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Fig. 3.1 Listera cordata . ( a ) Habit, scale bar = 1 cm; ( b ) flower, side view; ( c ) flower, front view,
scale bars ( b , c ) = 1 mm; ( d )-( f ) front view of column; ( d ) rostellum intact; ( e ) column after trig-
gering of rostellum and release of the pollinia; ( f ) column with an elevated rostellum and exposed
stigma, scale bar = 0.3 mm. an anther, po pollinia, ro rostellum, sg stigma, tr trigger hairs. ( d )-( f )
after Ackerman and Mesler ( 1979 ) with permission
species might also have such a breeding system. However, the tetrads of L. cordata
are more coherent, less likely to fragment, and less likely to autopollinate than those
of other Listera while cross-pollination is common and appears to be favored.
Melendez-Ackerman and Ackerman ( 2001 ), for example, found that subalpine
populations in Gunnison County, Colorado, although also self-compatible, pro-
duced a significantly higher percentage of fertilized ovules and more seeds per cap-
sule when cross-pollinated. Similarly, Ackerman and Mesler ( 1979 ), in a 3-year
study in Humboldt County, California, found that geitonogamy occurred but out-
crossing was promoted by protandry, and Mesler et al. ( 1980 ) estimated the relative
frequency of geitonogamous versus xenogamous pollination in northern California
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