Tuesday, May 5, 2015

Bees need dandelions but dandelions don't need bees

























If I was a fastidious gardener I'd probably have dug out the dandelions in our garden long before now because they seed themselves so prolifically, but instead I've been watching the constant stream of bees - with their pollen baskets stuffed full of orange dandelion pollen - visiting the flowers all afternoon. There are golden drifts of dandelion flowers everywhere just now - along road verges, on waste ground and in pastures - and every year they provide a reliable source of vast amounts of pollen and nectar for bees.



























The strange thing is, though, that dandelions don't need nectar, pollen or pollinators to produce a full crop of seeds. Bee and dandelion bloom might seem like a perfect example of a partnership between pollinator and plant, with generous rewards for services rendered by the insects, but this is really a very one-sided relationship.






















At first glance it might seem that the elaborate mechanism that dandelions Taraxacum officinale use for presenting pollen to visiting insects is a masterpiece of functional design. Look across the top of a dandelion flower with a magnifying glass and you can see a forest of stigmas, divided and curled back at the top of a long style covered in pollen. This is the last stage in a developmental process that begins in the flower bud ....






































 .... where at this stage the individual florets that make up the flower head (capitulum) are just on the point of flowering. From the bottom upwards in the photo above, first you can see the ovaries that contain the egg cells that will become the embryo in the seeds, then above them are the stamens, joined in a long yellow cylinder.....




... seen here in a single floret. Notice how even at this stage the ring of feathery hairs (the pappus), that will form the parachute that will carry the mature seed aloft on the breeze is already well developed. This floret is one from the centre of the flower and has no petal, unlike those around the edge that have ray petals for advertisement ....


























... like this one, where you can see the single petal attached. At this later stage of development the style has now elongated inside that cylinder of stamens, forcing its way upwards like a piston and sweeping out the pollen as it goes, then splitting at the tip to reveal the receptive stigma where pollen delivered by a visiting insect will germinate.


























The outer surface of the style is covered in a forest of short hairs that help to sweep the pollen out of that cylinder of stamens. Pollen adheres to the outseide of the style until an insect arrives and collects it, at the same time cross-pollinating the stigma with the pollen from another that it arrived with.









































But to the dandelions, all of this elaborate floral choreography is redundant - a waste of energy. 

At some point in their evolution they acquired mutations that allows their ovules (above) to develop into seeds without any need for pollination, producing clonal, identical copies of the parent plant. It's a process called apomixis, that's also found in several other plants, including some bramble species.

 So in dandelions all that complex and energetically expensive floral development and the provision of pollen and nectar to attract pollinating insects now serves no purpose - it's a legacy of an earlier stage in evolution, when dandelions did need to be cross pollinated. In some species of dandelion the pollination mechanism is still functional, but not in the apomitic common dandelion Taraxacum officinale

Nevertheless, all that redundant nectar and pollen is a wonderful asset for bees and for butterflies like....




















...the orange tip, and .......

























...the peacock

11 comments:

  1. Fascinating stuff Phil.
    A great site to see all the Dandelions blooming along the roadside verges at the moment. Sadly though, our local Council are out with the mowers this afternoon. Such a shame.

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    1. Long ribbons of golden road verge here too - a wonderful sight! Must be supporting uncountable numbers of bees.

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  2. Apart from that they are very pretty flowers too.

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    1. I always let then flower then keep them under control (and the neighbours happy) by pulling off the dead flower heads before they can scatter their seeds

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    1. Really interesting that you should mention that, Kate. One of the major objections to GM crops is that they can transfer their genes to non-GM crops via their pollen. An apomictic GM crop that was engineered not to produce pollen would set seeds reliably without insects and couldn't contaminate non-GM crops. There has been a lot of research into that.....

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    1. All insect life passes by when you sit and watch a patch of dandelions for an hour!

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  5. Great post with fascinating information. I didn't know that about Dandelions, but have been admiring them close-up in the garden here. I don't understand how we have created a situation where we call important wild flowers necessary to our insect life weeds and set out to deliberately destroy them, particularly when wild flowers are so aesthetically pleasing.

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    1. I think dandelions are a beautiful addition to a garden in spring and you can easily prevent them from taking the garden over just by removing the seed heads.

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  6. A brilliant and exceedingly interesting post with some great photos. When it stops raining I'll be out in the garden with magnifying glass and hand lens to look closely at the flowers :)

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