A violet-ant mutually beneficial symbiosis

 

Sweet violets Viola odorata produce two types of flower - the familiar scented kind that are amongst the first wild flowers to appear in spring and are pollinated by bumblebees, and a summer cleistogamous version that remains as a closed bud and self-pollinates without the intervention of insects. Insect pollination in early spring can be a chancy, hit-or-miss affair but, come what may, the cleistogamous flowers aways produce seeds in early autumn. This is a seed capsule of one such that I found in the garden, splitting open in three segments to release its seeds. Each seed has a small, white, oily attachment, an elaiosome, that ants find irresistible. They carry the seeds away so, if you grow violets in a garden, seedlings are likely to appear in unexpected places.

I’ve sown some freshly collected sweet violet seeds now, because they’ll need vernalisation, subjection to the freezing temperatures of winter, to break their dormancy.

 

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. 

 

Boot-mud seedlings

I first read Weeds and Aliens, a volume in the New Naturalist series written by Sir Edward Salisbury, a former Director of the Royal Botanic Gardens at Kew, almost fifty years ago. One particular section, where he discussed seeds distributed on footwear, has always stuck in my mind.

Salisbury went to some lengths to demonstrate the effectiveness of seeds dispersal by our feet as we walk around. He even swept up the soil on the floor between the pews of churches after services and germinated seeds of plantains, daisies, irongrass, pearlwort, chickweed and several grasses that had been carried in on the feet of the congregation.

He had a particular bee-in-his-bonnet about Roman soldiers carrying seeds across Britain in mud on their hobnailed sandals.

Early this year after a visit to Hayley Wood in Cambridgeshire  I followed Salisbury's example and scraped the mud from my walking boots into a seed tray to see what would germinate. The picture above shows some of the seedlings. So far I've been able to identify:

Broad-leaved dock
Broad-leaved plantain
Dandelion
Chickweed
A willow herb species
Mouse-ear chickweed
Wood avens
Herb robert
Hairy bittercress
Toad rush
4 unidentified seedlings

plus four species of grass that I haven't identified yet.

No real surprises but quite a mixture. The wood is famous for its oxlips but I don't seem to have had any luck picking up their seeds on my boots!

Yew

This is the time of year when yew 'berries' begin to become conspicuous. Yew is one of only three British native species belonging to that division of the plant kingdom known as the gymnosperms - seed-producing plants that don't have flowers and don't enclose their seeds in an ovary to form a fruit. The other two are juniper (whose seed has a succulent outer coating - seed picture at the bottom of this post) and Scots pine (which carries its seeds in woody cones - see picture below). 

In this picture you can seed the fully formed yew seed on the right, half covered by a pale green cup. This is the tissue that quickly expands to form the scarlet succulent aril, that you can see half-formed on the seed on the left. This squashy cup is attractive to birds that disperse the seeds, effectively judging from the numerous seedlings that appear in places where birds perch, such as crevices in old stone walls after the seed passes unharmed through the bird's gut. The seed itself is deadly poisonous to mammals but I've watched nuthatches wedge it in a bark crevice, hammer it open and eat it.

Here's a fully formed aril, which almost completely encloses the seed, like a scarlet doughnut.

Juniper 'berries'

Scots pine cone

The mysterious attractions of black berries

Thursday's Guardian Country Diary focuses on bird cherry Prunus padus, which has produced an exceptionally fine crop of its beautiful black fruits in Weardale this year. This is probably the result of excellent flowering conditions in late spring, fewer small ermine moths that often defoliate the trees and fewer instances of pocket plum disease which can destroy the fruits before they ripen.

Most wild fruits that are dispersed by birds are red (hawthorn, rowan, guelder rose and honeysuckle, to name but a few), so how come bird cherries (and also blackberries, buckthorn, deadly nightshade and elder berries) are black? Is fruit colour of any great significance for fruit-eating birds?

Over the last few decades there have been several scientific investigations into the ways in which fruit colours attract birds, addressing the question  'why are some ripe fruits red and others black?'

So here's a selective summary of some findings to date:

1983.
A study in the US using black cherry Prunus serotina concludes that it's not the final black colour of the ripe fruit that's important in attracting birds, but the mixture of intermediate red-coloured ripening fruits and black fruits that attract birds' attention. They observe the same results with pokeweed Phytolacca americana, whose fruits change from green to pink to black, and the conclusion is that the bicoloured phase of fruit presentation is important, rather than the final ripe colour. 

