Tribe Calypsoeae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 120

surface (Catling and Catling 1991 ). Each pollinarium is therefore able to contribute
pollen to 2-4 flowers.
Pollinator activity was related to the amount of nectar present. Whigham and
McWethy ( 1980 ) found that moth visits at their study site began 4-5 days after
anthesis and continued for 13-15 days, peaking on the 11th to 13th day. By about
the 16th day when nectar had dropped to approximately 20% of its maximum vol-
ume, very few pollinators visited the flowers, even though they remained open with
some nectar content for an additional 3-15 days. Apparently, P. unipuncta was sen-
sitive to the total amount of nectar available and broke-off visits when this amount
fell below some undetermined, minimum level. Willson and Bertin ( 1979 ) recorded
similar behavior of this moth on Asclepias where it functioned as a common polli-
nator for only 1 week of an extended flowering period.
Whigham and McWethy ( 1980 ) found the pollinators not only responded to the
overall cycle of nectar production, but also were able to concentrate their attention
on the portion of the inflorescence that produced the most nectar. Both nectar pro-
duction and anthesis occurred acropetally. Pollinator visits were initially restricted
to the basal portion of the inflorescence. After about 5 days, nectar production was
equally dispersed along the length of the inflorescence, and pollinator visits were
also equally dispersed. After 10 days, nectar production was largely limited to the
upper flowers, and pollinator visits were then concentrated in this area. All inflores-
cences were visited during the blooming period with a maximum of 25-45% of
available flowers visited daily.
Fruiting Success and Limiting Factors
In their Maryland study, Snow and Whigham ( 1989 ) reported that naturally polli-
nated plants each produced an average of 6-8 fruits per year, with 18-25% of the
flowers setting fruit. Eleven percent of the plants produced no fruit, 68% produced
1-10 fruits, and 6% produced over 15 fruits.
In their earlier study at the same site, Whigham and McWethy ( 1980 ) found that
once pollen was successfully transferred to the stigmatic surface, flowers produced
fruits an average of 70.7-93.5% of the time. However, pollinators were scarce dur-
ing the mid-summer flowering period, and fruits were set in only 24% of unbagged,
emasculated flowers. This result lies within the range of variation obtained for open-
pollinated plants in the 1989 study, and since it excluded fertilizations resulting
from facilitated self-pollinations, it is compatible with the postulated prevalence of
cross-pollination in this species.
Both studies imply that fruit set in open-pollinated plants is limited by pollinator
service. However, Snow and Whigham ( 1989 ) found that the potential advantage of
increased pollination in this species, as in others we have discussed, might not trans-
late into an increase in lifetime fecundity. Chances of sexual reproduction were
correlated with corm size and leaf area, and fruit and flower development reduced
the stored reserves available for corm and leaf growth (Snow and Whigham 1989 ;
Whigham 1990 ; Efird 1987 ). Plants that produced 1-10 fruits, as in most naturally