Biology Reference
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Ackerman ( 1981 ) noted that the size of the pollinator was variable, and the size
of the throat gap, measured from the rim of the lip to the top of the column in variety
occidentalis ranged from five to ten millimeters. He thought that the lack of preci-
sion in pollen transfer might be based on this variation, and Boyden ( 1982 ) sug-
gested that evolution of a more precise fit is unlikely because size in both the
bumblebee (Heinrich 1979 ) and flower are dependent on variable nutrient and
growth factors.
Nevertheless, Proctor and Harder ( 1994 ) reported that naturally pollinated stig-
mas of variety americana in their Alberta study usually bore at least two pollinia
(mean 2.53 ± 0.34, n = 30), the number considered sufficient to fertilize most of the
ovules contained in a single flower. Just the same, the low frequency of pollination
in both varieties very likely represents a limiting factor in the reproduction of these
orchids even if a sufficient amount of pollen is transferred when pollination does
occur.
Alexandersson and Agren ( 1996 ) found that although the effects of pollinator
limitation in variety bulbosa varied considerably from year to year, fruit production
over the lifetime of the plant was probably determined by the level of pollen trans-
fer. Over half of the plants that failed to produce fruit did so because they were never
pollinated, and hand-pollinated plants consistently produced more fruits than open-
pollinated plants, although the difference was statistically significant in only 1 year
of their 3-year study. At the same time, increased pollination intensity, based on
supplemental hand pollination for 5 years in one population, resulted in the cumula-
tive production of 1.8 times as many fruits as produced by the control plants, with
no reduction in flowering, growth, or survival. There is, therefore, no experimental
evidence indicating that fruit production in this orchid is resource limited.
Within season pollinator limitation is also suggested by a reported close correla-
tion of pollinator availability and pollination levels in a northern California popula-
tion of variety occidentalis. Ackerman ( 1981 ) observed two pollination peaks
corresponding with two episodes of bumblebee emergence. The first pollination
peak occurred in concert with the emergence of the first group of bumblebees, but
the bees soon shifted to other resources. There followed a drop in pollination in the
middle of the period of maximum flowering. The second pollination peak then cor-
responded with the emergence of the second group of bees and the process was
repeated.
Fruit production was also limited by bad weather and herbivory. In northern
Sweden, the amount of damage resulting from frost and drought was negatively cor-
related with tree cover, and animals sometimes consumed both the flowers and
corms (Alexandersson and Agren 1996 ; Pridgeon et al. 2005 ).
Tipularia Nuttall
Tipularia includes three temperate zone species: one in East Asia, one in the
Himalayas, and one in North America (Pridgeon et al. 2005 ).
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