I promissed yesterday the story of pollination of this orchid species.
The flowers are pollinated by bumblebees. When the flower opens the longboat-shaped rostellum lies close to the lip at the bottom of the tubular flower with the sticky viscidium facing downwards. In this position the rostellum blocks access to the stigma so that the flower cannot be self-pollinated. A bumblebee lands on the lip and inserts its proboscis in search of the nectar that is produced at its base. There is just enough room for the proboscis to reach the nectaries but in the process it pushes past the rostellum, and the pollinia are attached to the proboscis by the fast-drying flue of the viscidium. The bee moves on, carrying the pollinia with it. This can be seen in the image above: two pair of pollinia are attached to the proboscis of this bumblebee (Bombus lapidarius).
Over the following 24 hours or so the column and the lip move apart, creating enough space for a visiting bumblebee to insert its proboscis, with the pollinia attached, into the flower, where they rub against the stigma (which is now exposed and has become much stickier). The pollinia are brittle and break off in small pieces. A single bumblebee can pollinate several flowers. Due to the 24-hour interval, older flowers are always pollinated with pollinia from a younger flower. The lowest flower in a spike open first and, as bumblebees work upward from the bottom of a spike (i.e. from older to younger flowers), they visit the older flowers first and cannot pollinate them with pollinia taken from the same spike. In this way cross-pollination is virtually guaranteed. The mechanism is efficient and seed is set by almost all flowers.
Image: Zuid-Holland (NL) - 03/09/2010
© Johan Dierckx
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