Krynoids are bulky, mobile 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.
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.
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 lifecycle in a single hour, as compared to around a day in the case of normal Krynoids.
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 1970s, 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 Doctor, having been alerted to the situation by the World Ecology Bureau, traveled 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.