Sunday, May 10, 2009

Isotria verticillata (Orchidaceae)

Three weeks ago, while on a field trip with the Missouri Native Plant Society, we stumbled upon a large population of Isotria verticillata at the peak of bloom. Anyone that has conducted surveys for rare, threatened or endangered species in the Midwestern and eastern United States has probably had this search image in their head (though maybe more so for its relative the federally threatened Isotria medeoloides).

Before I learned the vegetative field characters that distinguish this species from the often sympatric Medeola virginiana, I agonized over its identification. With practice and a visit to the herbarium, I learned they are quite distinct even when found in the sterile state. When sterile, both have a whorl of leaves at the top of the stem. However, Medeola has three (or sometimes five) distinct veins in each leaf while the venation of Isotria is inconspicuous and more reticulate. The venation of Isotria reminds me of the organization of juice sacks you find in an orange when you peel the papery septa off a locular segment. Additionally, Medeola usually has some arachnose pubescence on the stem where Isotria is glabrous. Lastly, the leaves of Isotria are more obovate to oblanceolate and pudgy while the leaves of Medeola are more elliptic to narrowly oblanceolate.

Isotria verticillata has a stalked flower and long sepals which distinguishes it from I. medeoloides which as a sessile flower and much shorter sepals. Isotria verticillata is usually found in colonies while I. medeoloides is often found as single plants.

Isotria medeoloides has not been seen in Missouri since 1897. It is panning out to be a great year for orchids in Missouri. Perhaps it will show up.