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Tuesday, 27 May 2014

The science of nectar

Nectar is that sweet reward that flowering plants provide animals in exchange for their services as pollinators. It sounds incredibly simple on one level – much like rewarding a dog with a treat after it obeys a command. However, dig a little deeper and you realise that the reproductive success of the plant is dependent on very subtle yet complex characteristics of this substance – including when it’s produced and how much is produced, as well as its very composition.

Flowering plants will optimise the characteristics of their nectar in order to influence the foraging behaviours of pollinators and ultimately improve their reproductive fitness. The characteristics of the nectar not only determine which pollinators are attracted and when they come, but how frequently they visit and how long they stay. Suddenly one realises that there is an extremely complex system of regulatory mechanisms behind nectar secretion, which have not only influenced the evolution of flowering plants, but of the pollinators themselves.
Red admiral butterfly close up
Red Admiral butterfly (Vanessa atalanta) drinking nectar. 
Photo credit: Shelby Temple.

Nectar isn’t just about sugar

But before we get into the evolution, let’s first consider what nectar is, because as it turns out it’s not just about sugar – there are a number of things in nectar that are important for pollinators.
There is no denying, however, that carbohydrates – sugars such as glucose, sucrose and fructose – are usually the main constituent of nectar. Nectar will be anywhere between 7 to 70 % carbohydrates per water weight [1]. Other sugars might also be present in small amounts as well as sugar alcohols, such as sorbitol. It is these sugars that are the primary energy source for nectar consumers.

Amino acids and proteins are the next most abundant solute in nectar after the sugars. There are essential and non-essential amino acids, which are the building blocks for proteins and there are some non-protein amino acids that are constituents of enzymes and preservatives. It is thought that the amino acid and protein content of nectar may play a role in the taste preferences of insects [1], presumably related to their nutritional needs.

The water content of nectar may also be an important reward for pollinators, particularly in dry habitats.

Nectar also contains important ions, such as potassium, as well as antioxidants, trace amounts of lipids and some secondary compounds that seem to be associated with resistance to herbivory. 

Macro photography bee
A bee gathering its nectar reward in the Botanic Garden.
Photo credit: Shelby Temple.
Many species have also been shown to have antimicrobial compounds in their nectar, which prevents microbes from growing in the nectar as well as inhibiting florally transmitted diseases [2].

Terpinoids, which are the volatile organic compounds that give flowers their scent, also accumulate in the nectar.

The composition and consistency of nectar is extremely variable as it is tuned to the needs of the nectarivores (it’s a word...really). Flowers frequented by hummingbirds, for example, generally produce nectar in small amounts with high sugar content, while those frequented by more generalist passerine birds produce dilute nectar in large quantities. There has been some evidence that honeybees have a preference for warmer nectar that’s less viscous, regardless of the sugar concentration [3]. Bats also seem to prefer less viscous nectar, though will preferentially select more dilute nectar as the water content is extremely important for their rehydration.

Not all nectar is produced in the flower

Nectar is produced in glands known as nectaries. The glands are commonly found at the base of flowers, where they produce nectar as a reward for pollinators. However, there are also extrafloral nectaries located elsewhere on the plant, often on the leaves or petiole - the stalk that attaches the leaf blade to the stem. These nectaries provide a reward for mutualistic animals, almost exclusively ants, which benefit the plant. The ants help protect certain plant species by getting rid of the eggs of herbivorous insects deposited on the foliage and in return they feast on the nutrient rich nectar secreted by the extrafloral nectaries.

Extrafloral nectaries might be particularly critical at certain times in the plant’s lifecycle. For example, there are often nectaries located on the pedicel that secrete nectar when the flowers are in bud. This attracts ants, which help protect the vulnerable flower buds from herbivorous insects and improves the reproductive success of the plant [4].

Unlike nectar produced in the flower, nectar produced in the extrafloral nectaries is far less variable as it is attracting mostly ants.

Darwin's orchid: a classic example of the coevolution of flowering plants and their pollinators

Producing nectar may use up to 37% of a plant’s available energy [5]. This means that producing it comes with some cost to the plant, but these costs are clearly outweighed by the benefits of attracting pollinators that are far more efficient than relying on wind or water.

