ONE OF THE more extraordinary elements of "Geniuses," the play currently at Arena Stage, is the set and scenic effects, including a rainstorm that comes pouring down all four sides of the arena into a moat. (Indeed, some reviewers have called the set the best thing about the production.)

Tony Straiges is the man who designed this set, as well as the one for the next production in the Kreeger theater, "Buried Child," which opens April 20. "Geniuses" is the 11th set he has designed for Arena, and this year he has been given the title of resident artistic associate, thanks in part to a grant from the National Endowment for the Arts.

He designed the moving hedges for this year's "On the Razzle," the spare, existential void of "A Lesson From Aloes" and the military barracks for "Streamers," among others. Although he was an admiring student of Ming Cho Lee at Yale drama school, Straiges is not an idiosyncratic designer himself; his sets do not have a recognizable personal stamp, but rather he bends his choices to suit the play.

"When I first read a play I try not to design anything in my head, but rather, react to it. I think about what the playwright is trying to say. Then I read it again and underline all the stage directions and things that are necessary to the action, like steps. Then I do research."

In the case of "Geniuses," which is set in a Philippine jungle, the research included studying palm trees in National Geographic magazines and at the Botanical Gardens, and watching the current film "The Year of Living Dangerously," which was filmed in the Philippines.

"We had to get the right kind of palm tree," he said. "Although I think accuracy is important, the feel is more important. We want to get the feel of a place." Last year he researched vegetation in Brazil for a play at Baltimore's Center Stage called "Savages."

The "feel" he wanted for "Geniuses" was a "playful, King Kong, South Pacific quality. The director wanted the feeling of a Western civilization man trapped in this room in the jungle with their electric pencil sharpeners and hair dryers . . . There was also a feeling of 'M*A*S*H' that we wanted."

The rainstorm was accomplished by Arena's technical staff, which--having built a snow-covered mountain last year--apparently finds no challenge too great. They had to break into the theater's main line, then devised a system of concealed pipes with holes in them.

Straiges left Minersville, a coal-mining town in Pennsylvania, when he was 17 to work in Washington. "There was nothing for kids to do there after high school," he said. "I knew there was a big world out there."

He got a job at the Library of Congress photographing microfilm. A veteran of scenery painting for parochial school operettas in his home town, he gravitated toward the American Light Opera Company and offered his services. "At that time I didn't know there was such a thing as scene design, let alone that you could go to school to study it." At that time there was a scene design shop on East Capitol Street, and the proprietor told Straiges about schools like Carnegie Tech and Yale.

Meanwhile, in 1966 he and a few friends started the now-defunct American Puppet Theater. From an ad hoc workshop in his apartment, it grew to a successful company that spent a summer season in residence at Georgetown University's theater. Everyone in the group was between 17 and 20, mostly federal employes bored with their day jobs, who learned from books they found in the Library of Congress how to make puppets. It disbanded, Straiges said, when it became too much like a business.

He had decided to go to school, and went briefly to Carnegie before quitting and moving to San Francisco for an apprenticeship at the opera there. A Ford Foundation grant enabled him to go to Yale, and he stayed there more than five years.

"That was the best time of my life. All we did was theater." Although he was enrolled in a five-year program that would allow him to earn a master's degree without a bachelor's, he never did get the degree because he failed to turn in some light plots.

Now he works out of a studio in his Brooklyn Heights, N.Y., apartment, free-lancing for other resident theaters as well as Arena. His next project is a new off-Broadway musical by James Lapine and Stephen Sondheim based on a painting, "Sunday Afternoon on the Island of La Grande Jatte," by Seurat. The set requires a giant reproduction of the painting, which comes to life.