Tribe Pogonieae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 173

The initial pollinator removes only some of the pollen; the rest remains available
for transport by later vectors. The consequences of partitioning the pollen load are
discussed below for Cleistesiopsis . One result for flowers receiving repeat visits is
an increased probability of at least some pollen reaching the stigma of another
flower. An obvious disadvantage is that only one pollinator may visit the flower, a
particular concern in a species like I. verticillata where pollination is uncommon.
A second disadvantage is that the number of pollen grains transferred may be insuf-
ficient to provide maximum seed production. Naturally pollinated flowers of I. ver-
ticillata do, in fact, sometimes produce few or no seeds while flowers artificially
pollinated with whole “pollinia” produce seeds in abundance (Mehrhoff 1983 ).
Pollinators visited newly but fully opened fragrant flowers most frequently
(Mehrhoff 1983 ). The number of pollinator visits was influenced by illumination
and air temperature; insect activity increased with high values for both factors. Time
of day and relative humidity also affected insect activity but to a lesser extent. The
percentage of flowers pollinated per clone was positively related to the number of
flowers produced per clone, but the probability of pollination was unaffected by a
flower's proximity to other flowers in the clone or by its height above the ground.
In I. medeoloides , autogamy is initiated shortly after flower opening with a
decent of the anther out of the anther socket and an extrusion of pollen from the
pollen sacs. In a day or so following anthesis, these combined movements result in
contact of the pollen and stigma (Fig. 9.2b ) (Mehrhoff 1983 ; Vitt and Campbell
1997 ). Mehrhoff ( 1983 ) considers that such a mechanism might be easily derived in
an already self-compatible species by minor repositioning of the anther and reduc-
tion in the rostellar flap.
The flowers of I. medeoloides are open, and according to Catling ( 1984 ), occa-
sional cross-pollination could be promoted by higher growth rates of pollen tubes
from other conspecific plants. However, neither Mehrhoff ( 1983 ) nor Vitt and
Campbell ( 1997 ) observed any insect visitors. A single season survey of all flower-
ing individuals revealed pollinia missing from only four of 52 flowers (Vitt and
Campbell 1997 ), and these might not have been removed by pollinators. According
to Mehrhoff ( 1980 ), the pollen mass may occasionally break from the anther and
fall onto the labellum without contacting the stigma.
Fruiting Success and Limiting Factors
In I. verticillata , 21.0% of the flowers that were not lost to predation, abortion, or
other factors were pollinated and 43.3% of these produced mature capsules. In I.
medeoloides , the corresponding numbers were 83.3% and 80%, respectively
(Table 9.3 ). Although the pollination system in I. medeoloides is dramatically more
efficient than that in I. verticillata on a flower-by-flower basis, I. verticillata is capable
of producing a much larger number of seeds per plant if genets rather than flowers are
compared (Mehrhoff 1983 ). A higher percentage of I. medeoloides capsules survive to
maturity, but seedling recruitment may be limiting, possibly due to the encroachment
of surrounding vegetation in a species apparently adapted to relatively open seral
stages (Mehrhoff 1989a, b ).