Tribe Pogonieae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 178

transferred. The process is essentially identical to that found in Pogonia ophioglos-
soides and I. verticillata (Gibson 1905 ; Thien and Marcks 1972 ; Mehrhoff 1983 ),
except that tetrads rather than monads are transferred (Gregg 1991b ).
The pollination mechanism in Cleistesiopsis can dispense an average of 16
clumps of tetrads before the anther is emptied; thus, a number of successive pollina-
tors can receive pollen from a single flower (Gregg 1991a, b ). Gregg ( 1991a ) exam-
ined the effect of successive pollen dumps on capsule set, capsule development,
seed number, individual seed weight, and percentage of healthy embryos per fruit.
She found that a flower's first three pollen dumps, each of which contained more
tetrads than later dumps, produced heavier capsules with more seeds. Some first
dump pollinations contained enough tetrads to produce maximum seed set, and a
pollen to seed ratio of 4, very low for a nonautogamous breeding system, is proba-
bly related to the energy efficient production of pollen clumps large enough to fertil-
ize most of the ovules in an ovary (Cruden 1977 ; Gregg 1989 ). Pollination with
later, lower dosage pollen releases did not reduce percent capsule set, percent of set
capsules achieving ripeness, individual seed weight, or the percentage of healthy
embryos. Lower dosage dumps were, however, unexpectedly associated with
increased energy expenditure in the amount of pericarp produced per seed.
Flowers which received high pollen dosages all faded within 5-7 days, while
less than half of the flowers receiving low pollen dosages faded in this time period,
and many persisted as long as unpollinated flowers (Gregg 1991a ). More sparsely
pollinated flowers thus remained attractive to pollinators and sometimes received
additional pollen to increase their seed output.
The pollination mechanism in Cleistesiopsis probably also enhances paternal
success. When a single flower releases pollen in more than one package, the number
of pollinators is increased along with the chances of successful pollen transport
should any one vector fail to deliver pollen to a receptive stigma. Pollen would also
be distributed to a larger number of stigmas and variation in the sired offspring
would be increased (Catling and Catling 1991 ; Gregg 1991a ).
Harder and Thomson ( 1989 ) reported an analogous pollen dispensing relation-
ship in Erythronium grandiflorum Pursh. Here, the amount of pollen removed from
the flower was inversely related to the proportion deposited on subsequently visited
stigmas. According to this model, individual plants would improve their total pollen
dispersal when the number of pollen-removing visits increased and the amount of
pollen removed by each pollinator was limited.
A disadvantage of this pollination strategy is that when pollinators are limiting
the chance visitation of a single pollinator would result in only part of the pollen
load being transported (Gregg 1991a ). Moreover, although it might result in an
increase in variability among the offspring (Ter-Avanesian 1978 ), the presence of
relatively few pollen grains on a stigma receiving later, lower pollen dosages would
likely reduce competition among grains for ovules along with the advantageous
effect of such competition on fitness (Mulcahy et al. 1983 ; Gregg 1991a ).
The flowers of Cleistesiopsis , with their pollen-mimicking, fleshy labellar crest,
have often been assumed to function as bee-food mimics (Gregg 1989, 1991b ). As
such they would attract naïve bees and achieve pollination by deception much as in