16 Juni 2010

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

In the first chapter Darwin described the British orchids he had studied, giving detailed explanations of their various mechanisms for transferring pollen to insects.

The first mechanism described is that of Orchis mascula, which serves as an introduction to the explanation of other Orchidaceae. In the upper part of the flower a petal shelters the male organ which has two packages of pollen grains, held together by thin elastic threads. These pollen masses stand side by side and have stalks down to adhesive balls in a cup which keeps them moist and sticky. When an insect lands on the large projecting lower petal, the labellum, and pushes its head and proboscis into the centre of the flower and down to the nectary, it breaks the cup and the adhesive balls attach the pollen masses to the front of the insect. As the insect flies off, each stalk rotates the pollen mass downwards and forwards so that when the insect lands on another flower the pollen masses attached to the insect pass under the male organ and leave pollen on the female organ, achieving cross fertilization. Darwin envisaged:

A poet might imagine, that whilst the pollinia are borne from flower to flower through the air, adhering to a moth's body, they voluntarily and eagerly place themselves, in each case, in that exact position in which alone they can hope to gain their wish and perpetuate their race.

This is followed by descriptions of the differences in the mechanisms of several other orchids. In Orchis pyramidalis, the adhesive balls are combined together into a strap or saddle shape, which curls round the thin proboscis of a moth or butterfly to attach to it the pair of pollen masses, illustrated in the book by figure 4 showing a moth's head with seven pairs of pollen masses attached to its proboscis.

While the bee orchid showed adaptation for self-fertilisation, its mechanism also enabled occasional cross-fertilisation, creating the biological diversity that Darwin felt was needed for vigorous survival, which could not be provided by self-fertilisation. As an example of "how beautifully everything is contrived", Darwin described how he had found that in Spiranthes flowers the pollen is ready for collection before access is open for the female organ to receive pollen. At Torquay he had watched bees visiting spires of these flowers, starting at the bottom of the spire and working their way up to the topmost flowers. He speculated that if bees moved from top to top of the spires, the pollen clusters they collected from the most recently opened flowers would be wasted as the topmost flowers on the next spike would not be ready to receive pollen. A bee starting at the lowest flowers on the first spire it visited would continue up until it reached flowers that still had their pollen masses to attach to the bee, then would fly to the mature lower flowers on another plant, and fertilise them. By this co-ordinated process, the bee would add "to her store of honey" while perpetuating the flowers "which will yield honey to future generations of bees."

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

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