Orchid Pollination – Sexual Deception & other techniques
Chris Barker
Chris is now Chair of the BOC, but not really an orchid grower – which wife Jean is. Both are members of the RHS committee as well as both the Darlington OS and North east of England OS.
This is a slightly unusual lecture as it can easily apply to other herbaceous plants as it does to orchids – it is just that orchids have almost perfected the ‘dark arts’. Put simply pollination is the process of getting the pollen from the male ‘plant’ onto the female parts so that seed can develop to further the life of the species, or even start on a trail towards new species or adaptations of various types. This is generally carried out by the ‘birds and bees’ of our own teenage education along with other ‘bugs’ and wind. 90% of plants rely on ‘animals’, and over 200,000 species take part in this process with almost all of them being insects.
We generally think of pollen as being dust, such as that which drives our human hay fever; however in orchids it is generally 2 sticky packets or masses (some have 4-8 packets). We see pictures of (male) euglossine bees getting pollen masses stuck onto their thoraxes but tend not to think about how these are released. This is actually due to a deteriorating glue system that is geared to allowing the bee time to get to the next plant whence the pollen should easily become stuck to the stigmatic surface. These bees are naturally looking for dust pollen for food etc. We should appreciate that nature avoids self pollinating as this prevents evolution which is necessary to take account of natural cycles in nature such as climate changes etc.
Deception comes from a desire for insects to mate rather than collect food. This comes in a variety of ways with scent, shape and colour the main ones – often with each other. The important part of the orchid’s evolution is to resemble a female insect to attract the males that perform the fake mating but collect and transfer pollen. Drakaea glyptodon is a species from Western Australia that imitates a specific wasp in shape and colour on a hinged lip. When the wasp lands it swings backwards towards the column, and the pollen is transferred. At the next plant the glue has weakened and the pollen transferred to the stigma, and more pollen is collected. Cryptostylis leptochila is closely related from around Perth, and has a different shape to attract the Lissopimpla excelsa wasp. Diuris behrii is a yellow Australian species uses a different technique that means to have a successful mating the specific wasp needs to reverse into the flower. The fact that a queue can form is of little interest to the plant unless the next one has pollen from elsewhere.
Gongora bufonia is a species from Brazil that has a special relationship with the Brazil nut tree. Gongora has a scent that attracts male euglossine bees. The males attract the females to the canopy area where they then find the Brazil nut flowers and pollinate them. Once the nuts are ripe, another animal is used for seed distribution. The nuts are in a hard wood round case, and it needs the Aguti (rat family) to chew it open and take the nuts away and bury them for later food – if they haven’t already started to grow!
Reward is the technique that most of us tend to associate with orchid pollionation with many species having spurs on the lip that hold nectar. This includes hardy orchids such as Platanthera which uses the Sphinx moth that has the right length tongue so that as the tip of the tongue reaches the nectar then the head touches the column and catches the pollen. The best known of this technique is Angraecum sesquipedale – also known as Darwin’s Orchid. This led to his prediction of an unknown moth that was the pollinator, and only after his death was the moth discovered.Xanthopan morgani praedicta is the moth, a variety of the African Hawk Moth, and it has a 19″ long proboscis.
Moths aren’t the only spur pollinator as the humming bird does the same for the genus Comparettia. Having the ability to hover is essential whilst the process takes place.
Back to deception, Bulbophyllum phalaenopsis is a scented plant. Actually scent isn’t the right description as it smalls of rotting flesh. This attracts blowflies that walk all over the flower without finding anything to eat, but yet grab the pollen, and then move to another flower. Other Bulbos such as lasianthum have a similar smell. Only geographical separation prevents cross pollination.
Coryanthes is the final genus covered, and this is quite devious in its technique. It is known as the bucket orchids, and collects rainwater once open. It is surrounded by smooth waxy walls that insects can’t climb once they have slipped in. Their only way out is to crawl out via the horizontal column, which sticks the pollen to them. They then need to dry out prior to the next stage. Slipper orchids have a similar technique, but without the water.
Chris has used some wonderful images for this lecture, many of which are available online for those with lots of spare time. There are of course many more species and techniques that could have been included, but time has to be a limiting factor. One or two questions were asked and a hearty round of applause followed.