Science: Little Green Fern

When the great Kariba Dam on the Zambesi River was finished in 1959, every prospect about it was pleasing. Besides generating 1,500,000 kilowatts of power, the dam would create a lake 175 miles long where protein-starved black Rhodesians could catch fish and white Rhodesians could swim and sail. But Kariba Lake had hardly begun to fill with water when a vicious enemy showed its deceptively pretty face. A delicate, floating water fern named Salvinia auriculata appeared in patches that spread with astonishing speed. By last week dense mats, some of them strong enough...

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