The Krynoid is a botanical alien lifeform, a "galactic weed" with a hunger for animal life. The Krynoid travels through space as a tendril enclosed in a small pod, and they always travel in pairs. Their method of travel is unknown, but it may be a result of explosions on the surface of their home planet hurling the pods into space. The Krynoid pods are very hardy, and can easily withstand the extreme cold of space.
Once the pods land on a suitable planet and heat up, they begin to grow, presumably needing only to absorb the ultraviolet radiation found in sunlight. After reaching a certain size, the pod splits and releases a tendril, which immediately begins to seek a warm lifeform. When it does, the tendril attaches itself to the lifeform and begins to metamorphose it. The transformation begins almost immediately; first the skin begins to turn plant-like in color and texture, then it looks like it is covered in green tendrils, and finally all but the basic shape of the lifeform is lost under a mass of green plantlike growths. Both the body temperature and the pulse rate drop severely, and plant bacteria - schizophytes - appear in the bloodstream. The Krynoid actually takes over the mind of its host first, and by the time the body has completely metamorphosed into vegetation, the host's mind has disappeared. Initially, the shock of this transformation keeps the hybrid sedate, but once the Krynoid has taken over fully, it becomes mobile again, and seems to have great strength. During this early period, the animal-Krynoid hybrid is desperate for warmth and protein, some of which it absorbs as it converts its host. As it eats, it grows larger. The Doctor theorizes that amputating the area first infected might slow the rate of infection, but this is never put to the test.
Following the brief transition period in which it assumes the shape of the creature it infected, the Krynoid turns into a dome-shaped thing covered in tendrils, and makes a gurgling, raspy breathing sound. Some of these tentacles appear to end in red flower-like structures. As the Krynoid continues to eat, it will grow to the size of St Paul's Cathedral. It also has the ability to speak, although this may depend upon what sort of being it takes over. It can channel its power through nearby plants in order to help it achieve its aim - the extermination of all animal life, which it sees as perfectly justified, considering how animals exploit plants. The plants it controls seem to take on a life of their own, usually strangling their victims. The Krynoid can also possess the minds of humans.
The true danger of the Krynoid comes when its germination cycle is completed. At this point, having grown to enormous size, it can reproduce itself a thousandfold, rapidly covering the planet. Laser weapons do it no harm; however, it is sensitive to being cut and it appears to dislike fire. An efficient way to destroy a Krynoid is to blow it up.
