Philip K. Dick 
Piper in the Woods 

In Piper in the Woods, an army doctor is asked to treat a soldier from Asteroid Y-3 who claims he is a plant. This wouldn’t be a problem, but the soldier is incapable of any physical activity, preferring to stand still and point himself towards the sun. When more soldiers return from Asteroid Y-3 also claiming to be plants, Dr. Harris travels there to investigate. While there, he learns about a mysterious indigenous people living in the woods called Pipers, who might be able to shed some light on the subject.