The Seeds of Doom
Arnold Keeler was a botanist in the employ of Harrison Chase.
On Chase’s orders, Keeler and Scorby went to Antarctica to steal the Krynoid pod discovered there and bring it to England.
While performing experiments on the pod, Keeler became infected by the pod, and he slowly transformed into a Krynoid himself. His biology changed from that of an animal to a plant. Keeler grew angry at Chase, who refused him medical attention and locked him up to study, feeding him on raw meat. As Keeler fully changed, he effectively ceased to exist.
The Krynoid that used to be Keeler grew to gigantic size, became psychically linked to Chase, and took control of the nearby plant life as part of its plan to take over the world. It was destroyed when Major Beresford of UNIT, on the Fourth Doctor’s advice, called in an aerial attack to bomb Chase Mansion. (The Seeds of Doom)