Waiting for a breeze

A field on the edge of Durham city, full of thousands of dandelion 'clocks', waiting for a breeze to carry their plumed seeds aloft. 

The final phase in the dandelion life cycle, when the flowers are transformed into silvery spheres, dandelion ‘clocks’ composed of seeds each equipped with its own parachute, is a magical moment when an umbrella of hairs, a pappus in botanical parlance, carries the seed up and away on the wind, to pastures new. At sunrise in late spring, whole fields can shimmer with silvery dandelion clocks as their pappuses expand as they dry in the sun’s heat. Sometimes goldfinches arrive to feed on the seeds, releasing wraiths of downy seeds, ethereal ‘witches’ gowns’, into the rising thermals.

The name dandelion is a corruption of the French dent-de-lion, lion’s teeth, describing the deeply serrated leaf edges, but botanist Geoffrey Grigson also collected 52 parochial county names. Some, like Devil’s Milk-plant (Kirkudbrightshire), refer to the bitter milky sap. Many, like Schoolboys’ Clock and Tell Time (both Somerset) allude to the childhood game of guessing the time by the number of puffs needed to blow away all its seeds from the ‘clock’. Monk’s head (Wiltshire), likening the bare seed head left behind to a monk’s bald pate, is said to have medieval origins, while Wishes (Wiltshire) stems from the belief that the airborne seeds carry away hopes and dreams with them.

 And perhaps this is the best name of all, for these troubled times. Pick a dandelion clock, rediscover your inner child, blow as hard as you can and send dandelion seeds skywards, into the blue. 

 

Goldfinches, alders and dandelions

 

It's hard to over-estimate how important alder seeds are as food for small finches in winter. Siskins love them, but so do goldfinches, like these that I watched on a walk near my home yesterday. 

Goldfinches are also fond of dandelion seeds and, although the main season for these had long passed, there are always late-flowering dandelions around. This goldfinch was feeding on dandelion and lawn daisy seeds on a grassy verge at the beginning of December.

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

Gold standard

If I had any sense I'd probably have dug out the dandelions in the gardens 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 - for the reason why, click here.

Africa Gomez, over at Bugblog, has more pictures of some of dandelions' many insect visitors in spring.

A Dandelion with a Difference

I found this strange-looking dandelion in Newcastle this afternoon, growing beside a footpath in the city. Unlike typical dandelions (see picture below) which have a spiral whorl of florets each with as single long petal with a toothed tip, with very long petals around the margin of the flower head, this one had very short spoon-shaped petals throughout, so the flower resembled a pom-pom with numerous long styles, each tipped with a bifid stigma, sticking out of it. Quite decorative for a dandelion. There are mutant varieties of chrysanthemum sold commercially that are very similar in form - but a lot larger.

This (above) is a typical dandelion inflorescence. Normally each individual dandelion floret is quite a complicated affair (see here for details) with the long style elongating through a tube of pollen-laden stamens, followed by the splitting of the stigma tip to reveal its receptive surface - a complex arrangement that is totally redundant because dandelions set seed without the need for any fertilisation by pollen, by a process called apomixis. The seeds that are produced are all clones of the parent - so in a week or two, when the flowers have run to seed, I'll go back to this abnormal plant and collect some - they should breed true and produce exact copies if this is a genetic mutant.
 

The other curious aspect of this mutant, seen here from above, is that all the florets are female - there is no trace of stamens or pollen. It seems the mutation that truncates the petals also inhibits the development of the stamens.

There's more on dandelions here.

One-Sided Relationship

Dandelions are a terrific source of nectar and pollen for bumblebees ...

.... and for small solitary bees like those above. Can anyone help me identify any of these?

It might seem that this is a perfect example of a partnership between plant and pollinator, with generous rewards for services rendered by the insects, but this is a very one-sided relationship. Unlike typical flowers, most dandelions don’t actually need any pollen to set seeds.

Seeds are usually produced when male pollen fertilises a female egg cell inside the flower to produce an embryo inside of the developing seed, but in many dandelion species seeds are produced without the need for fertilisation, so the seedlings are genetic clones of the mother plant. All that pollen and nectar that sustains the bees, as well as the elaborate mechanism for presenting pollen to bees to maximise the chances of cross pollination, is redundant as far as the dandelion is concerned - a needless expense.

This production of seeds without pollination is called apomixis and in recent years it has attracted the interest of genetic engineers, because if it could be enginerred into GM crops they wouldn't require pollen for seed production. This would remove one major objection to these high-tech crops – that they can transfer their genes via pollen to organic crops or wild plant relatives. Without pollen, an apomictic crop could not contaminate non-GM crops, with the added bonus that it could reliably produce seeds without depending on insect pollinators.

Two heads better than one?

Found this twin-headed dandelion with a wide, flat flower stalk yesterday. There are usually four possible causes of this kind of abnormality: (1) A mutant plant – but if it was a genetic mutant all the flower heads would have been like this, not just one. All the other flowers on this plant were normal. (2) A sub-lethal dose of herbicide – but there were no signs of herbicide damage in any of the surrounding vegetation. (3) A soil bacterium called Corynebacterium, which causes the fasciation sometimes seen in twigs of woody plants like Forsythia, where the stem is wide and flat instead of having a circular cross section – a possibility. (4) Damage to the bud during the earliest stages of development, perhaps by an insect – my favourite theory, in this case.

The importance of the commonplace

Naturalists tend to be obsessed with rarities but it’s the common species that are the foundation of all ecosystems. The billions of dandelions flowering now – common and so taken for granted - provide a reliable source of pollen for honeybees and other insects just when they need it most. Rare species are generally those that are at the natural limits of their range of climatic tolerance anyway, and so come and go according to the vagaries of climate, but it seems to me that we should be most concerned when common species that play a pivotal role in ecosystems become less common – that is a symptom of potential catastrophe. Loss of old pastures has led to a rapid decline in cowslip populations in many parts of the country, but this can be reversed. I found this wonderful display of cowslips – deliberately seeded – just outside of Durham city today. This was a patch that was roughly the size of a medium-sized back garden; imagine what a five acre pasture with a population of cowslips like this would look like. They do still exist - I know of a few in the north east, although none are quite as denslely populated with cowslips as this. The wild plant conservation charity Plantlife has a well established common plants survey that anyone can contribute to – take a look, at http://www.plantlife.org.uk/uk/plantlife-get-involved-common-plants-survey.html

Now is the perfect time to get involved.