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I've been a Canberran since moving here from Adelaide on the first day of 1980. I now live in suburban Duffy with my partner Louise Maher, ABC 666 radio and on-line journalist. Among my early memories is following Sleepy Lizards (Shinglebacks) around the paddocks north of Adelaide, guarded by the faithful bull terrier. I have always been passionate about the natural world, trying to understand how it works, how the nature of Australia came to be, and sharing those understandings. My especial passions are birds, orchids and mammals. For much of my life I have been a full-time naturalist, running bush tours, writing books etc, doing consultancies, presenting a regular radio slot on local ABC, chairing a government environment advisory committee and running adult education classes. Recently I have eased back somewhat, but am still writing, teaching, doing some radio work and running overseas tours - as part of my fascination with our Gondwanan origins I've been running tours to South America for the past decade. I was awarded the Australian Plants Society Award in 2001 and the Australian Natural History Medallion in 2006, both for services to education and conservation.

Tuesday, 8 July 2014

The Pollination Story; part 1, beginnings

I think the story of flower pollination is one of the great narratives of our planet, and I love telling it, though preferably out in the bush surrounded by the flowers and their attendant animals. However that's not very practical for us, so let's make a start here. It's too big and beautiful a story to tell in one sitting and I anticipate it will take at least half a dozen chapters, which will appear from time to time.

The early land plants - whose ancestors came ashore into shallow estuaries and coast lines only some 450 million years, after nearly three billion years of life in the oceans - had no flowers, no seeds. Like their aquatic ancestors they relied on water to permit sperm to swim to eggs. In time dominant groups of plants developed the seed, a wonderful package of fertilised embryo, with food and water to start it out in life, insulated from the drought and cold of the world so that plants could at last spread into the bare inland.
An idea of how the entire world looked until about 360 million years ago, when the evolution
of the seed allowed plants to colonise the forbidding wastelands.
Lava fields, Bartolomé, Galápagos.
A related development was the wrapping of the sperm into its own tough packaging, which we call pollen, and which was cast to the winds. It's a hideously inefficient system - the chance of a pollen grain landing on the female receptacle of the right species at the right time is minuscule, and only a fraction of a fraction of  a percent of pollen produced forms a seed. As a result, vast quantities of pollen must be produced. Nonetheless, it works and conifers and cycads - the great dynasties of pre-flowering seed plants - dominated the world for some 250 million years. Indeed in vast areas of high altitudes and high latitudes where conditions are too harsh for animal pollinators, they still do dominate.
Black Cypress Pine Callitris enlicheri, Family Cupressaceae, Cooma, New South Wales.
Female (large woody) and male (small and pale brown) cones.
However, some 130 million years ago in China, according to the fossil record (longer ago, and perhaps in western Gondwana, according to some genetic evidence), plants took the next Great Leap Forward.

Archaefructus liaoningensis, one of the oldest known flowering plant fossils.
Courtesy Wiki Commons.
Pollen is high in protein, and doubtless early beetles - ancient insects - blundered around collecting some to eat, and in the process accidentally transferred some to other plants. And here was an evolutionary opportunity - if the plant could persuade the beetle to selectively take pollen to another of its species, it would be a massive advantage. It would be like addressing an envelope to a destination rather than dropping millions of identical letters from an aeroplane in the hope that one or two fluttered to the right doorstep, as the conifers were doing. 
Beetle on Xanthorrhoea flower spike.
The early beetles weren't the ideal carriers, relatively clumsy, hard-shelled and (it has been
unkindly suggested) not all that bright!
For effective pollination, plants need pollen-carriers which could visit relatively distant populations of the same plant species. The arrival of more mobile insect groups such as flies, bees and wasps, moths and butterflies, and even more modern beetles, with better sensory apparatus than the early beetles, provided an immense opportunity and, for most of the past 100 million years, the evolution of flowering plants and insects has proceeded as an inextricable partnership.
Native Bee on Xerochrysum sp., National Botanic Gardens, Canberra.
The underside of the body, especially the thorax, is covered with yellow pollen,
sticking to the hairs.
Fly, family Acroceridae (thanks Susan!), on Xerochrysum sp., National Botanic Gardens, Canberra.
Pollen can be seen adhering to the legs.
See-through butterfly on daisy, Milpe Reserve, north-west of Quito, Ecuador.
In particular, butterfly proboscises are known from 190 million year old fossils - much older than the oldest known flowers - presumably originally for taking up water and resin, but they were pre-adapted for nectar and as the flowering plants exploded in diversity across the world, so did the butterflies and moths.

Male Australian Yellow Admiral Vanessa itea on Xerochrysum sp., National Botanic Gardens, Canberra. Note coiled proboscis.
Pollen wasn't a great reason for insects to visit flowers, from the point of view of either party. It comprises complex proteins and isn't easy to digest, and of course the last thing the plant 'wants' is to have its pollen eaten. (I'm talking here in evolutionary terms, not really being anthropomorphic!) Further, the worst result of all would be having the insect carrier deliver the precious pollen uselessly to the wrong flower, ie of another species.

So two problems needed to be solved by the evolving plants. The insect had to be given another reason than pollen to visit, and the flower had to be visible, and recognisably different from the competition.

The first was solved by the development of a special gland called a nectary in the base of the flower, which produced a simple sugar solution, nectar - an energy source in other words, which was a great prize for any animal. Its sole purpose was to bribe the insects to visit. The second was by an increasingly complex system of 'flags', based initially on leaves, which we now know better as petals and sepals. And the pollinators, the early flies, bees, wasps, butterflies, moths and flower beetles, were quite capable of recognising and remembering these flag messages.

A mighty and earth-changing partnership was established.

Hoverfly, Syrphidae, on Bulbine bulbosa, Asphodeliaceae, Canberra.
As it accesses the energy treat in the nectary, it is encountering the pollen
on the fluffy anthers and the waiting club-like female stigmas.
Later, another and more distantly effective signal was added, to bring potential pollinators within sight of the petals. This was scent, another chemical released to the breezes. Poets have waxed lyrical on flower scents, but as usual they weren't developed for our benefit. The poets shouldn't be too disappointed at this realisation however - if blowflies were better pollinators, more flowers would smell of rotting meat rather than of roses!
Wilga Geijera parvifolia, Rutaceae, western New South Wales.
This rather lovely spreading tree of inland Australia, is one which does
attract blowflies to its somewhat putrid-smelling flowers.
(Ironically it is in the same family as famously sweet-smellers such as oranges and boronias!)




In the next episode, in a week or so, I want to explore how flowers and inflorescences (the arrangement of flowers on a stem) became more and more complex and specialised.

MEANTIME, BACK ON SUNDAY FOR AN ANNIVERSARY OF SOMEONE WHO IS A BIT OF A HERO OF MINE

2 comments:

Susan said...

Your Vanessa is V. itea, the Australian Admiral. V. kershawi sexes are identical.

The unidentified fly is Acroceridae.

Ian Fraser said...

Oops, yes, thanks Susan. I'd mislabelled the butterfly pic in my files and didn't check when I loaded it. And well done on the fly - they are mystery to me, to my shame.