Rheophyte

A rheophyte is an aquatic plant that lives in fast moving water currents in an environment where few other organisms can survive. Rheophytes tend to be found in currents that move at rates of 1 to 2 meters per second and that are up to 3 to 6 feet deep. The amount of force produced by these currents, and the damaging debris they can carry, makes this environment inhospitable to most plants. Rheophytes are able to live in such environments because their leaves are streamlined so as put up little resistance to the flow of water. The leaves tend to be quite narrow and flexible as well. In order to prevent the plants from being uprooted, Rheophytes have an extremely strong wide spreading root systems.

Many Rheophytes live in areas that sustain flash floods and they are dependent on the oxygenated water and buoyancy brought along with it. Simply being an aquatic plant with narrow leaves is not a sufficient condition for being a Rheophyte. Also, plants that grow in slow moving water that occasionally receive fast currents isn't a Rheophyte either if it doesn't need these fast currents to survive. Plants that fall into this category is known as facultative rheophytes. When low water levels occur Rheophytes often quickly begin to flower to take advantage of these occurrences.

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Wikipedia