16 Juni 2010

Exotic Orchids as Described in Darwin’s Book “Fertilisation of Orchids”

The book moves on to the various foreign orchids Darwin had received from others. His experiments showed that the "astonishing length" of the 111⁄2 inch (290 mm) long nectary hanging from Angraecum sesquipedale flowers implied the need for an as yet unknown moth with a proboscis 10–11 inches (250–275 mm) long to pollinate these flowers in Madagascar. He viewed this as the outcome of a coevolutionary race, writing that "there has been a race in gaining length between the nectary of the Angræcum and the proboscis of certain moths" disturbing to biologists of the time who believed that adaptations were the outcome of benevolent divine purpose.

Darwin described "the most remarkable of all Orchids", Catasetum, and showed how in these flowers, "as throughout nature, pre-existing structures and capacities [had been] utilised for new purposes". He explained the mechanism in which the pollen masses of the pollinium were connected by a bent stalk or pedicel to a sticky disc kept moist at the back of the flower. When an insect touched an "antenna" projecting from the back of the flower, this released the bent pedicel which sprang straight and fired the pollinium, sticky disc first, at the insect. In experiments, Darwin had imitated this action using a whalebone spring. He vividly illustrated how the flower ejected the pollinium with considerable force: "I touched the antennæ of C. callosum whilst holding the flower at about a yard's distance from the window, and the pollinium hit the pane of glass, and adhered to the smooth vertical surface by its adhesive disc."

Source: http://en.wikipedia.org/wiki/Fertilisation_of_Orchids

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