Tribe Calypsoeae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 112

In a study in the Rocky Mountain foothills of Alberta, Proctor and Harder ( 1995 )
noted that flowers receiving any amount of pollen senesced within 4 days, whereas
those that were not pollinated or only had pollinaria removed remained in good
condition for 8-11 days. The authors hypothesized that selection favors the onset of
senescence following completion of the female function because it is the less easily
satisfied. If removal of the pollinarium is significantly more likely to occur than its
deposition on the stigma, senescence after deposition will probably have been pre-
ceded by at least some pollen removal. They reasoned that in deceptive flowers
fulfillment of the male function was more likely to occur because it only requires
fooling the pollinator one time, whereas satisfaction of the female function requires
that the pollinator be fooled twice. Thus, senescence of the flower is triggered by
completion of the function less likely to succeed.
The fact that the onset of senescence in Calypso is independent of pollen load
may have further implications. We shall see below that the rate of flower senescence
in Cleistesiopsis bifaria is associated with the amount of pollen received (Gregg
1989, 1991 ). Proctor and Harder ( 1994 ) believe that this relates to the fact that pol-
len dumped on the pollinator of Cleistesiopsis is granular, and loads could be
received that contained far fewer tetrads than needed to fertilize the large number of
ovules in the ovary. In Calypso , on the other hand, a minimum of one pollinium is
deposited, and each pollinium is capable of fertilizing about 11,000 ovules (Proctor
and Harder 1994 ). If the deposition of one pollinium produces a satisfactory number
of seeds, selection for a more precise correlation between pollen load and floral
senescence might be relatively low .
Kipping ( 1971 ), Stoutamire ( 1971 ), and Gumprecht ( 1977 ) agree generally on
the pollination process. The foraging bee lands on the labellum and forces its head
and thorax beneath the column (Fig. 6.1a ). Discovering that no nectar is available in
the lip, it backs out of the flower. As it withdraws, the pollinarium is attached to the
hairless area at the rear of the dorsal thorax (Fig. 6.2 ). The placement is such that
the bee has difficulty removing it, and it may remain in place for many days
(Ackerman 1981 ; Boyden 1982 ). According to Ackerman ( 1981 ), attachment here
requires that the bee penetrate deeply into the flower and back out with its body
arched to bring the edge of the scutellum into contact with the viscidium. Moreover,
the size of the bee is important; it must be able to enter beneath the winged column,
and the latter must fit closely over the thorax. The stigma, viscid when receptive, is
proximal to the anther (Fig. 6.1d ). The withdrawing bee is, therefore, likely to
deposit pollen previously attached to its thorax before it contacts and removes the
pollinarium.
The mechanics of pollination clearly identify outcrossing as the predominant
mode of pollen transfer. Besides, the plants produce only one flower each year, and
bees do not usually reenter the same flower (Ackerman 1981 ). Kipping ( 1971 ) also
reported that the extracted pollinarium retains its anther cap for several minutes, a
feature that might prevent self-pollination should reentry occur. Geitonogamous
pollination is unlikely in variety occidentalis because, as already noted, vegetative
reproduction here is evidently rare. Ackerman ( 1981 ), for example, reported that it
did not occur at all at his study sites in Humboldt County, California. Geitonogamy