At the tree-ripened age of 49, Walter F. Mondale is becoming Vice President of the United States as one of the most prepared, qualified, self-controlled, and - to the public - little known men in recent political history.

It has long been assumed that his attitudes and ambition - not to say his liberalism - stemmed from Hubert Humphrey, his friend, tutor, mentor and patron for almost 30 years. That is not quite true. The attitudes, ambition, and certainly the liberalism predate the Hubert Humphrey of our times by a half-century. For they arise out of the time and place where Fritz Mondale was born and brought up.

The time and place was farm country of the Upper Midwest in the long Depression.

The Minnesota of legend - of lush farm belt stretching prosperously to the horizon - was bleached to nothing in the Midwest of the early 1920s. Instead the atmosphere was chill with realism, with apathy and resignation. "Pig-influenced country - all corn and pigs," says an older brother Clarence Mondale, now a professor of history at George Washington University. And Depression-influenced country. Behind the legend of happy times, there lay a layer of tragedy and near-despair. Walte Mondale's father was the victim of a series of such tragedies; they ruined his life and shaped Fritz Mondale's career.

The family had come from Norway in 1856, from the town that gave them their name, Mundal. (It was later "anglicized," so to speak, to Mondale - a fact which Fritz has said cost him tens of thousands of votes among the Scandinavians of Minnesota.) His father, Theodore Sigvaard Mondale was almost 52 years old when Fritz was born in 1928; he'd already raised one family; Fritz was the middle son of the second family.

Theodore Sigvaard Mondale had been a farmer, working a half-section of land near Walnut Grove (pop. 398) in 1909 when he had an experience which changed his life. "He was at the plow when he had a king of vision," as Clarence describes it. Lester, a half-brother who was an adult when Clarence and Fritz were born, remembers it as "a mystical experience, out in the fields one day."

A "born-again" experience akin to what Jimmy Carter experienced in the peanut fields of south Georgia? "A very central experience, a coming to one's senses," says Clarence Mondale. "A classic mystical experience," says Lester Mondale.

It moved him so deeply that - already in his middle 30s and with a family growing up - he decided to give up farming and go to the Black Hills of South Dakota to work among the silver and lead miners as a Methodist missionary. That was in 1911. "His first work in the ministry earned him $400 a year," recalls Lester Mondale. The work was not easy for him for he had a pronounced stutter. "He felt he was awkward verbally and physically," says Clarence. His gift was in the "caring" - in giving solace to the sick, the helpless. Bit by bit, despite his stutter, he rose within the church hierarchy, shifting from one small town in the Upper Midwest to a larger and then larger town until he was transferred to a pastorate in St. James, Minn., now a town of 4,027. It was the apex of his career.

In all these years, he sired three sons and adopted a daughter. (Her father had committed suicide by swallowing carbolic acid. "And Dad was the only one with him when he was buried" - a suggestion of the tragedy that was never far below the glossy layer of success in the hard years in the plains states.) The family never considered itself hard up. For the elder Mondale had given up farming but not the land. He'd rented out his farm and, when the demand for food erupted during World War I, he began acquiring enough money to speculate on land in a period of soaring prices. "He did a lot of praying over his investments," says Lester Mondale.

They needed prayer and something more. When European farms began producing again, the bubble of prosperity was soon to burst.

"When the depression came" - the farm depression, which preceded the Great Depression by nine or ten years - "the land all came back on my father," says Lester Mondale. He couldn't pay the mortgage or the interest charges. So he lost all the land and suddently he was poor.

Then illness befell his wife. Clarence describes it as a "progressive illness" which seemed to attack the central nervous system, paralyzing first her lower limbs and then her upper body. Lester remembers it as encephalitis. In either case, she soon needed nursing care for two shifts a day. Theodore Mondale took the third shift and when he could no longer pay for the nurses out of his minister's pay, he took over all three shifts - lifting her, carrying her, tending to all the intimate and necessary details of personal life. "it left my father permanently bent," says Lester Mondale. She died in 1923.

Then a dread disease hit Theodore Mondale himself: lockkaw.

"He was operated on for tonsilitis. On the kitchen table," says Lester. One of the instruments must have been infected. "When he came out of it, he couldn't open his jaws," says Lester.

The danger, of course, was starvation. "He lived on soup for a long while," says Lester. Then finally he was introduced to a device which would help: a wooden cone-shaped instrument which could be twisted between the teeth, like a screw, so that food might be fed through it into the mouth. The elder Mondale kept working and working with it, screwing it further into the mouth, until finally he could open his jaws and regain the use of them.