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sweet flag at the Kemper Center for Home Gardening (ignore the part about it being a member of the arum family – that's out of date) or North Carolina State University's factsheet on Acorus gramineus.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.
Daniel, as I recall, similar to Acrorus gramineus, Ginkgo biloba is the only species within the family Ginkgoaceae, the only family of the order Ginkgoales. I'm curious: do you know how many of these prehistoric species exist?
Sarah, I'm afraid I don't, but I'll direct the question to someone here at UBC who might be able to give some sort of numbers.
Hi Sarah, ginkgophytes (Ginkgoales) were never particularly species-rich, but they have a strong fossil record that extends at least into the Mesozoic (at least 225 Myr ago) and probably quite a bit earlier -- depends on how some of the earliest fossils are interpreted. Rothwell and Stewart (1993; Paleobotany and the evolution of plants, Cambridge U. Press) indicate that at least 19 species are known, and I think that's a pretty conservative estimate. These species are in several genera (not just in Ginkgo!).
In contrast to Ginkgo, there are at least two extant (living) species of Acorus. More than 30 Acorus species have been described in the literature, but most taxonomists today accept only 2-6 Acorus species. Often two species groups are recognized; Acorus calamus and Acorus gramineus (the North American A. americanus belongs in the former species group).
One of my graduate students (Jeff Saarela) has been working on this problem in Acorus. His initial molecular phylogenetic work suggests the existence of very substantial amounts of genetic/ phylogenetic variation within the genus, so there may actually multiple (more than two) living Acorus species. However, lots of work remains to be done to understand this genus!