Krynoid

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

Physical characteristics
Krynoids are bulky plants covered in tentacles. They have a very undefined anatomy, with no front or back. Though they have a high tolerance to the cold, high temperatures can damage them.

Krynoids can grow incredibly quickly, easily growing larger then a house in a few hours. Though they require animal proteins while they are young, they can survive on normal photosynthesis later in life. They also don't need roots to gather nitrogen and can gather nitrogen directly from the air.

The second Krynoid seen on Earth appeared to be as intelligent as a Human and could speak at least Human languages, although this may be because its host was a human being. Krynoids also have psychic powers, being able to control plants and some creatures. (DW: The Seeds of Doom)

Life Cycle
Krynoids start life as small pods, generally only about 20 centimetres in diameter. Possibly due to the Krynoids' home planet being an extremely volatile environment, many of these pods are launched into space (always in pairs). These pods can survive long trips in space, being able to hibernate for thousands of years.

Once the pod reaches a planet and is exposed to both sunlight and more tolerable conditions, it starts to grow. If a creature comes too close to the pod, it opens up and releases a long shoot, which grabs onto the victim. This infects the victim, slowly turning them into a Krynoid. The victim becomes weak and starts developing plant-like characteristics. At this stage, they still have most of their personality, if intelligent, though they crave animal proteins in order to grow. As time passes, they grow larger and begin to lose all vestiges of their original physical form. Their original personality is lost to the Krynoid sentience.

The Krynoid grows quickly, reaching the size of a house within hours. At this stage, Krynoids can control plants and will aggressively kill all creatures. Finally, the Krynoid will germinate, creating more pods to populate the planet. (DW: The Seeds of Doom)

History
The Krynoids were well known through-out the galaxy, though difficult to study due to their violent nature. Two of the Krynoid pods arrived on Earth in Antarctica, approximately 20,000 years ago. They lay dormant until the 20th Century, when they discovered by an expedition. One of them hatched after being exposed to ultra-violet light, and infected one of the Human scientists. The Doctor was sent in by UNIT and intervened in the nick of time, but the second pod was stolen and taken to the home of the botanist Harrison Chase. Chase allowed this pod to infect one of his scientists, Arnold Keeler. This second Krynoid turned against and controlled Chase and then went after the Doctor and Sarah Jane Smith. (DW: The Seeds of Doom)

Trivia

 * The humanoid Krynoid costume was a modified Axon costume.