Roberto Burle Marx, the Brazilian landscape architect and painter, threw his white mane back and thought for a minute why he liked the big square in front of St. Mark's at Venice.

Which, as every pigeon knows, has not got a single tree in it.

"It's beautiful," he said. He likes to bring up points that challenge what he has just finished saying, and just before St. Mark's he had said that in a fine design for public space in the city, "the plant must be the chief actor."

Well, maybe he had a great predecessor who thought byzantine arches and pigeons should be.

Marx is sure of one thing, "I don't like recipes for designing gardens. You remember what Picasso once said: "'It's better to imitate others than ourselves.'"

Marx has won gold medals for his paintings at international biennales, but to the world he is best known as a designer of gardens and parks and grand spaces.

He arrived for lunch yesterday at the Brazilian ambassador's house, and afterward explored what he should send for a one-man touring show of North America in 1978, with Annemarie Pope of International Exhibitions, and Dr. Jose Neistein of the Brazilian-American Cultural Institute, Insurance for traveling shows is shocking, but there it is, and amid ritual moans the outlines of the show began to take shape.

Then there was a gaggle of reporters, some speaking English and some Portuguese, who did not actually ask him to design their gardens but who took a lot of time, and then there was a bit of time to get ready for his lecture last night on "Parks and Cities."

The Brazilia ambassador, Joro Baptista Pinheiro, stood up and said Brazil could not have a better ambassador (of good will in the arts) than Marx, and Mrs. Pinheiro, sitting across the table the next to the artist, beamed, as she also did when guests worked into an astonishing chocolate bombe filled up with frozen pears or (some said) bananas and covered with sauce.

Marx provided quite inadequate in summing up in a line or two, the bright essence of his work. He might have said, perhaps, that his gardens are famous because he is simply a more inspired designer, but he didn't, and he has more sense, if it was clear, than to say that if you follow his style you will come out all right.

"It's not just textures," he said, rumbling in the back of his mind to try to explain how he works," but rather linkage. That is, the textures with the colors with the light, plus the use the garden will get.

"Look, if I design a garden for blind people, it will be different from a garden for children, and different from one for the old.

"At first - I will give you an example of my mistakes - I used everything I knew. Now I am inclined to think the minimum is the maximum. But that's hard if you love the plants just for themselves, as I do. I am a gardener."

"He paused to let the great word sink in - in his moments of greatest pride and assurance, he calls himself a gardener, but otherwise "landscape architect" will do.

Sometimes his clients want a European garden - some replica of a thing in Gloucestershire or South Wales - "but I deny that." It does not look right for Brazil.

"Five thousand trees in Brazil, 5,000 kinds, and I think the search for exotics can be a mistakes. No. I have never tried to design a garden without seeing the land. We cannot think in standardized ways, but have to think what the landscape is like, and how the garden will change through the year, and from night to morning."

His brother, John Burle Marx of Philadelphia, is a composer of symphonic and chamber music, and a great foe of the cigarette, wearing a quite large button in his lapel asking people not to smoke, and of course this intimidates the average cloudmaker.

"Our mother," he said, knowing Roberto would never get around to the trile has helped them was more perinteresting parts, "was a pianist and a singer. Roberto used to sing all the way through Wagner when he was 7, and Mother said he would ruin his voice.

"'Would you like to study music?' she asked him, and he said he would, so he did. But like all of us he had very bad eyes. Our mother had the theory you should not tell the eye doctor you couldn't see, because if you did he'd give you strong lenses and you'd get more and more dependent on them. Well, Roberto could not see the notes printed on the staff, and when at the age of 19 he finally got the right glasses, it was too late.

"He had been advised to get outdoors more and work in the garden and not strain his eyes.

"We were born in Sao Paulo but lived most of our childhood in Rio de Janeiro, and our garden was very beautiful. Our father had a tannery, and all the scraps and skins and manure were dug in - you cannot believe how things grew.

"Once I saw Roberto bring home 200 cactus plants in a sack over his back. Now that is not easy to do."

"It's certainly not the way Toscanini got started.

"Somebody seeing his passion for plants and design said to my brother, 'Design my garden' and Roberto said 'Oh, I don't know now.' The man said just go ahead and do it, and everybody liked the results. Roberto is completely self-taught."

A friend, Conrad Hamerman, landscape architect of Philadelphia and friend of Roberto's as well as current biographer, said most people know the curvilinear romantic gardens of Roberto's past, but do not know the ones he does now, with sharp diagonals and high drama. "As he gets older," Hamerman said, "his designs get more violent."

Asked about this, someone seated at a safe distance learned:

"What can he mean by that?" Roberto Burle Marx said, wondering for a brief second if his friend had by some chance been making sense, but deciding, "Drama, yes, and I hope passion. Believe me, passion is an important part of love."

Hamerman said Marx is gloriously inefficient, "and could not possibly survive in Europe or North America, where a nursery, for example, moves all the stock out and then moves completely new stock in.

"He has a sort of botanical garden, and money is always a problem, because of course the things have to be maintained, and Marx is forever bringing new things in. I guess you know he lives in the suburbs of Rio de Janeiro in something quite similar to a Florentine Renaissance workshop?

"That is, he has a room with a cot, and there are workrooms, and five or six on his team who help with the work.

"Men - laborers - from the nursery drop in to talk, and people from town come out. Maybe they talk about poetry or some piece of legislation, or some plant, or who knows what else. It is all very like the Renaissance. It is not much like Frank Lloyd Wright, who also had his assistants around and eating together like Roberto, but there were all kinds of things his assistants were not supposed to have opinions about, and Wright was the great god. Roberto is not like that at all. I am afraid the whole operation is very inefficient."

"Is it how they built Chartres?" someone asked.

"Ah," said Roberto's friend. "I think maybe so."