In an unprecedented act of cooperation between conservation organizations, the Martha’s Vineyard Land Bank and Sheriff’s Meadow Foundation have completed a $3 million deal which will see 145 acres at Quansoo fully protected from development.

The land was given to Sheriff’s Meadow by the late Florence (Flipper) Harris, whose great generosity was matched by the great prudence to realize the foundation might have to sell off part of the land if it were to raise the money to properly manage the rest of it.

Her will specifically provided for such a sale. But in a move that would no doubt gladden her conservationist’s heart, Sheriff’s Meadow elected not to sell to a private developer, but to sell 5.8 prime acres, somewhat below market value, to the land bank.

Under the agreement reached between the two organizations, and with other local landholders, both the 5.8-acre pond lot, which provides access to shellfishing in Tisbury Great Pond, and a trail around the whole farm, will be open to the public, probably in about a year, despite the fact that it lies beyond a locked gate at Quansoo.

During the high season, from June to October, when the gate is locked, visitors will park outside the gate and walk in. The rest of the year, people will be able to drive into the property, where there will be limited parking.

Sheriff’s Meadow president Steve Crampton described the arrangement, by which the two groups conspired together to protect the whole of Quansoo Farm as “a blueprint for the future” of such cooperative endeavors.

Mr. Crampton said the foundation began by setting up a committee to study how it might convey the pond lot to establish an endowment for the maintenance of the property “the way Mrs. Harris envisioned it.

“We got an appraisal, which told us the lot was worth $3 to $4 million, then began to explore alternatives, with the help of Edgartown realtor Tom Wallace,” Mr. Crampton said.

“So,” said land bank executive director James Lengyel, “Tom went looking for potential buyers, of which there were a plenty for house development, but he also came to us.

“It would have gone for a great deal more on the open market and we’re very grateful to Sheriff’s Meadow, who knowingly and deliberately took a lesser price because it was pleased to have this outcome.” He continued:

“I have the impression of Mrs. Harris — who I never met, I’m afraid — was a person of great foresight and that in particular she could see that handing over something as big as Quansoo Farm would require some cash and this is the way she decided it would work.

“The Great Pond does not need another septic system.

“In addition we know that Islanders are really going to really enjoy walking through Quansoo Farm and ending up on the edge of the Great Pond.”

He said the land bank will pay the money over several years. “Sheriff’s Meadow has taken a promissory note for most of the payment,” he said.

A press release from Sheriff’s Meadow announcing the deal quoted Mrs. Harris’s daughter, Starr Simpson, endorsing the cooperative venture.

“The family is pleased to see the property remain in conservation use,” she said.

“We are fortunate that the strong oversight of the land bank and Sheriff’s Meadow will protect such a beautiful part of the outwash plain that forms the southern shore of Martha’s Vineyard.”

Quansoo Farm is the second largest land bequest ever received by the foundation. Mrs. Harris, who was the sister of the late Polly Hill, died in June 2005.

The Quansoo property includes a rare coastal sandplain which fronts Black Pond on the western side of the Tisbury Great Pond, overlooking the Atlantic.

Among the rare plants harbored on the land are wood lilies, ragged-fringed orchids, goats rue and sandplain flax. Animal species of note include short-eared owls and northern harriers, and large numbers of seasonal tree swallows.

There is also a house, dating to the 17th century, one of the few left of original wattle and daub construction.