This question reminds me of Raymond Z. Gallun's Martian colony plants which were joined together in a single mind across the entire planet.
They appear in the short story "Seeds of the DusK" (1938) and also in the novel The Planet Strappers (1961). And possibly in other stories which I haven't read.
And I note that "Seeds of the Dusk" is said to be in the "When Earth is Old" series along with "When Earth is Old".
"Seeds of the Dusk", like The War of the Worlds, was based on obsolete ideas that the farther a world was from the Sun, the sooner it would have formed and acquired life and the soon it would die and become lifess.
When the world of the intelligent plants was almost dead, they would launch gazillions of spores into space and some of them would land on a world which was dying but not yet as far gone.
But the growths which were the last civilized beings of Mars had not originated there. Once they had been on the satellites of Jupiter, too. And before that--well, perhaps even the race memory of their kind lost the record of this dim, distant ages. Always they had waited their chance, and when the time came--when a world was physically suited tor their developlment--they had acted.
But who knew? Out of the void to Ganymede the invaders had come. Across space to Mars. Riding light to Earth. Perhaps when the time came--when Venus was growing old----
And I have read other stories about planets were all life was joined into a single organism, though I don't remember them as well.
One would be Murrey Leinster's "The Plants" (1946)