Subtribe Spiranthinae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 37

The bumblebees that arrive as pollen donors, thus, depart as pollen recipients, and
selfing and geitonogamous pollination are reduced while cross-pollination is pro-
moted (Catling 1983c ). The effectiveness of protandry has now been demonstrated
experimentally (Jersakova and Johnson 2007 ).
Darwin ( 1862 ) first drew attention to protandry and sequential flowering as fac-
tors promoting cross-pollination in S. spiralis . This flowering pattern has now been
confirmed for S. spiralis (Willems and Lahtinen 1997 ) and reported for other spe-
cies, including S. cernua and S. lacera var. gracilis (Gray 1862b ), S. romanzoffiana
(Godfery 1933 ; Summerhayes 1951 ; Larson and Larson 1987, 1990 ), S. sinensis
(Coleman 1933 ), and S. diluvialis (Sipes and Tepedino 1995 ).
In addition, Catling ( 1980b, 1982, 1983c ) confirmed the association of this flow-
ering pattern with pollination by Bombus , a few other Apidae, and some Megachilidae.
The relationship is documented for North American species of S. cernua var. cernua
from southeastern Ontario, southwestern Quebec, and New England; S. lacera var.
lacera from Ontario, Quebec, Nova Scotia, and Pennsylvania; S. romanzoffiana
from Ontario, New Brunswick, Newfoundland, and Quebec; S. vernalis from New
Jersey, Florida, and Georgia; and, based on more limited data, S. odorata from the
southern Coastal Plain. Floral morphology, acropetal flowering, and protandry
imply that bumblebees are also the primary pollinators of northeastern North
American species of S. lacera var. gracilis in New Jersey; S. laciniata in central
Florida; S. magnicamporum in southwestern Ontario (facultatively agamospermic
population) and western Illinois (sexual population); S. ochroleuca in Pennsylvania,
Massachusetts, and southwestern Ontario; and S. tuberosa in New Jersey (Catling
1980b, 1982, 1983c ).
The upward movement of bumblebees on the inflorescence of Spiranthes has
been verified in North America for Bombus terricola Kirby, B. vagans ssp. vagans
Smith, B. vagans ssp. bolsteri (Franklin) (= B. vagans Smith), B. flavifrons Cresson ,
B. bifarius Cresson, B. morrisoni Cresson, B. fervidus (Fabricius), and 15 other
unidentified Bombus species (Catling 1983c ; Larson and Larson 1987, 1990 ; Sipes
et al. 1993 ; Sipes and Tepedino 1995 ). Consistent movement in one direction may
benefit the forager by reducing its chances of revisiting newly emptied flowers (e.g.,
Pyke 1978 ; Heinrich and Waddington 1979 ). Corbet et al. ( 1981 ) found that bees
moved upward when flowers of the inflorescence were visited head up and down-
ward when they were visited head down. Floral morphology in Spiranthes requires
that bees of the Apidae and Megachilidae visit the flower in an upright position to
extract nectar (Catling 1983c ). In addition, it is probably more difficult for large
bees to crawl down a spike than up (Catling 1983c ). Although Corbet et al. ( 1981 )
found bee movements to be independent of the vertical pattern of reward, Catling
( 1983c ) maintained that the presence of more nectar in older flowers at the base of
the inflorescence makes it more energy efficient for bees to visit these flowers first.
Changes in the perianth may sometimes permit bees to identify the flowers at the
base of the inflorescence that contain the largest amounts of nectar. Thus, for exam-
ple, two characteristic brown spots appear on the lips of old or pollinated flowers of
S. vernalis (Luer 1975 ; Catling 1980b ). In most species, pollination simply leads to
a halt in nectar secretion followed by a wilting and fading of flower color within 1-3