We found a fine display of early purple orchids blooming on the cliff tops south of Seaham, along the Durham coast, today. They're beautiful flowers to look at but less attractive when you smell them, as they emit a strong aroma of cat's urine which is generally repugnant to people but attractive to bees........ which are in for a disappointment when they visit these flowers.
Charles Darwin spent many years studying orchids and was intrigued by their elaborate flowers and complicated pollination arrangements. In his seminal book in the subject, The Various Contrivances by which Orchids are Fertilised by Insects, first published in 1862 and which you can download here, he noted that although the flowers have a well developed nectar spur they don't produce any nectar. Bees that visit are duped into exploring one or two flowers in an inflorescence but soon leave when they find no sweet reward - but by then they've performed the essential pollination service that the plant requires.
This is Darwin's detailed dissection of an early purple orchid flower. The two important features to note are the hollow nectar spur in the uppermost illustration, which shows a longitudinal section of the flower, and the club-shaped structure in the centre, which is the pollinium containing the pollen grains.
Try as he might, Darwin could find no trace of nectar in the nectar spur and was reduced to suggesting that if visiting bees did extract any reward from the flower it could only have been by piercing the floral tissues with their proboscis, which has never been proved to be the case. He concluded that the basis of pollinator visits was deceit on the part of the plant, that relied on naive, newly-emerged bees in spring making exploratory visits to any flower that might contain a food source.
The other part of the flower that intrigued Darwin was the pollinium - the male stamen that is carried away in its entirety when a bee probes for nectar and the pollinium sticks to its tongue (click here for a picture of a pollinium-encumbered bee visiting an orchid).
Each flower has a pair of pollinia - they're the two dark, club-shaped structures under the hooded petal in the flower above. When they're carried away by a bee they bend forward within about a minute, so they're well positioned to make contact with the stigma of the next orchid flower it visits.
In his book Darwin demonstrated rather neatly how you could mimic the whole process by poking a sharpened pencil into the flower. Try this - it really works. When you withdraw the pencil with the pollinia attached their stalks dry out and they slowly bend forwards. Incidentally, the ever-meticulous Darwin dissected, counted and estimated the number of pollen grains in a single pollinium - a staggering 122,400.
This orchid's deceitful pollination technique is only partially successful because bees are smart and after visiting a couple of flowers look elsewhere for an energy reward. Darwin's work was followed up in the 1990s by Dr. L.A. Nilsson at the University of Uppsala who showed that bees would visit more flowers on an inflorescence if those empty nectar spurs we filled with artificial nectar.
Nilsson noted that early purple orchids only produce a few seed pods, developing from the first few flowers to open at the bottom of the inflorescence. The rest almost always remain unfertilised, although 80% of them could set seed if they were hand-pollinated. The first bees of spring may be naive but they learn fast and if they don't find the reward they are looking for they soon ignore that attractive scent of cats' pee and take their pollination services elsewhere, leaving many early purple orchid flowers unpollinated.
Fortunately the seed production from a single pollinated flower is quite large ..........Darwin counted 6200 seeds in a single capsule of spotted orchid and this species' seed production would be similar.... so early purple orchids manage to survive on deceit.