Phyllanthus fluitans

Lore & Background

The species was first discovered by English botanist Richard Spruce in the Amazon. Upon finding it, Spruce noted in his journal that despite being a flowering plant, it closely resembled the aquatic fern Salvinia, a case of convergent appearance across different plant groups. The generic name derives from Ancient Greek for 'leaf flower', and the specific name from Latin meaning 'floating'.

The plant's stems are 3 to 5 cm long with rootlets emerging from nodes. Its sessile, cordate-orbicular leaves have pockets on each side of the midrib that trap air, aiding flotation. The leaves are hydrophobic, likely due to the Salvinia effect, and vary from green to red depending on light brightness. The lower epidermis lacks stomata, while a few are present on the upper epidermis. It produces small white unisexual flowers on a cyme inflorescence, and seed capsules are depressed-globular with six triangular seeds each.

Taxonomically, it was formerly placed in Euphorbiaceae (hence the common name 'floating spurge') until a 1993 maximum parsimony analysis revealed that Euphorbiaceae contained multiple lineages, leading to its reclassification into Phyllanthaceae. It is apparently most closely related to P. caroliniensis.

Reader's Guide

Phyllanthus fluitans is grown commercially for use in aquariums, valued for its red roots and the reddish tint leaves gain under bright light. Its native range spans the Amazon basin across several South American countries, with the type location in the Rio Negro tributary. Outside its native range, it has become naturalized in southern Florida and Mexico (Tabasco), where it has invaded. The species has the potential to reach Puerto Rico, Hawaii, Louisiana, and parts of the southern United States, where it could become a problematic invasive species similar to water fern, water lettuce, and water hyacinth. By covering large swaths of stagnant backwater areas, it may limit or block ambient light penetration to the bottom, potentially stunting or killing submersed plants. It has also been recorded in Panama. The plant's anatomy, with air-storage cells and hydrophobic leaves, enables it to float and remain horizontal in aquatic environments.

Taxonomy, Naming, and Taxonomic History

The genus name Phyllanthus draws on two Ancient Greek roots: phyllon, meaning leaf, and anthos, meaning flower, essentially describing a plant that bears flowers among its foliage. The specific epithet fluitans is Latin for floating, a direct nod to the species' aquatic lifestyle. Described in March 1863 by George Bentham and Johannes Müller Argoviensis, this plant occupies a peculiar niche within its genus: it is one of only three non-terrestrial Phyllanthus species, the other two being P. leonardianus and P. felicis. For much of its taxonomic history it sat within the family Euphorbiaceae, a placement that gave rise to the misleading common name "floating spurge." In 1993, however, a maximum parsimony analysis revealed that Euphorbiaceae was actually a polyphyletic assemblage containing several distinct lineages, one of which was carved out as Phyllanthaceae. Despite the name "apple duckweed" also being applied, the plant belongs to neither the spurge family nor the duckweed group. Phylogenetically it appears most closely allied to P. caroliniensis, linking this Amazonian floater to a North American relative.

Morphology and the Engineering of Floating

Phyllanthus fluitans is a free-floating herbaceous perennial whose stems measure just three to five centimetres, studded with clusters of rootlets at each node. Its sessile, heart-shaped leaves are the key to its aquatic existence: the leaf surface forms a pocket on either side of the midrib that traps air, keeping the plant buoyant and horizontal on the water. Beneath the epidermis, large outward-bulging cells and one or two layers of green cells create a structure resembling palisade parenchyma, while broad gaps separated by single-cell-thick walls extend from the chlorenchyma down to the lower epidermis. The lower epidermis carries no stomata at all, with only a handful present on the upper surface. The leaves are hydrophobic, a trait likely attributable to the Salvinia effect. Flowering is modest: two to four tiny white, unisexual, actinomorphic blooms sit on a cyme, each barely a millimetre and a half long. Seed capsules are flattened-globular, roughly three millimetres wide, and hold six triangular seeds. Depending on light intensity, foliage shifts from green to a striking red.

Discovery and Native Range

The first recorded encounter with this species came from the English botanist Richard Spruce, who spotted it in the Amazon and was so taken aback by its resemblance to the swimming aquatic fern Sahinià that he doubted his own eyes. In his journal he marveled that a plant so structurally different in its flowers and fruits could mimic another species' overall form so convincingly, calling it one of many cases where evolution produces convergent devices. The type location for the species is the Rio Negro tributary, and its native range stretches across the Amazon basin, encompassing Peru, Ecuador, Colombia, Brazil, Bolivia, Venezuela, Paraguay, and northern Argentina. It has also been documented in Panama. Beyond its South American homeland, the plant has become naturalised in southern Florida and in Tabasco, Mexico, where it has established itself as an invader. These out-of-range populations mark the beginning of a broader story of dispersal that carries significant ecological implications for the regions it could potentially colonise.

Invasive Potential and the Aquarium Trade

Beyond its established range, Phyllanthus fluitans poses a genuine threat to freshwater ecosystems in the southeastern United States and beyond. Its potential distribution includes Puerto Rico, Hawaii, Louisiana, and the southern states of Texas, Alabama, Georgia, and Mississippi, where it could behave much like other notorious invaders such as water hyacinth, water lettuce, and water fern. By blanketing stagnant backwater areas, the plant can limit or entirely block ambient light from reaching the bottom, stunting or killing the submersed vegetation that depends on that illumination. Yet the same traits that make it ecologically hazardous also make it a favourite among aquarists. In the aquarium trade it is sold under the name red root floater, prized for the vivid red colouration its roots display and the reddish tint its leaves acquire under bright lighting. This duality—noxious in the wild, ornamental in a glass tank—highlights how a single species' value shifts entirely depending on the context in which it is encountered.