Krynoid

Krynoids were large, carnivorous alien plants which were often considered intra-galactic weeds with a hunger for animal life. The Krynoid travelled through space as a tendril enclosed in a small pod, and they always travelled in pairs. Their method of travel is unknown, but it may have been a result of explosions on the surface of their home planet hurling the pods into space. The Krynoid pods were very hardy, and could easily withstand the extreme cold of space.

Biology
Krynoids were bulky, mobile plants covered in tentacles, which may have been roots or vines. They had a very undefined anatomy, with no front or back. Though they had a great tolerance to cold, high temperatures could damage them. Some of these tentacles appear to end in red flower-like structures.

They could grow incredibly quickly, easily outgrowing a house in a few hours. Krynoids required animal proteins while young, but could survive on normal photosynthesis later in life. They gathered nitrogen directly from the air, without the need of roots.

A Krynoid seen on Earth in the 20th century appeared to be as intelligent as a human and could also speak English. This may have been a result of having taken a human host. Krynoids also had psychic powers and were able to control plants and some animals. (DW: The Seeds of Doom)

Krynoids were affected by high-pitched noises. (BFA: Hothouse) They could also be killed with special rockets. (IDW: Don't Step on the Grass)

Life Cycle
Krynoids started life as small pods, generally only 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, able to hibernate for thousands of years.

Once a pod reached a planet and was exposed to sunlight and more tolerable conditions, it started to grow. When a creature came too close to the pod, the pod would release a long shoot. This shoot grabbed and infected the victim, slowly turning it into a Krynoid.

The transformation began almost immediately; first the skin began to turn plant-like in color and texture, then it looked like it was covered in green tendrils, and finally all but the basic shape of the lifeform was lost under a mass of green plantlike growths. Both the body temperature and the pulse rate dropped severely, and plant bacteria — schizophytes — appeared in the bloodstream. The Krynoid actually took over the mind of its host first, and by the time the body had completely metamorphosed into vegetation, the host's mind had disappeared and 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, and seemed to have great strength. During this early period, the animal-Krynoid hybrid was desperate for warmth and protein, some of which it absorbed as it converted its host. As it ate, it grew larger. The Fourth Doctor theorized that amputating the area first infected might slow the rate of infection, but this was never put to the test.

Finally, the Krynoid germinated, creating hundreds of pods, ready to reproduce. (DW: The Seeds of Doom)

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

The Antarctic pods
Two of the 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, travelled to the base and investigated, discovering the second pod while doing so. 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 its belief that plants should be dominant over animals. The Krynoid was destroyed when UNIT bombed the mansion in an aerial attack. (DW: The Seeds of Doom) UNIT kept information on these Krynoids in the Black Archives, (IDW: Don't Step on the Grass) and pods were kept in the Vault. (CC: 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. Instead it produced a type of Krynoid that had a far faster metabolic rate and completed its life-cycle in a single hour, as opposed to around a day in the case of normal Krynoids. (BFA: Hothouse)

Other References

 * When Handrel was trying to give himself unlimited regenerations, he used a Time Scoop to capture a Krynoid for research. (ST: The Time Lord's Story)
 * After suffering from memory loss, Sarah Jane Smith was unable to tell the difference between Krynoids and Pescatons. (EDA: Interference - Book Two)
 * The Tenth Doctor compared the Swamp Monsters to Stage Trees, Comet Flowers, and Krynoids. (NSA: Wetworld)
 * The Tenth Doctor thought an occurrence of hostile plants was caused by a Krynoid, but he was later proven wrong. (IDW: Don't Step on the Grass)

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.