A battered sailing cap shades the sun from Henry Marien's eyes, his snowy, baby-fine hair cascading past its brim to form a silken collar around his ancient work shirt.

Impervious to the beads of sweat gathering on his aged, handsome face, Marien manipulates a hoe between rows of butter beans and fresh parsley. He pauses briefly to admire a spunky, purple-flowered weed that has barged into his carefully cultivated garden, and gingerly hoes around it.

"It's beautiful, why pull it?" shrugs the 81-year-old gardener as he straightens up and pulls off his earth-stained gloves.

"In the evening of your life one of the most important things is physical activity," he says. "I work in my garden to keep from dying in my rocking chair."

A conservationist before ecology became fashionable, a humanitarian and intense student of life, Marien's speech is peppered with Latin phrases and literary allusions, and he quotes great thinkers with the familiarity of an old friend.

Although his tone and manner suggest a retired professor, Marien is a grade school dropout. A reitred printer who quit school to work 58 hours a week as an apprentice "printer's devil" at age 13, Marien educated himself in botany, philosophy, Latin, Russian, music, cooking and horticulture.

And recently, Marien learned about acting and film-making as the star of "The Whidjit Maker," the Gold Medal winner of the 1978 Washington Film Festival.

"The Whidjit Maker" is the story of an aging violin maker who, feeling the rush of time, tries to rediscover a past love. It is a mini-feature that uses little dialogue. Instead, Marien was asked to express complex emotions with his agile frame, his weathered face and his penetrating eyes.

Local film-maker Polly Lewis Krieger discovered her star through a senior citizen's program of which Marien is a member. Although Marien admitted that he was "rather hesitant" to accept the role, he decided to try it because it would be good experience.

"I like anything that is a challenge," he said. "I took the invitation as a compliment, and even though I'd never acted before I said I'd give it a try.

Marien's family also encouraged his acting career. His wife of 43 years, Ida, a former employe of the Girl Scouts and of the Florence Crittenden Home, his daughter Rose Marien, a 36-year-old former New York schoolteacher, and hiis 40-year-old son Michael Marien, a New York social scientist, were all supportive of their family's "movie star."

"It was all very exciting especially going to Georgetown at 3 a.m. (for filming)," Marien said. Seeing the film was interesting too, because you see yourself almost as you do in a looking glass, but you see yout back and get angles of yourself you never see."

"During the shooting of "The Whidjit Maker," recalls Krieger. "Marien always brought a contribution from his garden to the crew's lunch table - a bag of fall lettuce so young the leaves were barely unfurled, a handful of radishes or an especially fine turnip he might persuade you to eat raw. He was curious about everything to do with the film-making process and asked keen questions, but he never got in the way."

Becoming a movie star is just one of the many extraordinary events of Marien's later years.

"The last quarter of my life has been the best," he says. "Most of the things I learned, I learned late in life and I've brought myself to a state now where I'm more at peace with myself than ever."

A conservationist who has assigned himself the task of taking a trash bag around the block to clean his Potomac Palisades neighborhood. Marien joined the Sierra Club last year and had his picture on the front page of The Washington Conservationist magazine.

His concern with western society's "wasteful pattern of life" prompted him to write a letter to a Mobile Oil executive which a friend suggested would make a perfect piece for the Washington Star's point of view column.

Shortly after Marien's piece was published came a telephone call from Admiral Hyman G. Rickover.

"Humble people like me just don't get phone calls of that caliber, so I was deeply moved. It was the finest compliment I ever got in my life."

Marien has also become an accomplished cook; his specialty is dandelion salad. He has taken up carpentry, refinishing an attic room, and has studied piano.

"But I don't give myself credit for anything or blame for anything," he is quick to add, "because I'm here by chance.

"I feel sorry for most elderly people, faced with so much tragedy - nursing homes, drugged with sophisticated medicines. Elderly people sit in rocking chairs too much when exercise and interests are so important.

"And like Lot's wife, they've got to be careful about looking back. The place to look is forward."