aspleneum, spelled with an e

The last two syllables are a blur of vowels, ee-um, and nothing in the sound says whether the first of them is written i or e. Latin settles it. The -ium ending is what botanical Latin puts on a genus it treats as neuter, and it keeps its i however lightly it is said. Polypodium and lycopodium end the same way and are written the same way.

The last syllable, where botanical Latin keeps letters that speech drops.