Krynoid

Krynoids are large, intelligent 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 getting 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.

Despite their appearance, Krynoids are as intelligent as a Human and can speak Human languages. 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 a 20 centimetres in diameter. Due to the volcanic nature of the Krynoid's home planet, many of these pods are launched into space (always in pairs). These pods can survive long trips in space, being able to go in hibernation 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 shoot can infect the victim, slowly turning them into a Krynoid. The victim becomes weak and starts turning green and growing small tendrils. At this stage, they still have most of their personality, though they crave animal proteins to grow. As time passes, they grow larger and grow even more tendrils. They start to lose their personality and become more like an adult Krynoid.

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 start to 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 laid dormant until the 1900s, when they discovered by an expedition. One of them hatched after being exposed to ultra-violet light, and took control of 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 new Krynoid turned againist 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.