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GREATER DUCKWEED Spirodella polyrhiza (Lemnaceae) Aquatic. Surface-floating, freshwater perennial that grows in still or slow-flowing waters of canals, ditches and ponds. FLOWERS are minute and seldom seen. FRUITS are seldom seen. LEAVES are up to 10mm across, flat and rounded or ovate with several dangling roots. STATUS-Locally common, mainly in the S.