Subtribe Spiranthinae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 38

days. In S. ochroleuca , however, 30-40-day-old, unpollinated flowers with a dry
stigma, nonfunctioning viscidium, and dead ovules still contain nectar and remain
fresh looking (Catling 1983c ). Catling ( 1980b, 1983c ) suggested that these flowers
might improve the chances of pollination in other nearby plants of S. ochroleuca by
rewarding and, thus, reinforcing pollinator visitations. Selection for this condition
might occur within a genet or in populations of closely related plants.
Depending on the species and the weather, all flowers in a spike become func-
tionally female after 5-30 days. However, a functional viscidium can still be present
(e.g., Catling 1983c ). Thus, although outcrossing is initially favored by protandry,
acropetally, and the predominantly upward movements of bumblebees on the inflo-
rescences, circumstances change if the pollinia are still in place a number of days
after the beginning of anthesis. Flowers then contain exposed, receptive stigmas and
fully viable pollen, and pollinators may effect self-pollination (Sipes and Tepedino
1995 ). The level of selfing may consequently be dependent on the frequency of pol-
linator visits (Sipes 1995 ; Sipes and Tepedino 1995 ). For example, when visitation
rates were low in populations of S. diluvialis from Colorado and Utah, more flowers
reached the female stage with a functional viscidium, and a higher percentage of
pollinaria (>80%) were removed during this hermaphroditic stage. When rates were
high, only 16% of pollinaria were removed during this stage (Sipes and Tepedino
1995 ). Of course, the number of pollinations per visit would be higher in an inflo-
rescence with functionally hermaphroditic flowers than in one with flowers in sepa-
rate male and female stages (Sipes 1995 ). As compared to obligate outcrossing, this
circumstance could provide a selective advantage in species with low levels of self-
incompatibility and few pollinator visitors (Sipes and Tepedino 1995 ).
Unlike the other examined taxa of Spiranthes , S. lucida is specifically adapted to
pollination by halictine bees (Table 2.7 ) (Catling 1983c ). The elongated viscidium
and protandry associated with bumblebee and megachilid pollination are absent: the
viscidium is oval (Fig. 2.3 ), and the inflorescence is relatively short and not
protandrous or only weakly so (Catling 1980b, 1983c ). The stigma is glutinous and
more upright and accessible in newly opened flowers, and pollinaria are easily
removed at this stage (Catling 1983c ).
Due to the relatively greater length of its column and the short claw of its lip, the
calli in S. lucida secrete nectar onto the ventral surface of the column rather than into
the base of the floral tube (Fig. 2.6 ) (Catling 1983c ). It is here more easily reached
by a short-tongued bee. In direct contrast with the Megachilidae and Apidae, the
basal parts of the proboscis in halictines are relatively long with a well-developed
cardo, prementum, and stipes (Fig. 2.6 ) (Catling 1980b, 1983c ). At the same time,
the terminal parts, including the galea, palpus, and glossa, are small (Catling 1980b,
1983c ). This morphology appears to be well-adapted to reach the nectar on the
ventral surface of the column behind the stigma. As the bee inserts its head into
the relatively open flower, the clypeus, between the eyes and below the antennae,
contacts the oval viscidium (Fig. 2.6b ), and the pollinia are extracted as the bee
withdraws (Fig. 2.6c ) (Catling 1983c ).
Catling ( 1980b, 1983c ) observed Augochlorella aurata [as A. stricta (Prov.)] and
Lasioglossum imitatum (Smith) [= Dialictus immitatus (Smith)] pollinating this
species in Ontario. Species of Bombus were present in the study area, but none