Krynoid

Krynoids were large, carnivorous plants that were often considered to be intra-galactic weeds.

Biology
Krynoids were bulky, mobile plants covered in tentacles. They had a very undefined anatomy, with no front or back. Though they had a high tolerance to the cold, high temperatures could damage them.

They showed an ability to grow incredibly quickly, easily outgrowing a house in a few hours. Krynoids required animal proteins while young, but can 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 1980s appeared to be as intelligent as a human and could also speak English, although this may have been because its host was a human. 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 20 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, being able to hibernate for thousands of years.

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

Victims of the infection became weak and started developing plant-like characteristics, such as plant bacteria in the bloodstream. At this stage, they still had most of their personality, if intelligent, though they craved animal proteins in order to grow. They grew larger and began to lose all vestiges of their original physical form as well as losing their original personality to the Krynoid sentience.

The infected would grow quickly, reaching the size of a house within hours. It could control plants, using them for various tasks. 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 Artic pods
Two of the Krynoid pods arrived on Earth in Antarctica, approximately 20,000 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, and infected Charles Winlett, one of the human scientists which 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, the home of botanist Harrison Chase. Chase allowed the pod to infect one of his scientists, Arnold Keeler. This second Krynoid grew to gigantic size and developed a psychic link with 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)

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 however, and instead produced a type of Krynoid that had a far faster metabolic rate and completed its life-cycle in a single hour, as compared 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.