Complex sentence: Apparently, most plants sold in America as "A. nidus" are actually "Asplenium australasicum", which has longer sori, and a differently shaped midrib.
Split into simpler sentences: Apparently, most plants sold in America as "A. nidus" are actually "Asplenium australasicum". But "A. australasicum" differs from "A. nidus" by having longer sori, and a differently shaped midrib.