"mel turner" schreef
This interesting story probably could use some elaboration.
A longer, but still short, version:

Although long classified in the arum family Araceae, "Sweet Flag"
[_Acorus calamus_ and a few other species of Acorus] have been
shown by molecular phylogenetic studies to be extremely distant
from the true aroids.

_Acorus_ is now placed in its own family Acoraceae, which is alone
in its own order Acorales.

[In contrast, it turns out that the duckweeds, traditionally a
separate family Lemnaceae, do belong within the arum family]

Acorus/Acoraceae/Acorales is evidently a very isolated, ancient
lineage, and according to several studies represents the living
sister group to all other monocotyledons.

http://www.efloras.org/florataxon.as...taxon_id=20008
http://www.mobot.org/MOBOT/Research/APweb/welcome.html

***
Yeah, what he said.

However, as /Lemnaceae/ were such a distinctive family it may be worth
considering splitting the new /Araceae/ to get them back. According to the
information at the APwebsite this would mean that /Lysichiton/ and allies
would then also need to become a separate family.
PvR