The evolution of flowering plants and their pollinators is the most frequently used example of coevolution – the physical characteristics of both flower and animal evolving to become more specialised. It was around 120 million years ago that honeybees developed longer tongues than their short-tongued ancestors in order to access the nectar reward flowers had started to produce. Their social structure became more complex and they became fuzzier and developed pollen baskets in order to carry protein-rich pollen, but also facilitating their role as pollinators.
Darwin's orchid in bloom at the Botanic
Garden last year. Photo credit: Andy Winfield.

The flowers also changed shape in response to the preferences of their pollinators. The most classic of these examples is Darwin’s orchid (Angraecum sesquipedale) with a flower depth of 20 to 35 centimetres. The Madagascar orchid was named after Darwin because he proposed, based on its shape alone, that it had to be pollinated by an insect with a proboscis of lengths unheard of at the time. Forty years later, Morgan’s sphinx moth (Xanthopan morganii), was discovered with an unusually large proboscis...and it was indeed the pollinator of this orchid.

It is also thought that nectar chemistry itself has evolved in response to pollinators. As mentioned earlier, bats prefer nectar with low sugar concentrations and as a result bat pollinated plants from very diverse and distantly related taxonomic groups have evolved nectar with low sugar concentrations.

Deceit and robbing

Not all flowers use nectar – some have non-rewarding flowers. Around 30-40% of species within the orchid family do not produce rewarding nectar in their flowers [6] and instead use different methods to attract pollinators. Orchid flowers may look like another species that provides nectar or they may mimic shelters or brood-sites or even pollinators themselves in order to draw the attentions of individuals looking for a place to shelter or for a potential mate (such as in bumble-bee orchids).

Just as plants have found ways to get pollinated without producing nectar, some animals have found ways to get nectar yet avoid being pollinators. Some flower visitors - known as nectar robbers - will avoid the normal route to the nectar, usually avoiding the floral opening all together and pierce or bite the flower elsewhere to extract the nectar directly without coming into contact with any of the reproductive parts.

For many years it was thought that nectar robbers had a negative or neutral effect on the plants, but over the last couple of decades, research has shown they can also have a positive effect on the plant. Firstly, some nectar robbers do ultimately end up pollinating the plants. Secondly, their presence can modify the behaviours of the pollinators. For example, if flowers have less nectar (because the robbers have extracted some) then pollinators will visit more flowers, increase their foraging range, travel further distances and spend less time at each flower – all of which could improve cross pollination and increase genetic diversity. Maloof et al [7] provide a good review on this topic.  

There has been extensive research done on the characteristics of nectar and its relationship with pollinators. More recent research, however, is starting to unravel the mechanisms by which plants produce nectar – identifying some of the pathways sugars are transported within the plant and concentrated in their nectar [8]. There is still lots to learn.


[1] Pacini E, Nicolson SW (2007). Chapter 1: Introduction, In: Nicolson SW, Nepi M, Pacini E (Eds.) Nectaries and Nectar. Springer: The Netherlands. ISBN: 978-1-4020-5936-0. (pages 8-10).

[2] Sasu MA, Wall KL, Stephenson AG (2010). Antimicrobial nectar inhibits a florally transmitted pathogen of a wild Cucurbita pepo (Cucurbitaceae). American Journal of Botany 97 (6): 1025-1030. (link)

[3] Nicolson SW, de Veer L, Köhler A, Pirk CWW. Honeybees prefer warmer nectar and less viscous nectar, regardless of sugar concentration (link).

[4] Bentley BL (1977). The protective function of ants visiting the extrafloral nectaries of Bixa orellana (Bixaceae). J. Ecol. 65 (1): 27.38.

[5] Pyke GH (1991). What does it cost a plant to produce floral nectar? Nature 350: 58-59. doi: 10.1038/350058a0

[6] Johnson SD, Hobbhahn N, Bytebier B (2013). Ancestral deceit and labile evolution of nectar production in the African orchid genus Disa. Biol. Lett. 9 (5): 20130500. doi: 10.1098/rsbl.2013.0500.

[8] Lin IW et al. (2014). Nectar secretion requires sucrose phosphate synthases and the sugar transporter SWEET9. Nature 508: 546-549. doi: 10.1038/nature13082

Friday, 16 May 2014

The Native Bluebell: Britain’s favourite flower in trouble

by Helen Roberts

It is a beautiful spring morning in May and I am taking my children for a walk. We are venturing to some local woods on the edge of the Mendip Hills, a stone’s throw away from our house.

