Water lettuce is a quirky plant. The leaves are spongy, filled with aerenchyma, a tissue that keeps the leaves afloat. The epidermis of the leaves has specialized trichomes (plant hairs). Each trichome splits into four 'hairs' that curl until their tips touch. Under electron microscopy, the trichomes look like cages and that appears their function. They trap air and repel water causing it to bead and roll off the surface.

Water lettuce blooms like other aroids with a spadix, but reduced to a single pistillate flower with 3 to 9 pollen bearing flowers above. The spathe is white with trichomes on the outer side. The inflorescence is tiny, barely emerging between the leaves. If one does not look for the 'flowers', one will never see them.

After pollination, the inflorescence twists until it is free of the leaves and dangling in the water. The fruit, enveloped in the remnants of the spathe, develops surrounded by the roots. If the waterbody should dry or the weather turns cool, the plant dies. The fruit will be carried to the bottom surrounded by the decaying plant.

When water returns and temperatures climb above 68 degrees F. (20 C.), the seed germinates and moves up to the surface. The single cotyledon (seed leaf) has no aerenchyma and no trichomes; it is trapped on the water's surface solely by the cohesive forces of water.

Water lettuce is pantropical and weedy. It is much better known by reproducing vegetatively. The plant produces stolons (runners) each with a plantlet at the tip. The runners snap readily and the rosettes are free to float away from the parent. When introduced to slow moving waters, water lettuce quickly covers the surface choking waterways, blocking sunlight to submerged plants, and slowing gas exchange which can smother aquatic animals. In many subtropical and tropical areas, the plant is illegal to buy, sell, transport, or even own.

Then, again, the plant has its uses and is sold for aquaria and garden ponds in temperate climates where it will not survive the winter. It removes ammonia and other nutrients harmful to fish from the water. But the plant requires a responsible water gardener. The plants should never be allowed to completely cover the surface. The excess plants (and there can be many excess plants) should be laid in the sun to dry until quite dead, then tossed in a compost bin.

The Center for Aquatic and Invasive Plants, IFAS, University of Florida (© 2005) has additional photographs of water lettuce. The second thumbnail from the right is a photo of the inflorescence. To view the photos, click on the link: http://aquat1.ifas.ufl.edu/pistpic.html Click on the thumbnails to enlarge the images.

Paul Venter has posted a fantastic Scanning Electron Micrograph of the trichomes on the surface of a leaf to Wikipedia, the Free Encyclopedia. To view his micrograph (© 2006), click on the link: http://en.wikipedia.org/wiki/Pistia Click on the thumbnail to enlarge the image.

(Compiled from: Hortus Third, Staff L.H. Bailey Hortorium, NY State College of Agriculture and Life Sciences, Cornell University, 1976; "The Genus Pistia Benth & Hook", Matyas Buzgó (Institute of Systematic Botany of the University of Zurich), International Aroid Society, 2006; "Closest relatives of Pistia stratiotes resolved with combined chloroplast and mitochondrial DNA sequences", Susanne Renner and Li-Bing Zhang, Aquaphyte Online, IFAS, University of Florida, 2003)