Weeds are considered undesirable plants that adversely affect the productivity of farm crops. They have been defined also as "plants growing where they are not wanted," or "plants growing out of place." Aquatic weeds are adapted to live in water. Certain modifications of structure and function permit them to grow in aquatic environment. Their stems are typically light and porous. Roots are used chiefly as anchoring organs and as organs for absorbing dissolved minerals and water. Submerged weeds have their leaves either ribbonlike or subdivided and dissected into threadlike segments. The flowers of aquatic plants are usually produced above water, but in some species the flowers begin their growth underwater.