The woods are secreted away in a limestone gorge. The stubby cliffs of limestone are clothed in ivy and gradually open up into a steep sided valley. A tiny stream channels through the gorge; tributaries often disappearing down sink holes. We trek across a ploughed field to the gate that lets us into the wood.

As we pass through the kissing gate, there is an overwhelming smell – it’s the heady perfume of the native bluebell, Hyacinthoides non-scripta. The woods are carpeted in vibrant blue (the colour almost glows it is so vivid), dotted with ferns and intermingled with wood anemones (Anemone nemorosa), Lady’s smock (Cardamine pratensis), wild garlic (Allium ursinum), greater stitchwort (Stellaria holostea) and yellow archangel (Lamiastrum galeobdolon). It is one of my favourite places for a walk in the spring and it is made special because of the sight and smell of bluebells.

Bluebell woods in Britain are under threat
British woodland with bluebells in bloom
Bluebells blanket the ground in British woodlands
this time of year. Photo credit: Shelby Temple

Bluebell woods are an iconic part of our natural heritage and are one of the most beautiful sights to encounter in the British countryside. They were voted Britain’s favourite flower in Plantlife’s ‘CountyFlowers project in 2002 and we have 50% of the entire world population in our country.

Sadly, the indigenous bluebell, Hyacinthoides non-scripta, is in danger because it cross breeds with the commonly planted Spanish bluebell (Hyacinthoideshispanica) and with the resulting fertile hybrid (Hyacinthoides x massartiana). Molecular studies have shown that the Spanish bluebell and the native bluebell have a shared ancestor [1], but Hyacinthoides non-scripta has developed in isolation over the last 8,000 years, its range to the north of the Spanish bluebell [2].

Polluting bluebell genetics

The Spanish bluebell has been grown as a garden plant in Britain since 1683 [3] and it and its hybrid have now ‘gone over the garden wall’ and are encroaching on our native bluebell woods. Its leap over the ‘wall’ has most likely been facilitated by bulbs being thrown out or dumped near native woodlands. The Spanish bluebell looks a thug of a plant next to our native one – being a much bigger plant - and is reported far more vigorous. 

Hyacinthoides non-scripta
Native bluebells are low to the ground and
deep blue to violet in colour. The flower spike
distinctly nods to one side. Photo credit: Glyn Baker
[CC-BY-SA-2.0 (],
via Wikimedia Commons
In its native range, the Spanish bluebell has a wider ecological tolerance to that of the native bluebell. It copes better with drier and more exposed conditions and can therefore grow in more open sites, such as roadside verges and waste ground. The Spanish bluebell is a garden favourite because it’s so much larger and can establish itself and grow quickly. Both the Spanish bluebell and its hybrid, however, have the ability to take over leading to the loss of genetic integrity of the native bluebell.

The native and bluebell hybrid are really difficult to tell apart even by expert botanists and sometimes the only way to distinguish between them is to apply DNA analysis. Many gardeners are sold the hybrid mislabeled as ‘English Bluebell’ and have planted them in good faith thinking these were the native bluebell.

The hybrids were first recorded in the wild in 1963, though they were likely there long before then as the Spanish bluebell was first recorded in the wild in 1909. The Natural History Museum gives good guidance on how to identify your bluebells with a supporting video given by botanist Fred Rumsey here.

Nation-wide bluebell surveys show extent of Spanish bluebell invasion

A survey performed by Plantlife International in 2003 found that one in six broadleaved woodlands surveyed were found to contain the hybrid or Spanish bluebell. The survey drew attention to the threat posed to our native bluebell as well as the need for more research in order to better understand species distribution, gene transfer across species and appropriate horticultural management of bluebell species.

Thankfully, it has been illegal (without a license) for anyone to collect and sell native bluebells from the wild since 1998 as they are protected under the Countryside and Wildlife Act (1981). Current legislation allows for the issuing of a special license to collect wild seed for commercial sale. These safeguards ensure that collection is done sustainably and protects wild bluebell populations.

