Biology Reference
In-Depth Information
simply depend largely upon the early, tentative explorations of potential new food
resources by recently emerged, inexperienced bees (Heinrich
1975, 1979
; Boyden
1982
; Little
1983
). In
C. tuberosus
and perhaps
Arethusa
, the occurrence of odor-
less or faintly scented flowers may force the pollinator to rely heavily on flower
shape or color, and the variation in the latter may extend the number of visits
required for the bee to learn that the flowers have no nectar (Heinrich
1975
;
Ackerman
1981
; Yannetti
2003
).
Fruiting Success and Limiting Factors
The data on experimental crosses can be compared to data on natural pollination
obtained from three widely separated locations (Table
8.2
). In Maine, the percent-
age of plants producing fruit in
C. tuberosus
over a 7-year period ranged from 12 to
40% with a mean of 26% (Firmage and Cole
1988
). In Wisconsin, the percentage of
flowers producing capsules under natural conditions was 5% in
A. bulbosa
, 16% in
C. tuberosus
, and 10-100% in
P. ophioglossoides
, the high percentages in some
populations of the latter apparently due to apomixes (Thien and Marcks
1972
; Thien
1973
). In Newfoundland, capsule set ranged from 16 (13-20)% in
A. bulbosa
and
19 (17-22)% in
C. tuberosus
to 30 (29-33)% in
P. ophioglossoides
(Boland and
Scott
1991
).
The relatively low level of capsule production in open compared to artificially
pollinated plants (Table
8.2
) suggests pollinator limitation (Thien and Marcks
1972
),
as might be expected if bees learn to discriminate between species offering abun-
dant and poor rewards (Free and Butler
1959
; Stoutamire
1971
; Firmage and Cole
1988
). Firmage and Cole (
1988
), for example, observed that in
C. tuberosus
only
one visit every 3-5 days per plant resulted in any pollen removal or deposition on
the stigma. However, low capsule set in these species may again be compensated by
the production of very large numbers of seeds in those plants that are effectively
pollinated (Stoutamire
1971
; Firmage and Cole
1988
).
Thien and Marcks (
1972
) found that bees in northern Wisconsin were most
attracted to flowering plants of
C. tuberosus
and
A. bulbosa
that occurred in groups.
Similarly, in Maine, Firmage and Cole (
1988
) found that in
C. tuberosus
the per-
centage of flowers setting fruit varied significantly with plant distribution. Flowers
on plants occurring in clumps of 2-8 within a 1 m radius had a higher probability
of setting fruit than flowers on either solitary plants or on plants in clumps of nine
or more. Presumably, groups of 2-8 attracted more pollinators than solitary flow-
ers, and larger groups were abandoned following a few unrewarding visits. In a
study of
Arethusa
and
Pogonia
in Newfoundland, Boland and Scott (
1991
) reported
an inverse relationship between fruit set and the percentage of plants producing
flowers. In
A. bulbosa
, about 27% of the plants flowered, and as noted above, a
mean of 16% of these set fruit. In
P. ophioglossoides
, only 3% flowered, but 30%
set fruit. Bowland and Scott reasoned that a higher number and density of
A. bul-
bosa
flowers may have allowed pollinators to learn more quickly that they were a
poor source of food.
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