Tiny plant family finds a home

One of the first things you used to learn in botany is that flowering plants can be divided into ‘monocots’ – grasses, lilies, orchids and the like – and ‘dicots’, the rest.

We now know that this is an artificial divide, but it’s still a useful way to group together similar looking plants. The Monocotyledons have either barely recognisable flowers (like grasses) or the floral parts arranged in threes or multiples of threes, and the leaves usually have parallel veins (hold the leaf up to the light and look for the solid lines).

The Dicotyledons, on the other hand, have flower parts usually in fours or fives and leaves mostly with a network of veins. They also provide wood tissue, so any true tree is a dicot.

There are a few plants that might seem misplaced, like the palms in the monocots (their trunk is quite different to a woody tree), but for most trained botanists or keen naturalists there is no confusion over this division.

So it was a great surprise to find that a family of tiny aquatic plants called the Hydatellaceae (don’t worry if you haven’t heard of them – they are really small and uncommon) were neither monocots nor dicots. It turns out they belong to a small group of plants that evolved before the diversification of the dicots and the splitting off of the monocots.

A paper published in the prestigious science journal Nature in 2007, with two Botanic Gardens’ scientists (Dr Barbara Briggs and Dr Adam Marchant) as co-authors, tells the story. Sequencing of the plants’ DNA showed quite clearly that they are more closely related to the water lily family, another in this so called basal group in the flowering plant tree.

This doesn’t necessarily mean the ancestors of flowering plants were aquatic or looked like the hydats, but it does mean this group is ancient and has not diversified much.

For the last 200 years or so the hydats had been misclassified in the monocots (close to, or within, a family of moss- or rush-like monocots called the Centrolepidaceae).

So why did we get it wrong for so long? The flowers have no petals, and the leaves are narrow and have either one vein or a few parallel ones, so there is little to go on. However there was a general ‘3-symetry’ about the flowers which suggested the monocot link.

In this case, looks were deceiving. But of course knowing they arose before the monocots were a twinkle in the evolutionary eye, means they may have some ancestral characters that carried through to the monocots or dicots.

The Hydatellaceae is small family of small plants, from Australia, New Zealand and India, all remnants of the once great southern land Gondwana. There are only 10 species in total, all but three of them in Australia, mostly in Western Australia and Tasmania but one of them also in inland New South Wales.

Image: At the top, the small plant (against a cm scale) at the centre of all the fuss, this one from a wetland in Western Australia (photo Jaime Plaza); above, Dr Barbara Briggs - previously Director of Science at the Botanic Gardens Trust and now full-time (honorary) researcher here - collecting in Tasmania. This post is from the Radio Archives (April 2007).