Pondweed. Plant and drug. Budding clearwater algae type plant. Eats bacteria both good and bad, as well as contaminants leeching out of the soil. Also require oxygen instead of producing it. Heat speeds reproduction and when out of control, overtakes ponds and lakes and drain them of oxygen, killing the body of water. In most cases stays within normal limits and is therefore harmless, even beneficial. Eaten by fish and birds. Can be made into a paste to apply to wounds and serves as an antiseptic. Full accounts in Book of People under diseases and treatments.