Hydrophytes

Hydrophytes are plants that live in water or in permanently wet places, and they are sub-divided according to exactly how much of the plant stays in contact with water, mud and air: free-floating hydrophytes float loose on the water surface, touching water and air but never soil (e.g. Eichhornia, Pistia, and Wolffia — the smallest flowering plant known); rooted-floating hydrophytes anchor their roots in mud while their leaves and flowers float on the surface, so they touch soil, water and air all at once (e.g. Nelumbo, Nymphaea, Potamogeton, Marsilea — lotus seeds, notably, have the longest recorded longevity of any plant seed); submerged-floating hydrophytes stay entirely underwater with no contact with soil or air (e.g. Ceratophyllum, Utricularia); rooted-submerged hydrophytes are rooted in soil but stay fully underwater, never touching air (e.g. Hydrilla, Vallisneria, Isoetes); and amphibious (rooted-emergent) hydrophytes grow in shallow water and are adapted to both an aquatic and a terrestrial mode of life at once (e.g. Ranunculus, Typha, Sagittaria). A related, smaller group called hygrophytes grow in moist, damp, shady places rather than standing water itself (e.g. the orchid Habenaria, mosses). Structurally, hydrophytes show a very consistent set of adaptations. In the root, roots may be entirely absent (Wolffia, Salvinia), poorly developed (Hydrilla), or well developed (Ranunculus), and root caps are often replaced by spongy root pockets (as in Eichhornia). In the stem, submerged forms have long, slender, spongy, flexible stems; free-floating forms have short, thick, stoloniferous, spongy stems; and rooted-floating forms grow a rhizome — vegetative propagation happens readily through runners, stolons, cuttings, tubers, dormant apices and offsets. In the leaves, shape varies with the exact hydrophyte type — thin ribbon-like leaves (Vallisneria), long linear leaves (Potamogeton), finely dissected leaves (Ceratophyllum), large flat floating leaves (Nymphaea, Nelumbo), swollen spongy petioles (Eichhornia, Trapa) — and emergent forms often show heterophylly, growing dissected leaves underwater but entire (undivided) leaves in air (Ranunculus, Limnophila heterophylla, Sagittaria). Anatomically, the cuticle is thin or absent, the epidermis is a single layer, the cortex is dominated by large air-filled aerenchyma for buoyanc …