National Geographic : 1976 Oct
dinner favors. She placed one plant in a la goon near her home in Jacksonville. Today water hyacinths blanket as many as 200,000 acres of Florida canals and lakes (pages 550 51). Hyacinths thrive in fertilizer-enriched waters. They impede boat traffic and shade out other plant life, sometimes to the extent that the water receives virtually no oxygen and biological deserts are created. Compared to hydrilla, however, water hya cinths are well behaved. Vinelike hydrilla perhaps a native of Malaysia or Africa that was brought into Florida as an aquarium plant in the 1950's-has within the past few years turned into an ecologist's nightmare. Hydrilla is an extremely adaptable plant. It grows an inch a day, can double its weight in a week when young, and thrives in almost total darkness (page 542). When its long green tentacles reach the surface, sometimes from a depth of 50 feet, they form dense mats that can stop an outboard motor dead. It simply overwhelms native plants. Hydrilla spreads by attaching bits of itself to boat propellers, birds' feet, and other ob jects that move from lake to lake. One tiny piece regenerates into a new plant. Conse quently, in only a few years it has clogged some 150,000 acres of waterways, and has moved into most other southern states. So far, it has been found as far north as Iowa, and biologists fully expect it to spread throughout the country. Hydrilla's reproductive potential is so great that it cannot be eradicated, just controlled. The chemicals to fight it can cost three hun dred dollars an acre, and must be applied at least twice a year. No wonder Floridians are looking at another new exotic, this one a vo racious weed-eating fish called the grass carp, or white amur, to clean out their hydrilla. The Oriental grass carp, which resembles a big silver-colored goldfish and can weigh 100 pounds, has been a hot issue (page 543). Con fronting the severe hydrilla problem, con cerned state officials favor releasing the grass OSTEOPILUSSEPTENTRIONALIS, 8 CM (THREEINCHES) When giant toads hop in for dinner, a wise dog abandons its bowl (left). Glands on the South American amphibians' backs (above) secrete a milky poison that has killed hun dreds of small dogs. People, too, can be affected if the toxin gets into their mouths or eyes. The everlastingly hungry toads gob ble almost anything in sight-insects, pet food, or whatever man eats. All eyes and appetite, a suction-fingered Cuban tree frog (right)-largest in the U. S. -intrigues Miami herpetologist Lewis Ober. It preys on native tree frogs that consume insects harmful to citrus trees.