Biology Reference
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each fruit produced 31.1 mg of seed, and 26.0% of the seeds were viable. Seed
weight and the level of viability did not differ significantly among open-pollinated,
unmanipulated and bagged, or artificial geitonogamous or xenogamous treatments,
and there was, therefore, no evidence of inbreeding depression.
Hogan (
1983
) recorded 0.00012 insect visits per flower per 10 min. This is about
1/4,000th the frequency observed for several species of earlier blooming spring
ephemerals at the same site (Schemske et al.
1978
). The difference cannot be
accounted for by the lack of nectar in
Aplectrum
because the visitation rates among
nectarless spring ephemerals were 0.03 to 0.71 per flower per 10 min (Schemske
et al.
1978
). Instead, Hogan (
1983
) suggested the reduced visitation rate might be
due to a change in pollinator availability between the blooming period for
A. hyemale
and the ephemerals: pollinators might have become less common or
Aplectrum
might
have faced increased competition from other, nectar producing plants (Schemske
et al
1978
; Nilsson
1980
; Motten
1982
; Hogan
1983
).
A limited number of pollinators and/or a high level of competition for their ser-
vices during the blooming period could have led to selection for autogamy. A change
in phenology might have provided an alternative (Hogan
1983
); however, the bloom-
ing time in this wintergreen orchid may be set by the period available for high car-
bohydrate production and accumulation in spring prior to flowering. This period is
restricted to the interval between snowmelt or spring warming and closure of the
canopy (Stevens and Dill
1942
; Adams
1970
).
According to Hogan (
1983
) the patches of
A. hyemale
are probably clonal. The
genetic variability resulting from autogamy (or agamospermy) would then approxi-
mately equal that resulting from pollen transfer within populations, and fruit set
could be increased with no loss in genetic diversity. A breeding system based on
clonal growth and autogamy and/or agamospermy is consistent with the limited
genetic variability in this species observed by Adams (
1970
) and Auclair (
1972
).
Restricted variability, in turn, may be reflected in the absence of diversification in
this monospecific genus (Stebbins
1957
; Hogan
1983
).
References
Ackerman JD (1981) Pollination biology of
Calypso bulbosa
var.
occidentalis
(Orchidaceae):
a food-deception system. Madrono 28:101-110
Adams MS (1970) Adaptations of
Aplectrum hyemale
to the environment: effects of precondition-
ing temperature on net photosynthesis. Bull Torrey Bot Club 97:219-224
Alexandersson R, Agren J (1996) Population size, pollinator visitation and fruit production in the
deceptive orchid
Calypso bulbosa
. Oecologia 107:533-540
Alexandersson R, Agren J (2000) Genetic structure in the nonrewarding, bumblebee-pollinated
orchid
Calypso bulbosa
. Heredity 85:401-409
Auclair AN (1972) Comparative ecology of the orchids
Aplectrum hyemale
and
Orchis spectabilis
.
Bull Torrey Bot Club 99:1-10
"ARRETT#&&REUDENSTEIN*60ATTERNSOFMORPHOLOGICALANDPLASTID$.!VARIATIONINTHE
Corallorhiza striata
complex (Orchidaceae). Syst Bot 34:496-504
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