the mind of the Hypothetical Gentleman, a Krynoid was among the images see saw. (COMIC: Hypothetical Gentleman)

The Eleventh Doctor once recounted saving the actor Peter Davison from a Krynoid. (COMIC: The Girl Who Loved Doctor Who)

The Twelfth Doctor recalled dealing with a Krynoid for Hyphen T Hyphen. (COMIC: Space Invaders!)

Behind the scenes

 * The humanoid Krynoid costume was a modified Axon costume painted green. Ironically, plans for a collect-and-build Krynoid toy were abandoned and the Krynoid toy was instead painted orange and pink and turned into an Axon toy.