Eel heads and floating flowers

All seems calm at the Ornamental Lake in Melbourne's Royal Botanic Garden. It's a wintery morning before a hot summer's day in late January. Fairly typical weather for this city.

There are a few (sub)tropical trees still flowering but the Jacarandas, Brachychitons and Cape Chestnuts have finished. Recent rain has kept the Gardens green but you can sense that things are about to curl and brown, with most plants shutting down for the summer.

In the lake itself there is plenty of algae, mostly submerged clumps of a green alga called Spirogyra, which do from time to time float to the top. The Purple Loosestrife (Lythrum salicaria) continues to do what it does best on the floating islands - bloom, bloom and then bloom some more.

In front of this floating wall of grass and loosestrife you can see mats of algal and aquatic plants accumulating at this north-east corner of the lake. The dominant aquatic plant is Eelgrass, Vallisneria australis, a Victorian native that extends all the way up to near the Queensland-New South Wales border. There are seven or so species of Vallisneria in Australia, mostly tropical, and mostly with long ribbon-like leaves - hence the common name.

For most of the year you can just ignore it. Eelgrass is there, doing all kinds of good things in the lake in terms of habitat and nutrient recycling, but for you and I its a case of move along, nothing to see here.

But in late January something does happen. The first thing you might notice are tiny eel heads poking up out of the water. I say eel* heads but that's just me being poetic and metaphoric. These are the, by the time I got to see them 'spent', female flowers.

The next thing you (might) notice is an intricate lacework of white fluff. These are the male flowers.

As you will have gathered, this is a plant with single-sex flowers. That's not unusual per se in the plant world but the way these flowers work is. Eelgrass has a particular method of pollination called, entirely unhelpfully, ‘Type III’. It's only found in Vallisneria and other members of its family Hydrocharitaceae.

The key feature is the complete detachment of male flowers as buds from a sheathed stalk at the base of the plant. These tiny flowers float to the surface where they open fully and then coagulate together in floating rafts, drifting around with the wind and current.

There are minor variations to 'Type III' pollination, and in Eelgrass and two other related genera, the pollen has to remain dry. So the flowers actually float so that the pollen bearing bits are held aloft, above the water! You see them here hovering around a slightly submerged eel head, a flower already fertilised and having lost its petals (and sepals).

The receptive part of the female flower (the style) also has to remain dry, before fertilisation. At first the style is protected by the tightly wrapped petals (and sepals). By the time the flower opens, its long flexible stalk has extended it through the water surface. There it eventually comes in contact with the drifting male flowers and fertilisation takes place.

All the female flowers I saw and photographed were fertilised and beginning to set seed. After pollination the stalk of the female flower becomes spiralled, dragging the developing fruit underwater. The seed are eventually released in the murk beneath, to be washed around and settle somewhere new.

Yes really. That's what is going on in and on the lake while the rest of the garden is bunkering down for late summer.

* As our website boasts, the Ornamental Lake is home to a healthy population of Short-finned Eels (Anguilla australis). These eels live hereabouts before the arrival of Europeans, and they enjoy the moderately shallow parts of the lake where they feed on crustaceans, frogs, insects and worms. So much so that they can reach 1.3 metres long. Eventually though they swim and slither their way into coastal estuaries where they swim 4,000 km to spawning grounds in the South Coral Sea. Because eels can breathe through their mucus-coated skin, they can in fact slither overland when it's damp (e.g. at night). At breeding time the mature eels are drawn towards flowing water, and find their way through drains to the Yarra River. The leaf-shaped larvae (soon becoming what we call 'glass eels') hatch off New Caledonia and are carried southwards on ocean currents, until at the age of one to three years when they (now 'elvers') migrate up Victorian rivers and streams, until some call our Ornamental Lake home. 

[Thanks to Neville Walsh for directing my attention to the curious botanical phenomenon.]