1999.
Unlike humans, birds can perceive ultra-violet light so the colours that they see don't correspond to our perception. Blue, violet and black berries reflect ultra-violet light, which may make them more conspicuous to birds and indicate their stage of ripeness. The bloom on the surface of bilberries (which is caused by a surface layer of yeast) reflects UV. When redwings were allowed to choose between bilberries with the bloom on the surface and others where the bloom had been polished away adult redwings preferred berries with the UV reflecting bloom, but inexperienced juveniles didn't distinguish between the two, suggesting that the birds learn to associate UV reflectance with fruit ripeness. 

2003
Another study revisits the bicoloured fruit hypothesis from 1983 and finds evidence to support it. 

2004.
Another study with juvenile redwings, which initially prefer red fruits but learn to associate black/UV reflective fruits with palatability, suggests that learning plays a key role in fruit colour choice. The study suggests that the colour of the fruits, not the contrast between their colour and the background, is particularly most important. 

  
2004
Another study that examines the relative significance of fruit colour and the contrast between fruit colour and background colour concludes that red and black fruits contrast more strongly with background foliage colours than any other fruit colours, which may be why they predominate. Tests on four bird species indicate that it's conspicuousness relative to background colours, not fruit colour itself, that's most important to foraging fruit-eating birds.

2006
Tests with crows in a flight cage conclude that artificial  red fruits are more conspicuous to them from a distance than artificial black fruits. When the crows were offered blueberries with or without their UV reflecting bloom, they were more successful in finding the UV reflecting ones - but that also depended on how UV-reflecting the background was.

2009.
A study concludes that blackcaps preferentially eat black or UV-reflecting fruits, choosing those with the highest intensity of colour, and that these darker fruits have high purple anthocyanin pigment antioxidant levels (good for their health) and also higher energy content. 

2009
A  study shows that birds can preferentially select ripe elder berries that have red stalks (indicative of high anthocyanin content) or ripe elder berries with green fruit stalks (that make the fruits more conspicuous and indicate a higher sugar content) - all suggesting that birds don't just gobble fruit down, they can be picky eaters and make dietary choices based on fruit quality.

2011
Studies using captive blackbirds and redwings that are offered artificial fruits show that blackbirds preferentially choose red fruits whereas redwings prefer black ones, but that these preferences can change depending on previous experience - they tended to prefer fruits of a colour that they had previously fed on, so learning was important.

2011
A review of the literature on the subject concludes (a) there is evidence that fruit colour is an important signal that birds react to; (b) that black fruits high in antioxidants (purple anthocyanin pigments), which tend to be formed in the fruits in bright light under cold conditions, enhance the immune systems of blackcaps; (c) that high levels of antioxidants reduce the fruits' proneness to fungal attack, benefiting the plant and suggesting that black fruits might have originally evolved for this reason, rather than to attract birds. 


So, back to the original question :'why are some ripe fruits red and others black?'. In summary, it's all to do with - 

- the fact that these are the two colours that birds find most attractive

- that the mixture of reds and blacks in a branch of ripening fruit (think of blackberries ripening from pink to black) may be important in attracting birds' attention

-  UV reflectance (that we can't see) is very important especially in fruits like blueberry, bilberry and sloe that have a UV-reflecting surface 'bloom'.

- that the contrast between the fruit colour and background colour is important

- bird species may have innate preferences but these can change when they learn to associate colour with fruit quality

- some birds can select fruit, on the basis of colour signals, that has higher nutritional value

. black colour, with high antioxidant anthocyanin pigment content, might have evolved in plants as a defence mechanism against fungal disease rather than to attract birds.

As is so often the case, there are no easy answers; scientific research rarely delivers simple explanations but always produces a whole raft of more interesting, more sophisticated questions. 

Which is why science is so addictive - the natural world is always far more complex than anyone can possibly imagine.

As for the birds in my garden:

- the blackbirds and thrushes never touch yellow-berried holly but eat all the red berried holly as soon as it ripens.

- waxwings and bullfinches only eat the yellow fruit of 'Golden hornet' crab apple when it goes brown and begins to rot. If you want to attract birds to a garden in autumn, don't plant yellow-fruited trees and shrubs

- blackbirds go mad for the blue, bloom-covered UV reflecting, high anthocyanin containing berries of red flowering currant and Berberis - there's purple-stained bird poo all over our garden right now!


Click here for two more long-standing botanical mysteries

Sources:

Willson,M.F. and Melampy, M.N. (1983) The effect of bicolored fruit displays on fruit removal by avian frugivores. Oikos 41: 27-31.

