Short story: man rescues alien from cruel hanging ceremony, but alien turns into a shrub

Question

I am seeking a short story where a man rescues an alien from a cruel hanging ceremony, but the alien turns into a shrub. It turns out the hanging ceremony was a way the aliens,of humanoid form, had developed to prevent themselves turning into a non-motile mature form and to retain their humanoid form.The young ones were muscular and agile, but after the ceremony the older ones were emaciated and weak. By saving the young alien, the man is horrified when the rain starts and the young alien turns into a shrub ! The joke at the end is that the spaceman telling the story recounts how the unfortunate kind man carts the shrub around the universe with him from job to job, and doesn't know he has the wrong shrub. He had grabbed it in the rain storm as they hurried to take off in their space ship while the angry aliens approached to rescue their young one. This was probably written in the late 1950's. One phrase in the story describes the spaceman doing maintenance, tightening bolts in rotation as per the service manual.

Answer 1

"Unhuman Sacrifice" by Katherine MacLean, first published in Astounding Science Fiction, November 1958, available at the Internet Archive. Here is a plot summary from jennre:

“Unhuman Sacrifice,” Katherine MacLean, 1958 — A missionary and two crewman arrive on an as-of-yet uncolonized planet. The missionary–who thinks he’s been given a great honor–is obnoxious, nearly oafish in his over-eager attempts to open a dialogue with his future converts. The crewmen find the missionary “relentless” even by normal religious standards; they assume his superiors gave him the position to be rid of him. They tolerate him until he begins preaching through a box that only transmits a few words successfully, subjecting the natives to long, loud, nonsensical sermons. Concerned about cultural contamination and the misunderstandings that might occur, they finally convince him to try another strategy.

Meanwhile, a native boy fishing at a nearby river is approached by one of the crewman. The boy assumes that the crewman is a ghost, due to his white skin, and that his strange words mean that the ghost has forgotten how to speak. Fascinated and a little frightened, the boy speculates that the crewman’s moist skin must mean that he died by being drowned in the river. Moved by pity and a sense of responsibility, the boy agrees to teach the crewman to learn to speak again. Due to the boy’s tutelage, during which they spend many happy hours together, the crewman learns about the local culture, which includes a rite of passage ritual in which boys are hung upside down during a flood. Survivors are left elongated and sickly but considered adults; those who fail enter the spirit world.

At the conclusion, a flood bears down on the valley cradling the ship. The natives anticipate the flood and, before leaving, hang their boys from trees and leave them to their fate. The crewmen rescue the boy, as they’ve grown very fond of him, but this delays their return to the ship, which is now half-submerged. They almost make it to the ship, but the preacher–who can’t swim–has lagged behind, and they must both rescue him. They instruct the boy to wait for them on the ramp. The boy is fatalistic; he expects death and fantasizes that the two crewmen are ghosts who have found him unworthy of adulthood. He lets himself sink in the water; when he reaches the bottom, he feels the urge to take hold of his feet. Gradually, he turns into a plant. When the crewmen return, his friend is devastated.

Flash forward: The two crewmen endure an awkward reunion on another ship. They’re civil, but they know they can’t serve together. The memory of the boy’s death has alienated them from each other. The crewman who was closest to the boy still hasn’t recovered; he takes the “plant” with him on all of his trips. The other crewman doesn’t have the heart to tell his former friend that it’s the wrong plant.