Tribe Neottieae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 76

Fruiting Success and Limiting Factors
Although precise details are lacking, pollination is often frequent, and Mousley
( 1927 ) reported finding three or four sets of pollinia on a single stigma of E. helle-
borine in southern Quebec. However, in Israel, where syrphid flies are the primary
vectors, only 15.5% of 724 flowers on 24 plants were pollinated naturally (Ivri and
Dafni 1977 ). In Europe, Richards ( 1982 ) found some capsules of E. helleborine
containing nearly 100% viable seed and others with embryos present in only 1-15%
of the seeds. He assumed the former to result from wasp visits and the latter to be a
product of less-efficient autogamy, where the amount of pollen transferred to the
stigma was inadequate to fertilize 1,000 or more ovules present in each ovary.
!TASTUDYSITEIN'ATINEAU0ARK1UEBEC,IGHTAND-AC#ONAILL 1990, 1991,
1998 ) found that about 25% of the plants that emerged each year flowered. All pol-
linated flowers set fruit, and 4-79% of flowering plants developed capsules. The
number of seeds produced per capsule decreased from the bottom toward the top of
the inflorescence. However, because the decrease in seed number was abrupt and the
capsules toward the top of the inflorescence were oddly shaped, they did not believe
this decrease to be a result of resource limitation.
As in Spiranthes spiralis (Wells 1967 ), Isotria medeoides (Mehrhoff 1989 ),
Tipularia discolor (Snow and Whigham 1989 ), Cypripedium acaule (Cochran
1986 ), and many other orchids, Light and MacConaill ( 1990, 1991, 2006 ) found
that plants sometimes failed to appear above ground for 1 or more years, even
though about half of the nonemergent plants still had living rhizomes. Three plants
reappeared following an absence of 3 years and one of these flowered. This species
may be able to obtain sufficient nourishment from its mycorrhizal fungus to remain
underground and forego photosynthesis for several seasons before emerging to
bloom. Most (62%) emerged only once over a 20-year period, and 27 plants
reemerged after an absence of 7-18 years. The probability of any plant reappearing
was highly variable from year to year, but was not related to whether or not it had
bloomed in the previous season. According to Light and MacConaill ( 1990 ), the
perennating bud, containing the next year's floral primordia, develops prior to flow-
ering and fruiting. They believe that there is little evidence to indicate that the costs
of flower and fruit production affect differentiation or development of this bud,
although they have not yet ruled it out.
Kindlmann ( 1999 ), on the other hand, considered that the most likely explana-
tion for yearly transitions between flowering and sterility in a European species of
Epipactis , E. albensis Novakova and Rydlo, was based on the costs of sexual repro-
duction and leaf herbivory, as reported in a number of other orchids (Snow and
Whigham 1989 0RIMACKAND(ALL 1990 ; Whigham 1990 ; Whigham and O'Neill
1991 ; Zimmerman and Whigham 1992 ; Calvo 1993 0RIMACKETAL 1994 ; Gill
1996 ). Additional work based on demographic modeling and costs associated with
fruit rather than flower production in this species is needed to further test this idea
(Kindlmann 1999 ; Ehlers et al. 2002 ).
)NASTUDYIN3USSEX%NGLAND0IPERAND7AITE 1988 ) found that rates of pollen
import and export in E. helleborine were positively correlated with two secondary