Subtribe Spiranthinae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 36

Fig. 2.5 Spiranthes romanzoffiana and Bombus vagans . ( a ) Lateral view of flower with part of the
perianth removed; ( b ) bee removing nectar, showing the position of head and mouthparts; ( c ) lat-
eral view of flower following removal of the pollinarium and the head of B. repens with the polli-
narium attached to its galea, scale bar = 5 mm. an anther, cl callus, cr cardo, ga galea, ne nectar, po
pollinia, sg stigma, si stipes, vs viscidium. After Catling 1983c , © 2008 of NCR Canada. Reproduced
with permission
attached to the upper surface of a bumblebee's proboscis cannot readily reach the
stigmatic surface but are instead diverted above and around the column by the
downward pointing rostellum at its tip (Fig. 2.4a ) (Catling 1980b, 1983c ). However,
the galea of probing bees can easily contact the exposed viscidia at the tip of the
rostellum and extract the pollinia (Fig. 2.5 ) (Catling 1980b, 1983c ). After 2-4 days,
the column and lip separate and the stigma is exposed so that pollinia are easily
deposited on its surface (Fig. 2.4b ). In some species (e.g., S. lacera var. lacera from
Ontario, S. vernalis from New Jersey), the stigmatic surface becomes stickier after
the column separates from the lip (Catling 1983c ).
Because of the bumblebee's tendency to move from the base toward the top of
the inflorescence, in full anthesis the older, functionally female flowers at the base
of the inflorescence are visited first and receive pollen from other plants (e.g.,
Darwin 1862 ; Gray 1862a ; Catling 1980b, 1983c ; Corbet et al. 1981 ; Willems and
Lahtinen 1997 ). The younger flowers near the top are functionally male and donate
pollen prior to the maturation and exposure of their stigmas (Catling 1983c ).