A look at the genus Acorus is a glimpse at the past. It is the sole genus within the family Acoraceae, which in turn is the only family within the order Acorales. Recent work suggests that the Acorales is the sister group to all other monocots. What does this mean? I'll borrow a passage from the Flora of North America on Acoraceae: “Evidence based on DNA sequences fails to show any close relationships between Acorus and other genera, and instead supports Acorus as the oldest extant lineage of monocotyledons (M. R. Duvall et al. 1993)”. Other orders of monocots have evolved more recently (though this is relative – the scale is in the tens of millions of years).
From a gardening perspective, Japanese or grassy-leaved sweet flag is a striking groundcover. As its cultivar name implies, this particular selection is both dwarf and golden-coloured. For more, see grassy-leaved sweet flag at the Kemper Center for Home Gardening (ignore the part about it being a member of the arum family – that's out of date) or North Carolina State University's factsheet on Acorus gramineus.
Botany / horticulture resource link: There's a new(ish) blog carnival on the topic of trees: Festival of the Trees will reach festival #6 at the beginning of December. I'll link to it again when I submit for the next festival, but in the meantime, there is plenty of Sunday reading if you follow the links to the various weblogs.