Salvinia
Salvinia molesta is part of a complex group of closely related floating fern species. Salvinia, apparently native to southeastern Brazil, has become a serious pantropical freshwater weed. Over the course of the twentieth century, the fern has been introduced to many countries, via the international trade in aquatic plants for ponds and aquaria.
Salvinia leaves are arranged in groups of three along a thin rhizome. In each group, two green, flat, undivided aerial leaves float on the water surface. The third leaf is non-photosynthetic, highly divided and hangs, root-like, underwater. At high light intensities, salvinia aerial leaves are partially folded, raising the plant off the water surface. At low light intensities, such as in the Botanic Garden glasshouse, leaves tend to lie flat on the water.
Biological, morphological and anatomical features of salvinia have contributed to the fern's success as an invasive species. Salvinia is only known to reproduce vegetatively, from rhizome fragments; even tiny fragments are capable of producing new plants. Spore-producing structures, for sexual reproduction, form underwater but the spores are infertile; this has led to the suggestion that salvinia may be of hybrid origin. Furthermore, under appropriate nutrient conditions, salvinia grows rapidly; some studies have shown that salvinia plants may double their size in just over a week. The effects of heavy salvinia infestations are profound. Thick carpets of plants clog lakes and block sunlight to other plants that oxygenate the water. Eventually, as the plants decompose lakes stagnate and overall biological diversity drops. Infestations of salvinia also interfere with water management, for example, through flooding and irrigation controls, and provides conditions for mosquito breeding.
The aerial leaves show clear adaptations for buoyancy and water repellence. The two leaf surfaces are separated by rows of cells that divide the leaf into a series of air-filled chambers, which creates a buoyant platform. The surface of the leaf in contact with the air is densely packed with long-stalked, wax-covered hairs that look like egg whisks. The main part of the hair is hydrophobic but the extreme tip is hydrophilic. The effect is to produce an extremely water-repellent surface from which water droplets are readily lost and when submerged the plant is covered by an air film. Curiously, from a developmental perspective, salvinia leaf surfaces are reversed: the surface in contact with the air is the lower surface, whilst the surface in contact with the water is the upper surface.
Further reading
Barrett SCH 1989. Waterweed invasions. Scientific American 261: 90-97.
Kaul RB 1976. Anatomical observations on floating leaves. Aquatic Botany 2: 215-234.
Stephen Harris