A plant’s ability to survive submerged in water depends on specialized biological adaptations. Water severely limits two necessities of terrestrial plant life: oxygen and efficient gas exchange. Diffusion of gases like oxygen and carbon dioxide is approximately 10,000 times slower in water than in air, making underwater survival challenging for most species. Plants adapted to aquatic environments are known as hydrophytes, and they exhibit a spectrum of adaptations based on whether they are fully submerged, partially submerged, or tolerant of temporary water exposure.
Plants That Live Fully Submerged
True aquatic plants, or submerged hydrophytes, spend their entire vegetative life cycle completely underwater, sometimes sending a flower above the surface for pollination. These species have evolved profound adaptations to overcome environmental limitations, primarily the lack of atmospheric oxygen and slow gas diffusion. To facilitate gas exchange, these plants have significantly reduced their cuticle, the waxy outer layer terrestrial plants use to prevent water loss.
The thin or absent cuticle allows the plant to absorb dissolved gases and nutrients directly through the leaf surface. To maximize absorption, many submerged plants exhibit finely dissected, ribbon-like, or thread-like foliage. Examples include Hornwort (Ceratophyllum demersum) or Anacharis (Egeria densa). These plants lack external structural support and rely on the surrounding water for buoyancy; when removed from water, they appear limp.
A primary adaptation is the development of specialized spongy tissue known as aerenchyma, which forms air channels throughout the stems and leaves. This network acts like an internal snorkel system, transporting oxygen produced during photosynthesis down to the roots buried in low-oxygen sediment. Common fully submerged plants include Cryptocoryne species, Vallisneria, and Water Wisteria (Hygrophila difformis). These species are popular in aquariums because they oxygenate the water and absorb excess nutrients.
Plants That Thrive at the Water’s Edge
Marginal or emergent plants are semi-aquatic species that anchor their roots in saturated soil or shallow water, but their leaves and flowers extend above the waterline. Although their roots are permanently submerged and face low oxygen challenges, the foliage remains in the atmosphere. This allows them to perform efficient gas exchange and photosynthesis like terrestrial plants, circumventing the need for specialized leaf adaptations.
Marginal plants combat waterlogged conditions with highly developed aerenchyma tissue. This extensive air channel system channels atmospheric oxygen, collected by the leaves and stems above the water, down to the submerged roots. This prevents oxygen deprivation that could lead to tissue death. The roots primarily serve for anchorage while absorbing water and nutrients from the surrounding environment.
Robust examples include Cattails (Typha species), recognized by their tall, narrow leaves that offer minimal resistance to wind. Pickerelweed (Pontederia cordata) and various Water Iris species also thrive where the water table is consistently near the soil surface. Taro (Colocasia esculenta), with its large, distinctive leaves, relies on aerial leaves for survival while its root system is underwater. These marginal plants play a significant ecological role by stabilizing shorelines and filtering pollutants.
Terrestrial Plants Cultivated in Water
A distinct group of plants that can be submerged are terrestrial species, typically houseplants, grown with only their root systems in water. This practice is known as water propagation or hydroponics. While not naturally adapted to aquatic life, they tolerate submerged roots because they are often grown from cuttings that develop specialized, delicate “water roots.” These roots differ structurally from thicker soil roots, as they do not need to search for water.
The submerged roots of these terrestrial plants, such as Pothos (Epipremnum aureum), Philodendron, and Lucky Bamboo (Dracaena sanderiana), survive because their leaves and stem remain completely in the air, ensuring a steady supply of oxygen. Water alone lacks the complete spectrum of nutrients found in soil. Therefore, for long-term growth, these plants require the addition of liquid fertilizer.
While many cuttings are rooted in water for a few weeks, robust species like Pothos and Philodendron can live in water for extended periods if nutrients are provided. Spider Plants (Chlorophytum comosum) and certain Begonia varieties can also be successfully grown this way. The practice utilizes the plant’s natural ability to form adventitious roots at the node when a stem cutting is placed in a moist environment, making propagation simple for home gardeners.