Monday, 22 November 2010

Plants in search of water




Sometimes the topic of my regular radio chat with Simon Marnie is inspired by a question I get from a visitor, a staff member or Simon himself. In this case, Martin Corben (then Online Producer, now reporter and presenter, for 702 ABC Sydney) wanted to know where water plants go when there is no water? My response to a lot of questions starts with "it's complicated" or "it depends". In this case it was the latter.

Some aquatics have seeds or vegetative fragments that are easily spread, perhaps by water birds or the wind, and they travel easily between catchments. These plants, including pondweed (Potamogeton) and waternymph (Najas), usually have relatively short-lived seed and depend on a network of wetlands to survive.

An extreme example of this gypsy life style is the aquatic sundew (Aldrovandra). Its seeds are carried around by birds, sometimes over great distances, but it rarely persists in one habitat for many years, and never survives drying out.

Aquatics with heavy seeds, such as native water lilies (e.g. Nymphaea gigantea), will tend to stay within a catchment, and are usually spread by floodwaters. The tough seeds can also lay dormant in mud or baked earth until the next rains come.

Others, like the ribbony Maundia triglochinoides, you could describe at ‘stay at homes’. They persist vegetatively or from seed, but rarely escape from their home pond. Obviously this group is most at risk if conditions change, and it includes some of the rarer species.

The Lotus (Nelumbo nucifera) has large long-lived seeds and would normally not move from a single catchment. Humans have, since prehistoric times helped it to move freely around the world.

A special group of aquatic ‘plants’, the charophytes or stoneworts, are a kind of algae. You can find them attached to sand or mud all over Australia but what you won’t see are the up to one million spores per square metre of wetland soil.

These ‘spore-banks’ are the charophytes investment for the future – a few will germinate in a single flood but most will remain for the next flush. In the meantime, birds and other animals carry them from one water hole to the next.

Like the charophytes, other algae spread by spores and small fragments. Some however have spores that can’t survive desiccation and they have to live in permanent waterbodies, often with quite restricted distributions (e.g. one catchment). These algal species can have evolved over a long time in relative isolation like our many of our native animals and plants.

Image: Lotus in the Main Pond of Sydney's Royal Botanic Gardens. This posting is from the Radio Archives (April 2007).

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