In the shirt-sticking heat of south Florida, jade spires, thick as a man's forearm and bristling with spines, grow 8 feet tall, shooting straight up from the ground like giant exclamation points.

Found in only four other places in the United States, these tree cactuses stand in danger of disappearing forever. Little studied, they have probably never been tested for their possible benefits to mankind.Enter the Nature Conservancy. One of the youngest, least known and most successful environmental groups in the United States, it combines business and biology to rescue plants, animals and their habitats from extinction.

Buying, selling, trading, cajoling, the 37-year-old nonprofit organization has brought protection to 3.5 million acres, saving land in all 50 states. It completes an average of one U.S. land deal a day, adding an average of 1,000 acres daily.

The Nature Conservancy itself maintains almost half of the 3.5 million acres in about a thousand preserves, the largest private sanctuary system in the world, Noel Grove reported in a National Geographic magazine story.

Biggest of the preserves is 55,000 acres on Santa Cruz Island off the California coast. Smallest is just under an acre of Connecticut marsh where herons feed. Thousands of acres more, bought by the conservancy, have become state and national parks and wildlife refuges. An international program protects species and ecosystems in Latin America.

In its Arlington, Va., headquarters, the conservancy keeps a computerized inventory of more than 50,000 plants and animals, ranking each according to the extent of its peril. The current status of any of them can be checked within seconds. "It's the most complete data base on biota in the world," chief scientist Robert Jenkins said.

The "ark" the conservancy is building has an urgent, specific mission: to preserve the world's biotic diversity. Hundreds of species have proved valuable to humans - the blood of the horseshoe crab in the diagnosis of meningitis, bee venom in the treatment of arthritis - but only one in every 100 known species has been intensively screened for potential use.

"Creative land deals are at the core of conservancy work," Grove wrote. Ready cash for large acquisitions is available from today's $85 million land preservation fund. In the early years, conservancy members mortgaged their own homes for such funds.

To preserve a remnant of pure Eastern prairie, the conservancy bought a small-plane airport on Martha's Vineyard, the Massachusetts vacation island, and then leased the airport's operation.

To protect ospreys nesting on the 2,039 acres of a hunting club on Shelter Island at the eastern tip of New York's Long Island, it had to buy all the holdings of the firm that owned the club. The conservancy then sold off everything but the club property, where active osprey nests have since doubled.

To save the sand-dune habitat of the fringe-toed lizard in California's Coachella Valley, the conservancy negotiated a solution that is held up as a model of compromise. The 9-inch lizard was threatened with extinction when a host of satellite towns began expanding beyond Palm Springs into the valley.

The U.S. Fish and Wildlife Service attempted to shut down development but the developer of the luxury homes threatened to sue. The county government, eyeing tax revenues, sided with the developer. Environmentalists called in the conservancy.

The result: 13,000 acres in a preserve that includes more than 5,000 acres of lizard habitat. That open space increased the nearby land's value for development.

Stewardship of the lands it acquires is an increasing financial burden on the conservancy; more frequently, preserves are trying to pay their own way. In western Montana, the operation of a dude ranch covers expenses for 18,000-acre Pine Butte Swamp Preserve, where the real guests are grizzly bears.

In its campaign to prevent extinctions, the conservancy has found that practicality is the most convincing argument. Perhaps the point was best made by botanist Peter Lesica to a Montana rancher on whose land grew a threatened prairie carnation.

The conservancy wanted to protect it with a conservation easement, restricting some use of the carnation's surroundings but allowing the rancher a tax deduction.

"This flower," asked the rancher testily, "is it good for anything?"

"We don't know yet. But if you see a bolt on the ground, do you throw it away?"

"Course not. I might need it some day."

"We feel the same way," said the botanist, "about the prairie carnation."

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CONSERVANCY ACTIVELY PURSUES UTAH PROJECTS

The Nature Conservancy, which purchases property to preserve threatened habitat, has been quite active in Utah. Just in the past month the environmental group bought a 3,200-acre parcel of private property, called "The Basin," in the Deep Creek Mountains near the Utah-Nevada border. The property is to be traded to the Bureau of Land Management to be added to a surrounding wilderness study area. Also, the group has been active recently in securing wildlife preserve sites along the Lower Strawberry River.