polietrichum, and why the ie looks so plausible
heard as polietrichum
spelled polytrichum
The sound is right and the letters are not. Polytrichum has y in that slot.
What the ear did
English uses ie for this vowel in words it made itself, such as pliers or friendlier. Greek loans keep the y they were transliterated with, and the botanical name never anglicised.
P-O-L-Y-T-R-I-C-H-U-M
The full entry for polytrichum covers the name itself.