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S at two and N at three leaves one word standing, and it is a lichen rather than anything you would say.
Usnea is a genus of pale grey green lichen that hangs in strands from tree branches. Old man's beard is the common name for it, and it grows across much of the world.
USNEA turns up regularly in herbal medicine, where the lichen is sold as a tincture, and in ecology, where its presence is taken as a sign of clean air. It is not a word invented for word lists.
U is the only opening letter here, E the only fourth letter and A the only ending. Every box holds exactly one option.
So there is nothing to work out. Confirming S at two and N at three finishes the puzzle, which happens on only a handful of pages across this site.
Usually as UZ nee uh, with the stress on the first syllable. The S sounds like a Z.
The lichen absorbs everything it needs from the air and cannot filter out contaminants. Where sulphur dioxide levels rise it disappears quickly.
No. You would never reach it by reasoning, so it belongs in the category of words to recognise rather than deduce.
Several unrelated plants are called old man's beard, including a clematis. Only the lichen is a usnea.