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Catling (
1983
) found that cleistogamous flowers of
C. odontorhiza
essentially lack
a hamulus and that autogamy occurs as a result of the direct growing together of the
pollinia and the stigmatic surface rather than by a rotation of the hamulus as in many
other species of
Corallorhiza
. More specifically, a small flap of the rostellum that
separates the pollinia and stigma in chasmogamous flowers (Fig.
6.5c, d
) is absent
or poorly developed in cleistogamous flowers (Fig.
6.5e, f
). This allows the pollen
mass to contact the viscid secretion of the stigmatic surface leading to pollen expan-
sion and germination of the pollen tubes. In
C. bentleyi
, a hamulus is present, but as
the flowers remain closed it is not used. The pollinia germinate in place, and the
pollen tubes grow down to the stigma (Fig.
6.6c
) (Freudenstein
1999
).
In
C. maculata
and
C. trifida
, on the other hand, Kipping (
1971
), Catling (
1983
),
and Freudenstein (
1994b, 1997
) consider autogamy to occur through rotation of the
pollinia onto the stigmatic surface. The pollinia in both are positioned near the apex
of the column (Figs.
6.7b
, c and
6.8c, e
), where they are retained until the anther cap
degenerates and falls away (Catling
1983
). They are then free to rotate forward and
downward on the hamulus through an angle of as much as 270°, bringing them into
contact with the stigmatic surface on the underside of the column (Fig.
6.8f
).
Claessens and Kleynen (
1998
) described a somewhat different process in
C. tri-
fida
. Following opening of the anther cap and connection of the stipes with the cau-
dicles and pollinia, the elevated anther cap dries out and falls off. The pollinia rest
unattached in the clinandrum (Fig.
6.7d
), and in response to the slightest vibration
fall onto the stigmatic surface below (Fig.
6.7e
). The caudicles are of the proper
length and elasticity to assure that the pollinia land on the stigma. The stipes (hamuli)
remain in place.
Since self-pollination does not occur until after the flower has opened and the
anther cap has fallen away, exclusive insect pollination is possible during the inter-
val separating these events. The anther cap may be retained for up to 48 h after the
flower opens in
C. maculata
(Catling
1983
), whereas, according to Freudenstein
(
1997
), it is fugacious in
C. trifida
. The pollination mechanism could therefore be
considered more truly facultative in
C. maculata
than
C. trifida.
At the same time,
only half of each pollinium often touches the stigma and swells, leaving the other
half free (Catling
1983
). Insects are therefore able to remove fragments of the
pollinia after, as well as before, self-pollination has occurred (Catling
1983
).
In his study of coastal populations in Marin County, California, Kipping (
1971
)
captured an unidentified species of
Empis
L. (dance flies, both sexes; Emperidae;
Diptera) (Fig.
6.10a
) carrying pollinia of
C. maculata
on its dorsal thorax (nota).
Subsequent observation in a terrarium revealed that after landing on the labellum,
Empis
advanced slowly toward its base and probed the nectar opening with its pro-
boscis. The proboscis was guided to the entrance of the nectary by the two ridges,
about 3 mm long, on the surface of the labellum (Fig.
6.8a, b
). These movements
resulted in contact of the fly's back with the column. After feeding for about 3 min,
it withdrew with a pollinarium attached to its dorsal thorax and canted slightly for-
ward. In one case, Kipping (
1971
) observed transport and deposition of pollinia on
the stigma of a second flower. Species of
Andrena
(Andrenidae; Hymenoptera) are
also capable of removing the pollinaria of this orchid as illustrated by Luer (
1975
).
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