Cool Roots

A story for a hot summer's day. A few years ago Angus Stewart was answering a gardening question on Simon Marnie's ABC Sydney Radio Weekend Show and mentioned contractile roots in Gymea Lily. He said they draw the plant down into the soil and are important in helping the plant surviving fires.

Contractile roots are common in many different plants, and not just as a response to fire. They pull a plant closer to the ground, or in the case of bulbs, deeper into the soil.

You can usually pick contractile roots by their wrinkled exterior. At a certain stage in the roots development, some cells expand radially and contract longitudinally. This means that other, less flexible, parts of the root get contorted and wrinkled. It also means the plant is drawn downwards.

In the Australian heathlands, it’s a good idea to have your growing point below the soil surface during a raging fire. So a variety of plants, the best known being the grass tree and cycads, send down a taproot to anchor the young plant and then pull the growing tip a cm or two below the surface. Sometimes the stem itself also gets into the act and contracts a little.

For cold climate bulbs, the contraction into the soil is to protect them from freezing temperatures in winter.

Compare this to some of the Trigger Plants (Stylidium) and other species in the heathlands of Western Australia. If you look carefully, you find a kind of mini mangrove – these tiny plants look like they are on stilts and scared to touch the sandy soil.

In fact they are. The stilts keep the plant cool in summer and away from the waterlogged soil in winter. Around the world you can find larger palms and trees propped up by stilt-like roots in tidal or swampy areas, but almost only in south-west Australian heathlands do you find these ‘micro stilt plants’.

The stilts are special roots initiated from the stem above the ground, and only produced in winter when the sensitive root tip is safe from drying out before it hits the soil. Micro stilt plants don’t do so well in fires, but they like open spaces and establish almost exclusively in the season after a fire – in fact their age can tell you when the last fire occurred.

Image: A Trigger Plant from Western Australia keeping cool with its ‘micro stilts’. *This posting is from the Radio Archives (November 2007).