‘Floral diversity shows evolution is an ongoing process — with certain unpredictable aspects’

David Baum is professor of botany at the University of Wisconsin-Madison. Speaking to Srijana Mitra Das at Times Evoke, he discusses floral variation — and its fascinating reasons:

What is the core of your research?
My work is about evolution and how this has generated the amazing diversity of life around us. I study plants but I’ve also become interested in the evolution of life and early steps in life’s emergence.

What are some important kinds of diversity found in the floral world?
A wonderful thing about flowers is that they vary in so many ways — there is great diversity in colour and shape, how deep a flower is and the extent to which it’s easy or hard for an animal to access nectar. In many groups, there is fascinating floral symmetry — these can be radially or bilaterally symmetrical — and variation in size. I specialise in baobab trees — their flowers are massive but they differ greatly in shape, colour, smell and nectar. There is so much visible variation in flowers which you can study through an evolutionary lens.

HOW DOES YOUR GARDEN GROW: Earth is a veritable garden — and the amazing diversity seen in its flowering plants, from antirrhinum (L) to baobabs (R), reflects both severe evolutionary pressures as well as strategies of resilience (Photo credits: iStock)
What does such diversity tell us?
The floral world shows how evolution is an ongoing process of exploration — and there are nonpredictable aspects of it. It is predict able to have lineages of plants interacting with different animals — selection will then favour the traits which improve that fit. So, if a bird becomes a primary pollinator of a flower, the latter will produce a lot of nectar, grow longer and become red, yellow or orange to attract it. If it’s a bee, the flower can become shorter and take on ultraviolet colours. On the other hand, whether individual lineages switch pollina tors can be idiosyncratic, based on where they live, the climate changes they face, etc. Floral diversity thus holds both a predictable dimension — and a random part which we call ‘historical contingency’, where lineages over long periods of time can do very unexpected things.
What shapes floral symmetry?
If you look at flowers from the front, most look like a star — they have multiple planes of symmetry.
You could spin a blossom around and it won’t change its appearance. But some have a top, a bottom and two sides. The genetic basis for the control of floral symmetry is most likely in relation to certain pollinators — if you have a radially symmetrical flower, a pollinator can enter at any orientation. So, the flower must produce enough pollen to put this at every possible place. In a flower with a top and bottom, the pollinator consistently visits with one orientation, so the flower can produce less pollen, put this in fewer places and have more efficient pollination.

The snapdragon (antirrhinum) and orchid are bilaterally symmetrical. Phlox are radially symmetrical, with five petals on top, all equally spaced around the centre.

FAR FROM FRAGRANT: Rafflesias grow upto 3 feet wide and smell of decay (Photo credits: iStock)
Do flowers then involve physics?
Yes. All organisms are physical beings, constrained by the laws of physics. Plants control flower shape by regulating where cells are growing and where they’re not — that generates the pressures which yield a flower’s shape, consistent with physics.

We think of flowers as delightful — but can you tell us about rafflesias?
Found in southeast Asia, these are possibly the world’s largest single flower — the plant body is parasitic and lives inside another host plant. It has a huge bloom — the pollinators which visit rafflesias are carrion flies though and to attract them, the flower grows extremely large, smells awful and generates heat, mimicking a perished animal on a forest floor. The rafflesia shows us the business of flowers is not to simply please — it is to achieve fertilisation or pollination.

BAE-BEE: A flower’s shape is for its ‘Before-All-Else’ (BAE) pollinators (Photo credits: iStock)

Can you share insights on baobabs?
There are eight species of baobab trees globally. One is native to continental Africa, six to Madagascar and one is restricted to north-western Australia. Their closest relatives are in South America, so it’s puzzling how this distribution happened. Somehow, the fruit got to Australia in the last 20 million years or so. We don’t know if it floated directly from Africa or if now-extinct baobab species existed around the Indian Ocean and gave rise to the Australian species. These trees tend to be predomi nant species as they’re huge and extremely long-lived — some can live upto 2,000 years. Their flowers are large, some almost 30 metres in size. These are active only for a single night though, opening after dusk. The African baobab has spherical flowers hanging down on stalks, visited mainly by bat pollinators. The Australian species is pollinated by hawk moths. In Madagascar, two species have upright flowers, visited by bats and nocturnal lemurs. The other four have very elongated yellow and red buds, pollinated by hawk moths.

Many forests where these trees live are needed for agriculture — hence, most baobabs are threatened. When I first visited Madagascar in the 1980s, there were baobab forests — now, there are very few left. As baobabs don’t have useful wood, they are left standing while the forests around them get cleared — we don’t know how long they can keep going in that state.

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