Spirodela polyrhiza

One of the world's most widely distributed aquatic plants, Greater Duckweed floats on still and slow-moving water across nearly every continent. Its round, dark-green fronds — 2–10 mm across — grow in a flat, overlapping mat on the water surface, distinguished from other duckweeds by the 7–21 roots that trail from each frond like fine threads. The underside is typically a rich reddish-purple, a detail that becomes visible whenever fronds are disturbed. Under warm conditions it can double its colony in as little as two days, making it an efficient nutrient exporter for paludariums and aquariums; in autumn it forms dense turions — starchy, compact resting buds — that sink to the bottom and re-sprout in spring.