Montgomery County planners are proposing to spend $100 million over the next decade to preserve vast stretches of undeveloped land and historic property, hoping to significantly expand conservation efforts at a time of rapid growth and prosperity.

The program, known as Legacy Open Space, would rely on a mix of public and private money to connect trail networks, protect stream beds and preserve environmentally and historically unique land from development.

The targeted parcels would be scattered across a broad swath from the Potomac River to Georgia Avenue in western and northern Montgomery, and include land near Travilah, Sandy Spring and along the Patuxent River. The purchases would require a dramatic increase in county funding for land purchases, an effort now financed almost entirely by the state government.

"I don't think that the small amount of county funds committed to land preservation really reflects the values of our community and our expectations," said William H. Hussmann, chairman of the Montgomery County Planning Board. "This request is warranted and justified given the rate at which critical resources will be lost if we don't."

The proposal comes as Montgomery and the rest of Washington's suburbs search for ways to contain new growth and rejuvenate urban neighborhoods. Last year, Fairfax County voters approved a $75 million bond program to buy land and create parks. In Maryland, Gov. Parris N. Glendening (D) has aggressively pursued a policy of using public funds to protect sensitive land from development, but counties have been reluctant to use their smaller treasuries for that purpose.

Montgomery County Executive Douglas M. Duncan (D) has signaled his interest in increasing money spent on land conservation. To date, the county's efforts have relied mostly on state largess. Montgomery receives about $3.5 million a year from the state to preserve undeveloped land, a source that would not be tapped to help fund the county's Legacy Open Space program.

The county has a 96,000-acre agricultural preserve, created through restrictive zoning rules and the purchase of development rights. It also has accumulated more than 50,000 acres of parkland and private open space.

Now county planners propose selling bonds to raise money for the new initiative, including $18 million worth in the next fiscal year. The financing strategy is intended to allay concerns that money would be diverted from public school construction or transportation projects.

"That, frankly, would be the only way this would be achievable at the level they are thinking about," said County Council member Steven A. Silverman (D-At Large), the panel's lead member for parks. "But we need to make a greater commitment to buying open space and preserving open space. I just want to know how financially doable this is."

Already county planners have assembled a list of more than 4,000 acres of "open space lands of exceptional significance" for possible purchase, according to a staff report that will be presented today to the planning board. Planners have placed the land in two categories -- the first to be purchased over the next two years, the rest after that. The property is scattered across the county, and much of it would be connected in what planners say would be a broad, circuitous band of green space.

In the next two years, planners propose spending $36 million in county, state and federal funds and private money raised from corporations and foundations on land purchases. The property selected for the first round includes four parcels that would join 100 miles worth of county trails, connecting the Potomac, Northwest Branch and Patuxent rivers in a crescent stretching across central Montgomery.

Planners also want to buy property in northern Montgomery that was part of the underground railroad -- used by African American slaves seeking their freedom -- and a site commemorating the county's Quaker heritage, which together would cost as much as $12 million. Planners also will look into buying closed school campuses with an eye toward turning them into parks.

Environmentally significant sites are among the most expensive on the list. Planners propose buying 258 acres near the Potomac known for rare rock formations and an additional 335 acres ripe for future residential development in western Montgomery. Also, the plan calls for spending as much as $3 million to buy 900 acres along the Patuxent River to protect the watershed from development.

Duncan said yesterday that he had not reviewed details of the proposal but that he wants to increase funding for preserving land within critical watersheds.

"We are leaders in the country in preserving open space and farmland," Duncan said, crediting past planning policies. "If we have dollars to spend, we need to spend it on protecting water that is in our streams and runs into the [Chesapeake] Bay."

Word of the idea has spread quickly through the environmental community, which has endorsed the concept wholeheartedly.

"We spend too much time fighting about development," said Ed McMahon, a senior official at the Conservation Fund in Arlington. "If the region had a blueprint for conservation, it would become much easier to facilitate development in the right places."