The Pollination Story; part 3, specialising

This is chapter 3 in the fascinating - to me anyway! - story of pollination; see here for the previous episode. It didn't take plants long, in evolutionary terms, to devise numerous ways, visual and chemical, to be more obvious to compete with their neighbours for the essential insect pollinators. We looked at some of these strategies last time.

Another is to put out advertising hoardings - "get your lovely fresh energy-enhanced nectar HERE!" - in the form of nectar guides on the petals, to direct their customers straight to the source. They weren't the last advertisers to assume that their clients weren't bright enough to work things out for themselves!

Alpine Gentian Gentianella muelleriana, Kosciuzko National Park, New South Wales.

Pelargonium rodneyanum.

Lilac Lily Schelhammera undulata, Family Colchicaceae, Budderoo NP, New South Wales.

We see these as contrasting colours, and it's likely the insects do too, but it's not safe to assume that a butterfly sees the same colours that we do - it probably doesn't in fact. For instance many, perhaps most, insects can see much shorter wavelengths than we can - once they get shorter than what we interpret as violet, we just lump them all as 'ultraviolet', but if a butterfly could speak it would probably have names for another half dozen or so colours that we could never imagine. By viewing flowers under ultraviolet light we can see nectar-guide streaks otherwise invisible to us - but we still have no way of seeing what a butterfly or wasp sees. 

But all this was but a prelude to more and more sophisticated specialisation - after all the point is not just to have the pollen taken from you, but to be reliably delivered to another flower of the same species. Colour is one way of narrowing the field of overlap with competitors; insects see best at the yellow-blue end of the spectrum. Another is petal number (and for current purposes I'm using 'petal' loosely to include both petals and sepals). Some insects can in fact 'count' to some degree, so a major direction was towards reducing the number of petals and keeping them constant; insects learnt to associate these petal numbers – 'iconic numerals' – with a favoured food source. This was a big step forward from earlier flowers with no regular shape, and varying numbers of petals clustered randomly. It led to flat flowers with set petal numbers.

The next major move was into three dimensions - ie a tubular flower like a Daffodil or Correa. It not only excludes most pollinators - ie assisting the goal of specialising - but more accurately guides the pollinator past the flower's sexual organs. 

Brachyotum quinquenerve Melastomaceae, Manu NP, Peru.

So far, all the flower shapes I've considered have been radially symmetrical ('actinomorphic') - ie any line drawn across the flower will divide it in half. This limits the potential for variation.

Correa barkeriana Rutaceae, Barren Grounds NR, New South Wales.

The next stage of complexity was to a flower that is bilaterally symmetrical - only one line, down the middle, can divide it in half.

Wedge Pea Gompholobium huegelii, Canberra, is relatively simple.
Carousel Spider Orchid Caladenia (Arachnorchis) arenicola Perth, (below) is more complex.
In each case only one pair of mirror images can be obtained, by running a line down the
centre of the front of the flower.

The advantage of this may not be immediately obvious, but it removes the limitations on flower shape variations imposed by the requirement that the flower has to be uniformly shaped. Evolution can now tweak infinitely by altering the top or bottom of the flower without changing the other, or by changing each differently.

So, why not free yourself entirely of restrictions on variation by having no symmetry? It is intriguingly rare, but apparently some tropical bird- and bat-pollinated flowers have indeed taken this path. Unfortunately I can't offer you any examples, and any help with finding some would be greatly appreciated! 

Next time, a whole new suite of bigger and better customers!

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