This is a partial answer, which I hope someone can flesh-out. I will give what detail I can.
The story you are after sounds like one that I read perhaps as long as 30 years ago. I could have sworn it was by James Blish, but I can’t find it in any of my own Blish collections.
I’m not certain of the initial setup, but I think that it involves a newly-discovered planet, and an explorer helping a young humanoid native to escape from something or someone. The native goes along with this quite happily, I think out of cooperative innocence.
At the climax they are running through some open landscape, which I think must involve crossing a stream or river. The story ends with the native responding to contact with water by coming to a halt and reflexively bending over to grab his own ankles: his body quickly fuses into this new shape. His ‘feet’ do what they were going to do all along, and put down roots. The natural mature stage of his existence is to stop being a mobile ‘animal’ and become a ‘plant’. I suppose his mobile form achieves something a bit like thistledown, and once he reaches a suitable place to germinate (or whatever we should call it) its job is done.
The ‘rescuer’ (or perhaps just the reader, if I have invented this rescue story) suddenly has to realise that he or she was massively misinterpreting an alien life cycle that’s actually fairly straightforward once you have an informed perspective on it. Classic hard-and-soft sf.