Some aquatic plants float on the surface of the water thanks to special structures called hydrocytes or pneumatophores, which contain air and ensure their buoyancy.
Aquatic plants float due to the principle of Archimedes: they experience an upward buoyant force equal to the weight of water they displace. In other words, if the plant is less dense than water, it remains on the surface. Many of them contain special tissues with air-filled spaces, which decreases their overall density. The result: they float easily without sinking. It's a bit like when you float thanks to an inflatable float, except here, the "float" is integrated directly into the plant.
These plants often have broad, flattened leaves that create a sort of natural raft, distributing their weight on the water's surface. Some have parts filled with air, called aerenchyma, which act like small integrated buoys and thus promote their buoyancy. Their epidermis is typically covered with a waterproof waxy layer that prevents water from penetrating, helping them stay dry, light, and efficient at floating. The roots, on the other hand, hang freely below the surface to absorb available nutrients directly from the water, without needing to anchor in soil. All of this forms a lightweight, balanced structure perfectly adapted to remain still at the surface without sinking to the bottom.
Floating aquatic plants provide a practical and secure shelter for many animals. Fish, insects, and amphibians hide there to avoid predators or to lay their eggs in peace. These plants also limit the appearance of algae by creating shade that slows their growth. Additionally, they act as a true eco-filter, absorbing excess nutrients like nitrogen or phosphorus — which helps prevent eutrophication (excessive algal blooms). Their floating roots also help purify the water by trapping pollutants and particles. Finally, they provide a direct food source for herbivorous animals such as certain waterbirds or some greedy fish.
Among the floating plants that are often encountered, the water lentil is surely the most well-known: very small and oval, it sometimes completely covers ponds, forming a smooth green carpet. Then there is the water hyacinth, with its lovely purple or bluish flowers and its swollen leaves at the base that act as natural floaters. There is also the salvinia, an aquatic fern with small hairy leaves, extremely light, that drifts freely. Or the pistia, sometimes nicknamed "water lettuce" due to its tender green appearance and thick leaves vaguely resembling a floating salad. All these plants benefit from a system of natural floaters to remain peacefully on the water's surface, effortlessly capturing light.
The sacred lotus (Nelumbo nucifera) has leaves covered with tiny bumps and a special wax that allows water droplets to roll off while carrying away dirt particles. This is known as the 'lotus effect,' which is now used in self-cleaning technologies.
The Victoria amazonica holds the record for the largest known floating leaf, sometimes reaching up to 3 meters in diameter and capable of supporting the weight of a small child!
Some floating aquatic plants, such as duckweed, are used in wastewater treatment due to their exceptional ability to absorb pollutants such as nitrates and phosphates.
The water hyacinth (Eichhornia crassipes), although beautiful, is often regarded as one of the most problematic invasive aquatic plants in the world. It can double its biomass in just two weeks!
They are generally beneficial because they provide shade to moderate water temperature, limit algae proliferation by reducing light availability, and offer valuable habitat for aquatic wildlife. However, excessive growth can block the surface and decrease the dissolved oxygen levels in the water.
To control their proliferation, it is recommended to remove them regularly by hand, introduce specially adapted herbivorous species, or regulate the nutrients present in the water, as their excess promotes their rapid growth.
Even if they are not used to anchor the plant in the ground, these roots play a key role in the absorption of nutrients directly present in the water and provide an ideal refuge for various aquatic organisms.
In general, these plants are specially adapted to an aquatic environment and cannot survive for long outside of water, as their cellular structure and tissues do not withstand desiccation.
Floating aquatic plants often develop large, flat leaves to maximize their exposure to sunlight, allowing them to efficiently carry out photosynthesis.
Many floating plants do indeed produce flowers. For example, water hyacinth and water lentils produce clearly visible flowers at the surface, whereas others do so rarely or in a very discreet manner.
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