Wednesday 1 June 2011

Bee Orchids

The wonderfully intricate and beautiful flower of the Bee Orchid
 Earlier in the year a very kind friend brought me three plants of Ophrys apifera, the Bee Orchid. She is fortunate enough to have a healthy wild population in her grass and has learnt that they make very good bargaining chips! Far from being difficult to move, as is often thought, they can in fact be transplanted quite easily, should the necessity arise. It is a winter-growing species, producing a prostrate rosette of leaves in the turf in autumn and remaining in that state all winter before becoming more active in spring, when the leaves elongate somewhat and the inflorescence begins to develop. The presence of this winter-green rosette dictates the plant's growing conditons: the turf must be cropped short - by rabbit, sheep or lawnmower - so that it is not overshadowed while in growth. The species, in Britain at least, is most often associated with short chalk or limestone downland, but it can also be found in dune slacks, and even on heavier, clayier soils, but very rarely if ever in acidic soil.

An extra bee
With its remarkable flowers, the Bee Orchid is one of our most charismatic native plants and I don't like to miss seeing them in season (late May-June) - I can remember theexcitement of seeing my first one, aged six or seven and that thrill never diminishes. As with all members of the genus Ophrys - however many there may be -  the flower is adapted for pollination by insects, in this case through the development of the labellum (lip) into a vague bee-shape, which, coupled with hairy bumps and markings, results in a remarkable resemblance to a bee. In most species of Ophrys this insect-mimicking labellum, with the addition of pheromone-laden scent, attracts a male wasp or bee which, seduced into thinking that it is a female, attempts to copulate with the flower and in the process collects pollen for transmission to another plant. In Ophrys apifera such pseudocopulation seldom occurs and the flowers are almost always self-pollinated. The series of images below shows how this happens. 

In a young flower the yellow pollen sacs (pollinia) are held in a chamber at the tip of the anther column. At the base they are attached to the column by a long slender thread or caudicle. At the base of the column is the sticky stigmatic surface, visible here as the yellow area below the transverse brownish line.

As the flower develops the pollinia drop out of the chamber at the tip of the column, and dangle on the caudicles, moving in any breeze.

The inevitable happens: the dangling pollinium becomes stuck on the stigma and self-pollination occurs
The mechanisms by which orchids are pollinated were studied by Charles Darwin in great detail, and he found in their intricacies great support for his theory of evolution. This was no less true in Ophrys apifera, which he found to be perfectly adapted for self-pollination in the positioning and size of the various parts, with further evidence from the conspicuous success-rate of the flowers in developing a full capsule of seeds. He also watched flowers patiently and never observed a single insect visitor. But this was all at odds with the evident evolutionary trend toward cross-pollination, for which practically every other orchid is so interestingly adapted, and it bothered him, imagining that by this repeated self-pollination and apparent inbreeding the species would become weakened and ultimately go extinct. This has not yet happened, but it is notable that certain mutations have become the norm in some populations - the so-called 'Wasp Orchid', O. apifera var. trollii with a narrow labellum is the best known case, but there are several others. The consequence is that a full crop of seed can be expected here in due course, and then I must decide where to sow it...

Bee Orchids, Ophrys apifera

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