Siitari, H; Honkavaara, J; Viitala, J (1999). Ultraviolet reflection of berries attracts foraging birds. A laboratory study with redwings (Turdus iliacus) and bilberries (Vaccinium myrtillus). Proc.Roy. Soc (B) 266,2125-2129

Jennifer M. Cramer, Maria L. Cloud, Nathan C. Muchhala, Anastasia E. Ware, Brent H. Smith and G. Bruce Williamson (2003) A test of the bicolored fruit display hypothesis: Berry removal with artificial fruit flags. Journal of the Torrey Botanical Society 130, 30-33

Honkavaara, J; Siitari, H; Viitala, J (2004) Fruit colour preferences of redwings (Turdus iliacus): Experiments with hand-raised juveniles and wild-caught adults. Ethology, 110, 445-457

Schmidt, V; Schaefer, HM; Winkler, H (2004) Conspicuousness, not colour as foraging cue in plant-animal signalling. Oikos 106, 551-557

Schaefer, H. Martin; Levey, Douglas J.; Schaefer, Veronika; et al. (2006) The role of chromatic and achromatic signals for fruit detection by birds. Behavioural Ecology 17,784-789

Schaefer, H. M.; McGraw, K.; Catoni, C. (2008) Birds use fruit colour as honest signal of dietary antioxidant rewards. Functional Ecology 2, 303-310


Schaefer, H.M. and Braun, J . (2009)  Reliable cues and signals of fruit quality are contingent on the habitat in black elder (Sambucus nigra). Ecology 90, 1564-1573.


Larrinaga, Asier (2011) Inter-specific and intra-specific variability in fruit color preference in two species of Turdus. Integrative Zoology 6, 244-258


Schaefer, H.M. (2011) Why fruits go to the dark side. Acta Oecologica 37, 604-610.

Botanical artillery...

Cranesbills are wild flowers with a particularly violent method of dispersing their seeds, that involves a catapult mechanism and is particularly well developed in bloody cranesbill Geranium sanguineum.

Cranesbills belong to the genus Geranium, a name that's derived from the Greek word geranos, meaning 'crane' (the bird, not the mechanical device) and nicely describes the central axis of the fruit, which resembles a crane's beak.

There are five egg shaped seed containers at the base of that long beak, which is formed from five long strips of plant tissue that are firmly attached to the tip of the beak and develop tremendous internal tensions as they dry out. They are botanical springs and when their inner tensions reach a critical point they break free at the base and curl away from it with enormous force......

.... so that they curl upwards with the egg-shaped containers attached, hurling out the seed which lies in each container, in much the same way as those dog ball throwers that dog walkers use. After the seeds have been dispersed this elegant candelabra-like structure is left behind.

Bloody cranesbill is the official county wild flower of Northumberland.

Violent violets, with Seeds as Slippery as a Bar of Soap ....

Summer is over and it's time for plants to disperse their seeds. Some just release them on a feathery parachute, to drift away on the breeze. Others hook themselves into fur or feathers and let animals carry them passively. Others are enclosed in succulent fruits and pass through an animal's digestive system.

And then there are those with a violent tendency, with exploding seed pods and catapult mechanisms or, in the case of some members of the genus Viola (pansies and violets), a mechanism that's easier to demonstrate than describe. 

It works like this. Grip a wet bar of soap as tightly as you can, then watch the soap fly out from between your fingers. Much the same thing happens when a pansy disperses its seed. It's globular seed capsule splits into three boat-shaped sections that fold outwards and curl inwards at their edges, exerting ever-increasing pressure on the smooth-coated seeds that they enclose ..... until the seeds are violently shot outwards.

Crane'sbill Catapults

Meadow Crane'sbill catapults discharged
Meadow Crane'sbill catapults primed and ready to go

The meadow crane’sbill Geranium pratense in my garden (see http://cabinetofcuriosities-greenfingers.blogspot.com/search/label/Geranium%20pratense) has now run to seed and is using its natural catapult mechanism to hurl seeds around the flower borders. There are five seeds, each inside an ovary that splits open, arranged around the bottom of the central ‘beak’ of the fruit and connected to it by long strips of dead cells that become tensioned like springs as they dry out. When the tension reaches a critical point each ovary breaks free and is flicked upwards violently by its ‘spring’, hurling out the seed – just like a rock launched from a Roman siege catapult. It’s a highly effective mechanism – from one original plant, I now have meadow crane’sbills all over the garden. The seeds of this species have a hard, water-repellent outer coat and are slow to germinate, so if you want to raise it from seed gently abrade the seed coat with a piece of sandpaper, then it will germinate very quickly.