The weeping, talking trees in Virgil and Dante suggest that the idea of communication with plants is of great antiquity, but only in the sense of transmigration of human souls into plants; the subject is not yet real plant intelligence in its own right.
Then comes the transitional example in the early part of William Hope Hodgson’s The Boats of the Glen Carrig (1907). In the chapter ‘The Land of Lonesomeness’ we are taken to an island in which there is a wailing during the night, and evil trees are prone to wrap their branches round the unwary traveller. The narrative suggests that human souls are somehow sucked into the trees and then beckon for more to join them. The sense of horror is peculiar and powerful. The atmosphere is that of supernatural fear, but the work can marginally count as science fiction.
Then comes the great age of magazine science fiction, and all sorts of portrayals of intelligent plants blossom out into the literature.
Murray Leinster’s ‘Proxima Centauri’, dating from the early years of pulp SF, depicts malevolent space-travelling plants attacking human explorers. A more subtle approach comes from the planet-wide vegetable intelligence in the 1931 story ‘Seedling of Mars’ by Clark Ashton Smith, where humanity is subjugated by the promise of Utopia. Raymond Z Gallun, another vintage 1930s writer, produced a more evocative variation on this theme in ‘Seeds of the Dusk’, where this time humanity is gassed to peaceful death by an alien vegetable invader in the far future. In this last story, the reader is made to feel that the removal of the last degenerate humans is no great loss to the world.
As a change from these threats, in Clifford D Simak’s All Flesh is Grass (1965) we actually enounter a benevolent (though somewhat ruthless) intelligent life in plant form, though the form it takes is that of a planetwide biological computer that works through photosynthesis, and is only outwardly similar to the plant life we know. All Flesh is Grass is one of Simak’s best novels, a joy to read. Proclaiming the brotherhood of all species in his gentle, humane, inimitable style, there is nevertheless nothing soft or flabby about it, and it contains plenty of excitement, menace and that impingement of a strange cosmos upon ordinary life, which is the hallmark of a certain subgenre of science fiction – what one might call the small-town cataclysm.
What of plant civilization considered in itself, without regard to its impingement upon humanity? For this you have to go to Olaf Stapledon, to the 8 pages in Star Maker (1937) in which he narrates the rise and fall of the ‘plant men’ of a small, hot, energy-rich world. The story of the beings he describes is dominated by the tension between their active night-time and their contemplative day-time natures. The balance is eventually lost, and first one, then the other nature predominates, leading to the doom of the plant men and their world. In 40 years of reading science fiction I have never come across anything remotely comparable in intensity to these 8 pages, as far as the theme of plant intelligence is concerned. It is a parable of universal relevance to all cultures, in the stress it lays on the vital importance of fidelity to one’s natural origins.