Krynoids are a hostile plant life which consume, and control, the native animal life of the planet on which they germinate. Krynoids are able to take over a human body, whereby it evolves into a full size Krynoid.
Two Krynoid pods were discovered in the permafrost in the Antarctic. As the pod was unique the fourth Doctor was called in to identify the pod and advise the government about it. One pod germinated and took over one of the scientists at a base in the Antartic. It was destroyed along with the base in an explostion.
The second was stolen for the millionaire plant-lover Harrison Chase where it took over one of his scientists and ran amok in England. It controlled other plant life in Chase's mansion's grounds and eventually the whole estate became hostile. The final Krynoid was destroyed by a UNIT airstrike.