The native bluebell is a priority species under the UK BiodiversityAction Plan (BAP). Plantlife International states that it's vital that the horticultural industry stop the deceiving sale of the Spanish and hybrid bluebell as native bluebell. Plantlife has also worked with Flora Locale to set up an industry code of practice. Flora Locale helps people get in touch with suppliers in their area who sell seeds of local provenance. Another initiative between Landlife and the Mersey Forest produces a legitimate source of bulbs grown from seed with a long term programme running to plant them in new woodlands. Plantlife International also gives advice about making sure that gardeners check suppliers of bluebells and how to remove Spanish or hybrid bluebells from your land - read more here.

The Natural History Museum launched a bluebell survey in 2006 (of which you can take part) to look at the extent to which non-native bluebells have spread into the British countryside. Results from the last eight years show that most bluebells in urban areas are now hybrids, but fortunately there are still large areas of countryside containing our native species.

Since 2010, the survey has concentrated on comparing the flowering times of native and non-native bluebells to understand how they will each respond to climate change. By comparing recent surveys with past data, it is possible to find out whether the flowering season is changing. These data need to be collected over many years in order to tease out any real effects of climate change from the natural fluctuations inherent in any population.


[1] Grundmann, M. et al. (2010). Phylogeny and taxonomy of the bluebell genus Hyacinthoides, Asparagaceae [Hyacinthaceae]. Taxon, 59 (1): 68-82.
[2] Natural History Museum [website] Hyacinthoides non-scripta (British bluebell).
[3] Pilgrim, E. and N. Hutchinson. Bluebells for Britain: A report on the 2003 Bluebells for Britain survey.  Plantlife International. <>

More sources of information on bluebells:

Preston C.D. et al. (2002). New Atlas of the British and Irish Flora: An Atlas of the Vascular Plants of Britain, Ireland, The Isle of Man and the Channel Islands. ISBN: 9780198510673. [Provides information on each taxon]

Tines T.D.. et al. (2012). The Wild Things Guide to the Changing Plants of the British Isles. ISBN: 9781905026999. [Provides information on the spread of non-native bluebells]

Monday, 12 May 2014

The 2014 Easter Art and Sculpture Exhibition at the Botanic Garden

This is the second year in a row my family has worked off some Easter chocolate by biking to the University of Bristol Botanic Garden for the Easter Art and Sculpture Event. Unlike last year, however, when we were bundled against the cold, this year we basked in glorious sunshine. We’ve been so impressed with the quality of the art that this will be an annual event for my family...whatever the weather!

The Garden’s annual sculpture event was combined with the Friends of the Botanic Garden’s annual art exhibition, for the first time this year. This gave visitors a visual feast of outstanding artwork set in the backdrop of the garden in its full spring glory.

Over 2,800 visitors came to the gardens over the 4-day event. Events such as this, as well as the Bee and Pollination Festival, are important for the Botanic Garden as they reach beyond those interested in gardens – drawing in new audiences. This event brought in a range of West Country artists working in diverse mediums, from watercolours to ceramics and stained glass to metalwork – there was something for everyone.

Stainless steel sculpture of water lilies by artist Ian Marlow
Ian Marlow's lilies were on display in the University
of Bristol Botanic Garden pond during the
Easter Art and Sculpture Exhibition.

A taste of some of the artists and artwork on display

One of the first pieces to catch my eye as I entered the gates of the garden was, of all places, in the pond. Stainless steel lilies, created by of Ian Marlow (, shone in the sunlight – a stark contrast to the dark water. Ian was also the creator of Sir Gromit ofBristol, which was certainly one of my son’s favourite stops along last year’s Gromit Unleashed trail.  

Willa Ashworth's working display of one of lovely
metal open fire-pits offered warmth as well as
the promise of a sausage and hot cuppa.
Willa Ashworth ( was there again this year, with a number of new pieces. I went home last year with one of her beautiful garden wind chimes and I still adore it! This year, she had one of her open fire-pits set up with sausages cooking on a grill and a kettle bubbling away. Her working display created a multi-sensory experience – the smell and sounds of sizzling food, the warmth and glow of fire, the beauty of the fire-pit itself. I watched as people gazed at the fire, no doubt envisioning (as I was) the lovely little set-up in their own backyard. Willa’s functional metalwork sculptures are inspired by her love of gardening and one of her pieces is now permanently displayed next to the lake in the Botanic Garden.

