Tribe Pogonieae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 168

Isotria verticillata (Muhlenberg ex Willdenow) Rafinesque
(Large Whorled Pogonia) and Isotria medeoloides (Pursh)
Rafinesque (Small Whorled Pogonia)
Distribution and Habitat
Both species are eastern woodland plants with a preference for acidic substrates. I.
verticillata occurs in sphagnum bogs or dry to mesic, forested uplands from south-
ern Maine and Michigan to Georgia and east Texas (Luer 1975 ). It can be abundant
with a single genet producing as many as 300 closely spaced flowering shoots (ram-
ets) (Mehrhoff 1983 ). I. medeoloides , considered one of the rarest orchids in tem-
perate North America (Mehrhoff 1983 ; Vitt and Campbell 1997 ), occurs at scattered
sites from Maine to Georgia and west to Michigan and Missouri (Luer 1975 ; Vitt
and Campbell 1997 ; Mehrhoff and Homoya 2002 ). Each site usually has only 3-10
plants, although some northern stations include over 100 (Mehrhoff 1983 ; Brackley
1985 ; Vitt and Campbell 1997 ). Often occurring in disturbed areas, its preferred
habitat appears to be relatively open, dry-mesic to mesic, second growth deciduous
or deciduous-coniferous forest (e.g., Gregory 1988 ; Mehrhoff 1989a, b ; Mehrhoff
and Homoya 2002 ). Prolonged periods of dormancy have been reported (Brumback
and Fyler 1988 ; Ware 1990 ; Vitt 1991 ).
Floral Morphology
Both species produce one, occasionally two, terminal, resupinate flowers on each shoot
(Mehrhoff 1983 ; Vitt and Campbell 1997 ). The flowers of I. verticillata are multicol-
ored and distinctly larger than the uniformly light green flowers of I. medeoloides
(Table 9.1 ). The sepals are widely spreading, and the lateral petals converge over the
column (Figs. 9.1 and 9.2 ) (Luer 1975 ; Correll 1978 ). The lip is obovate and apically
3-lobed with a rounded middle lobe. The middle lobe in I. verticillata is expanded and
white with a broad, fleshy, green ridge along the middle of the disk and a revolute,
undulate margin. The lateral lobes, with purple margins, are turned upward, forming a
floral tube with the other two petals (Fig. 9.1c ). The rounded, yellowish-green to green-
ish-white lip of I . medeoloides is less ornate with a slightly emarginated middle lobe
and yellowish-green, keel-like projections extending from the base and expanded
above into blunt, elongated, wart-like processes. The narrowly triangular lateral lobes
are involute and again form a floral tube with the lateral petals (Fig. 9.2a ) (Luer 1975 ;
Mehrhoff 1983 ; Mehrhoff and Homoya 2002 ). The column is white and has a white,
terminal, incumbent anther containing two, soft, mealy pollen masses with upwards of
2,000 tetrads in each (Figs. 9.1b and 9.2b ) (Luer 1975 ; Mehrhoff 1983 ). True pollinia
are absent (Pridgeon et al. 2003 ); granular pollen in this group is again thought to rep-
resent a reversion rather than a primitive condition. The stigmatic area is well defined
and pubescent in both, but the rostellar flap, between the anther and the stigma, is
reduced in I. medeoloides (Figs. 9.1b and 9.2b ) (Mehrhoff 1983 ). Viscidia are absent
(Dressler 1993 ). Young flowers of I. verticillata produce a scent (Andrews 1901 ;