A plant was a kind of life-form.
Around 10,000,000 AD, the Earth was in danger of falling into the Sun. The entire human, Monoid, animal and plant population of the planet was loaded onto the Ark, a gigantic spacecraft, to take them to a new home on Refusis II. (TV: The Ark)
Krynoids were sentient plants which could control normal plants. They fed on animals and nitrogen. They travelled through space in pods. When they landed on a planet, they grew and germinated, spreading until all animal life was eradicated. (TV: The Seeds of Doom)
The Master Plant, like the Krynoids, could control other plants. It was artificially created by Professor Rayner. When it had no further use for him, it killed the Professor. (COMIC: The Plant Master)
The Raab were a plant species whose favoured habitat was low-gravity environments. They travelled through space and scattered their seeds when they impacted an asteroid or moon. Once they had grown to maturity, they would fire their seeds into space, to begin the cycle anew. (PROSE: Frontier Worlds)