Tribe Arethuseae (Calopogon R. Brown and Arethusa L.) and Subfamily Vanilloideae (Part One) (Pogonia Jussieu) - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 154

and Marcks 1972 ). The chromosomes of Pogonia (2 n = 18) average 18 Pm in length
versus 3.8 Pm in Arethusa (2 n = 40, 44) and 4 Pm in C. tuberosus (2 n = 26, 40 or 42)
(Baldwin and Speese 1957 ; Thien and Marcks 1972 ; Love and Love 1981 ; Goldman
et al. 2002b ; Sheviak and Catling 2002a, b ). The number and size differences
between the chromosomes of Pogonia and the other two species apparently inhibit
hybridization.
Pollinators and Pollination Mechanisms
Since the flowers of all three orchids produce little or no reward, it is likely that they
are pollinated primarily by deceit. The pollinators are apparently attracted to the
large, colorful perianths and the ultraviolet absorbing, anther-like brushes on their
lips (Stoutamire 1971 ; Thien and Marcks 1972 ). Thien and Marcks ( 1972 ) studied
the pollination of all three species in a bog in Vilas County, Wisconsin. Visitors
were infrequent, but all were bee pollinated (see below).
In C. tuberosus , the base of the upright lip is narrow and functions as a hinge
(Fig. 8.3a ). A pollen-seeking bee of the proper size and weight landing on the ultra-
violet absorbing tuft of anther-like hairs causes the lip to fall forward and down
(Fig. 8.3b ), and the pollinator is deposited on the upper surface of the column.
According to Thien and Marcks ( 1972 ), it lands on its back and skids downward
along the column, where it first contacts the stigma and then the anther (Fig. 8.3c ).
The stigma is viscid, and a sticky secretion is deposited on the bee's dorsal abdo-
men. As it slides past the anther, pollinia may adhere to this secretion and be
extracted from the anther.
The process sounds untidy, but the column, shaped like a slide, has ridges along
both margins that constrain and position the body of the pollinator. As a result, an
effective pollen vector, a bee of the proper size, usually receives the pollinia on the
dorsal surface of its first abdominal segment (Fig. 8.4b ). The lip snaps back to the
vertical position when released by the bee, and the flower is ready for another visi-
tor. The entire process is completed in less than 4 s. If the bee, now carrying pollen,
visits another flower and the process is repeated, pollen may be deposited on the
stigma during the slide down the column.
In Arethusa , the pollinator lands on the anther-like cluster of ultraviolet absorb-
ing hairs and crawls toward the base of the lip searching for nectar (Fig. 8.2a ) (Thien
and Marcks 1972 ). It follows the ridges of the crested lip beneath the over-arching
winged column, thrusting its head and thorax deep into the flower (Stoutamire
1971 ). As the bee backs out, a downward curvature near the center of the lip
(Fig. 8.2a ) forces its thorax against the column. It first brushes against the stigma
(Fig. 8.2b, c ), which deposits a sticky secretion on the hairless rear edge of the tho-
rax and on adjacent hairs. Next, it contacts a small protrusion on the underside of the
hinged anther case; the anther case is pulled open, and the pollinia are attached to
the stigmatic secretion (Fig. 8.4a ) (Stoutamire 1971 ; Thien and Marcks 1972 ). Once
the insect has passed, the anther case snaps shut (Stoutamire 1971 ). Since pollinia