Science Fiction and Intelligent plants:
Edmond Hamilton, Alien Earth, in Isaac Asimov Presents the Golden Years of Science Fiction: Sixth Series, Eds: Isaac Asimov and Martin H. Greenberg, 1984, Bonanza Books, pages 135-159. First Published: Thrilling Wonder Stories, April 1949.
A serum makes men a hundred times slower until "you live at the same tempo as plants." At that speed the jungle plants appear active, and humans can even hear the thoughts of the trees.
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Murray Leinster, Proxima Centauri, in Before the Golden Age, edited by Isaac Asimov, Doubleday & Co., New York, 1974, pages 606-648. First published 1935.
The major race, Centaurians, evolved from plants. They value any animal product above gold. Structures (even spaceships) are grown, not built.
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Ursula K. Le Guin, Vaster than Empires and More Slow, in New Dimensions 1, edited by Robert Silverberg, Avon Science Fiction, New York, 1971, pages 99-133.
A ten-person deep-space survey team lands on a planet with only plant life, no animal life. They come to realize the entire planet is one huge being, and the ganglionic structures at the root of every plant are like nerve cells in human brains. Having no senses, the being is essentially unconscious - no concept of "other," change, cause or effect, universe, or even more than one. When it senses the team's presence, it is terrified: even the possible existence of anything (else) is an utterly alien thought. The team's antagonistic empath, who had been literally driving the rest of them crazy, is reluctantly left behind when he disappears into the forest to calm the entity and avoid further confinement with the team's emotional effluvium.
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J. R. R. Tolkein, The Two Towers, Houghton Mifflin, 1987, (III):4, pages 75-81.
The Ents are a race of old, wise, very slow moving, tree-like creatures.
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A. E. van Vogt, Process, The Magazine of Fantasy and Science Fiction, December 1950, pages 565-571.
A world is covered by a few enormous, sentient forest/trees fighting each other for land. They discover how to make atomic explosions by piling enough radioactive material together. The ensuing nuclear war reduces them to near nonsentience, only remembering that radioactives can be useful.
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Black Orchid, DC comics.
She is a living hybrid of human and plant consciousness.
Swamp Thing, DC comics.
Alex Holland was in a fire in the Louisiana bayous. But he didn't die; his consciousness was imprinted in the flora. Now he is a living plant creature.