Karen Edwards ( was another return artist this year, displaying her nature-inspired ceramics. Each of Karen’s pieces are hand-built and unique. I was particularly attracted to her planters, which appealed to my functional side. The organic textures in the ceramics not only drew my eye, but beckoned me to reach out and touch them.

Karen explained how she created the lovely textures in one of her pieces:
One of Karen Edwards' nature-inspired ceramics with
imprints of ammonites and bark.
“The doors of my studio are clad with unstripped half logs,” explained Karen. “I pressed some clay onto one area, then made that into a cylinder that I biscuit fired to use as a small hand roller. I then pressed in some shell and ammonite textures. The textures are highlighted with metal oxide wash and slip (liquid clay).”

One of Jude Goss's stained glass pieces hanging in the
Chinese herb garden.
The Chinese herb garden once again hosted the stained glass art, including that of Jude Goss ( My six year old was thrilled to once again see Sam Bailey’s ( metal dinosaur sculpture in the Evolution of Land Plants Display (nicknamed the grotto).

Stonecarver Tom Clark ( had handed his chisel over to a pair of children when I got around to his display. The youngsters seemed to be having a brilliant time chipping away at the block of stone. Tom served an apprenticeship at Chichester Cathedral and has since worked on many large and interesting restoration projects including WestminsterAbbey and The National Gallery.

While I was circulating through the outside gardens, my husband managed to get into the glasshouses where he was captivated by the stainless steel pieces created by Julian P. Warren (

My son, Morgan, was equally enthusiastic about Julian’s work and he told me all about it on the bike ride home, with the unbridled enthusiasm that comes with being six.  “There was this amazing dragonfly mum...and a bird of paradise with those little sticky-up feathers on his head and everything!”

Sadly, I didn’t get to all the exhibits as I was keen on joining a tour set up by Andy and Nick for a number of bloggers (more on that below). I missed the botanical artists displayed in the Linnaeus Study room entirely! However, I have listed all the artists with links to their websites at the bottom of this post as they all made incredible contributions to the weekend event.

Oh...and I also got to hear about some delicious carrot cake on my bike ride home. Another thing I missed out on...the delightful refreshments being served on the newly finished west patio of The Holmes.

It wouldn’t happen were it not for the volunteers

As with any event of this magnitude, there is a whole lot of work that happens behind the scenes. With the Garden’s small staff, it is thanks to the many volunteers that it all comes together, and they are the first to admit it.

“It’s all those people out there that are welcoming people as they enter, giving tours, serving cake and refreshments and helping direct people around the garden that make this work,” said Nick Wray, the Garden’s Curator. “Without these volunteers, we simply couldn’t host important events like this.”

A personal tour for local bloggers

As part of the Garden’s plan to increase its digital presence and reach out to wider audiences, Andy and Nick hosted a tour of the garden for a number of local bloggers during the event.  Helen and I, both bloggers for the Garden, tagged along too!

We started in the meeting room with delicious cakes and a cuppa, while Nick gave everyone a briefing on the history of the garden, its core collections and more generally the role of botanic gardens. After lots of healthy discussion about the Botanic Garden and the role of social media in marketing, we all followed Andy into the garden where he briefed us on each of the main displays.

Fused and slumped glass artwork
Adele Christensen's 'Blomsters': Fused and slumped glass
with mild steel support.
Despite having been to the garden many times before and having had lots of discussions with Andy and Nick...I still learned a great deal, and found new inspiration for blog stay tuned!

Hopefully some of the other bloggers will have found some inspiration from the tour also – the Botanic Garden is, after all, a place for education and research, stewardship and conservation, but it is also a place of great beauty.

Artists on display were:

Willa Ashworth (
Susan Bracher (Email: susanbartle (at)
Pamela Clogstoun
Sarah Easby (
Anne Girling
Nick Hasell (Email: woodbarnfarm (at)
Emma Jean Kemp (
Florence Maggs
Betty Marten
Connie Ridge
Cynthia Skinner
Dorcas Sohn
Jac Solomons (aka J Zulka)
Sheila Southgate
Julian P. Warren (
Frankie Wild
Refab Arts (