Wednesday, 9 September 2009

All is Leaf


I’ve been waiting to use this quote, attributed to the German poet, thinker and kind-of scientist, Johann Wolfgang von Goethe. And this picture is one I took last year at the first botanic garden in the world, in Padua near Venice – it’s of a glasshouse built around a palm admired by the very same Goethe.

The story, though, is about the origins of flowering plants, a topic I’ve touched on before.

In a very nice article in Monday’s online New York Times, Carl Zimmer first reminds us of Charles Darwin’s interest in this topic – “the abominable mystery” as he called it.

In Darwin’s day there were no fossils from rocks older than 100 million years ago, the time when the flowering plant architecture arose.

Not only do we have flowering plant fossils from older rocks (to 136 million years), we read the story of a plants history in its DNA. The latest discovery is that, as Zimmer puts it, “flowers evolved into their marvellous diversity in much the same way as eyes and limbs have: through the recycling of old genes for new jobs”.

The early fossils are still open to interpretation and the signal is too noisy, so far. Comparison of DNA sequences has identified long unforked branches (sometimes called basal branches because they are represented in most “trees of life” as arising from the base of the trunk) that tell us something about the early flowering plants – see my earlier posting on Amborella.

Although Amborella from New Caledonia excites evolutionary botanists, it’s the fruit fly of the plant world, Arabidopsis (see my posting on this genus) that has given us clues to where the flower itself came from. We’ve known since the time of Goethe in the 18th century, and probably before, that flower parts probably started as a weird kind of leaf. You get a hint of this occasionally when a deformed petal or other floral bit becomes leaf-like. “Alles ist Blatt (All is leaf)” said Goethe.

Zimmer describes some of the genes responsible for flower parts, reporting on research showing how petals in some flowering plants have evolved from different but related genes.

Then there is the famous ‘double fertilisation’ of flowering plants – the egg, as well as the sack of food reserves around the egg. Zimmer gives the latest theories on how this arose and discusses the debate among scientists as to whether this is much of an advantage to flowering plants and the secret to their dominance of forests on earth today.

Read Zimmer's article...

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