Floral fragrances

In ‘Why are flowers pretty?’ I introduced the notion of pollination syndromes – the idea that natural selection has resulted in flowers being adapted to the pollinator which is most likely to bring about effective pollen transfer and fertilisation. This has led to particular families of plants having distinctive flower traits and relationships with specific types of pollinator.

One of the most obvious features of flowers can be their scent, and this is also one of the traits which clearly separates plants pollinated by flies and some beetles from those pollinated by other insects. Both flies and beetles are attracted by the smell of rotting meat or fruit, because they lay their eggs on sources of rotting protein. We’ve all seen enough episodes of CSI to know that forensic pathologists can sometimes determine time of death from the stage of development of fly larvae (maggots) found on a body left in the open.

The most spectacular plant like this I’ve come across was the Dragon Arum (Dracunculus vulgaris) we found on Crete. You can smell the unpleasant odour of the large, dark red flower head long before you see it.

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Dracunculus vulgaris with wild flower book for scale

Like many Arums, the small flowers are carried in a dense spike or spadix, around which a large bract called a spathe is wrapped.   The spadix is apparently supposed to look like the tongue of a small dragon hiding in the spathe, though I think you need a good imagination to see this!   Of course these plants are tricksters, every bit as much as many orchids – the carrion flies which visit then receive no reward for carrying pollen between plants. The foul smell (produced by amines such as putrescine and cadaverine) disappears once the flowers are pollinated, having served its purpose.

Flies tend to be important pollinators at high altitudes, where other insect groups are relatively scarce and there are certainly fly pollinated Arums in the Indian Himalayas – so maybe we will see the Dragon Arum’s cousins.

Other flowers have a scent we find much more attractive and these tend to be pollinated by insects in search of a sweet reward in the form of nectar.   The bee and wasp-pollinated Mahonia described in ‘A case study in evolution – Podophyllum hexandrum is a case in point.  A large range of different types of flowers are pollinated by bees and wasps – some are bowl shaped or radially symmetrical (think mallows) whereas others are bilaterally symmetrical or zygomorphic, for example foxgloves.

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Musk mallow (Malva moschata) and foxglove (Digitalis purpurea) being pollinated by bees

Of course there are many different types of bee and wasp and some will have preferences for specific flowers, which make them more specialised pollinators.

Another feature common to many insect pollinated flowers is the presence of ‘nectar guides’ to direct the insect towards its reward at the base of the flower and, more importantly, to position it such that pollen can be picked up from the anthers or deposited on the stigma. The nectar guides in a foxglove look a bit like stepping stones to me, but more commonly they take the form of lines on the petals which converge on the nectaries at their base.

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Nectar guides in Grass of Parnassus (Parnassia palustris)

Because bees don’t see light towards the red end of the spectrum but do see in the ultraviolet range, we may not see the same patterning on a flower as an insect does, however. A crocus flower, for example, looks much more dramatic to its pollinators than it does to us – the image on the right, below, is the flower seen under UV light.

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Crocus flower in daylight and under UV light.  Image: Utah Pests News, Winter 2012