So you think you know Georgia O'Keeffe.

That's the starting point for both museum visitors and exhibit curators, in exploring 16 years early in the artist's career, when she spent summers that stretched into fall in the Adirondack Mountains about 180 miles north of New York City.

There's not a sun-bleached animal skull in sight among the 55 paintings that make up "Modern Nature: Georgia O'Keeffe and Lake George." The collection, covering her career from 1918 to the early 1930s, runs through May 11 at the de Young Museum in San Francisco.

True, there are paintings (from the Jack-in-the-Pulpit series) that display the sensuality that has been analyzed as sexuality since the 1920s. But just as clear, among bits of O'Keeffe's evocative writing posted throughout the exhibit, is the artist's response to the psychoanalyzing:

"You write about my flowers," she said, "as if I think and see what you think and see -- and I don't."

What we do see is an enthralling, wide-ranging examination of "modern nature," with colors both blazing and subtle, landscapes and close-ups, hyper-realistic and abstract works, images that are both simple and unsettling.

The exhibit confirms O'Keeffe as one of the pioneers of American modernism, directly in what might be called the avant-garde mainstream. Scholars -- and museum visitors -- will continue to analyze O'Keeffe's life and work, but she's bigger than her categories: female artist, wife of photographer Alfred Stieglitz, later icon of the Southwestern desert.

These paintings, along with other material on display and a superb catalog, underscore O'Keeffe's relationship with a wider artistic community. There are ideas in common with American artists Arthur Dove, Charles Sheeler and Charles Demuth, whom she knew. On display is a woodblock print by Russian-born colorist Wassily Kandinsky; O'Keeffe was reading his book "Concerning the Spiritual in Art" by 1915.

Some images are placid and dreamlike, such as the soft brushwork and pale blues and greens of "Lake George," from 1922. "Storm Clouds, Lake George," painted the following year, is more of an abstract nightmare in black, dark blue and gray with a slash of rusty red sky.

"Trees in Autumn" (1920-21) and "Autumn Leaves" (1924), bold and blazing with reds, browns and acid green, threaten to leap out of their frames. "Starlight Night, Lake George" (1922) is both calm and ominous, with splotchy stars reflected in the water -- an approach suggesting a van Gogh starry night.

Of course, there is a selection of O'Keeffe's celebrated flowers, fruit and plants as well, some in the intense close-ups (inspired by photography) that turn realism into abstraction. But it's not photorealism.

"Nothing is less real than realism," O'Keeffe said. "It is only by selection, by elimination, by emphasis, that we get at the real meaning of things."

How's this for real: In 1924, O'Keeffe began planting beds of purple and blue petunias at the Stieglitz family estate on Lake George, where she spent summers to study their subtle, radiant hues. As co-curator Erin Coe points out in the exhibit catalog, the growing of petunias that pivotal summer provided the impetus for her first enlarged flower painting.

"Petunias" (1925) is radiant, with its subtle gradations of purple and blue within the blossoms, and an intensity of color balanced by the velvety surface. Coe points out that these enlarged flowers were O'Keeffe's most original contribution to 20th-century art. Also on view is the glowing, jewel-like "Apple Family -- 2" (1920) and the Jack-in-the-Pulpit series from 1930 (one looking almost like an autopsy).

Making something of a circle, McCall's magazine reproduced "Petunias" in 1927 to illustrate an article on the use of blue and purple petunias for the home garden. O'Keeffe was not merely the "illustrator." Coe explains that her years at Lake George coincided with her first critical success as an artist. As even the caption writer for McCall's wrote, "Never before has her chosen art attained higher perfection than in the flower study of Blue Petunias."

O'Keeffe's years at Lake George -- though later overshadowed by her iconic presence in the Southwest -- were among the most prolific in her seven-decade career. She created about 200 paintings on canvas and paper, in addition to sketches and pastels. Her art from Lake George, Coe notes, reveals her "intense connection to nature, which is often cited as fundamental to her art."

This exhibit, although not meant to be an O'Keeffe career retrospective, nevertheless packs a remarkable range of styles and approaches into the years from 1918 to 1934, with one additional painting from 1959.

"Corn No. 2" (1924) seems to take on the multiple perspectives of cubism. "Cow Licking" from 1921 -- and you can't come up with a title more direct from nature than that -- might be a take on Picasso. "Lake George Barns" (1926) conveys a soulful abstraction that suggests the period after World War II.

Yet it's all O'Keeffe's world, as much as the cityscapes she painted from the window of her New York City hotel and the animal skulls and cloud patterns from her later years.

The exhibit at the de Young, organized by the O'Keeffe Museum in Santa Fe and the Hyde Collection in Glens Falls, N.Y., near Lake George, is rich in O'Keeffe's written narrative as well as paintings. Sometimes they coalesce, as in this letter she wrote to Stieglitz from Lake George in 1931:

"I have never seen a more beautiful night than last night -- a breathless kind of stillness -- the mountains a pale gray blue in very white moonlight. ... I walked down to the lake -- almost went out on it -- it was so still and so warm. ... Stood a long time looking at the perfect reflection of the mountain ... I walked about up in our woods -- to all the places I like. ... Then painted."

'Modern Nature: Georgia O'Keeffe and Lake George'
Through May 11
Where: De Young Museum, Golden Gate Park, San Francisco
Hours: 9:30 a.m.-5:15 p.m. Tuesday-Sunday
Admission: $16-$26 weekdays, $19-$29 weekends and holidays; 415-750-3600, http://deyoung.famsf.org