Given Milton Avery’s fondness for broad arabesques and liberated colors, it’s no surprise that he felt completely at home with the medium of watercolor. Starting in his 40s, Avery (1885–1965) turned regularly to the medium when his oils were unavailable or inconvenient, and in his later years often combined it with opaque media such as crayon and pastel. Knoedler’s selection, limited to the artist’s pure watercolors, spans four decades with more than 30 works. These include many from the private collection of the artist’s family that have never before been exhibited.

The watercolors’ subjects are the still lifes, interiors, and landscapes familiar from his oil paintings, and they reflect the same trend toward increasing abstraction. With its elementary composition of gamboling shapes and colors, “Little House by Purple Sea” (1958), produced when the artist was in his 70s, is classic Avery. At once fanciful and resolute, it locates the essential aspects of a scene — a house perched on a shore, spreading water, a crowding background — with an elementary scheme of purples, blues, and greens. With the even more reductive “Edge of the Lake” (1953), Avery shows off more of watercolor’s unique qualities with blended, layered, and dry-brushed strokes that build as bands of water, trees, and sky. Some of the later watercolors, animated by scratchy textures rather than a counterpoint of tones, seem more tentative in design, but even these resonate with the artist’s appealing blend of obtuseness and grace.

Most surprising are works from the ’30s. The very earliest cityscapes, naturalistic in color and modeling, appear to predate the artist’s conversion to Modernism. But by the time Avery produced the striking “Drawbridge” (c. 1930s), he had begun to simplify and flatten, while still observing dramas of tone and texture; its fluid mixing of light and dark blue-greens wonderfully evoke overcast sky and reflecting water. The artist, moreover, exploits the tensions of shapes, conveying the drop from the parting black ramps of the bridge to the paper-white of a tugboat far below.

Another remarkable work from this period, “On the Boardwalk” (c. 1930), orchestrates a crowd scene with humorous verve. A large umbrella shelters a foreground couple from bits of humanity all around: at left, the lumpy curls of a sunbather, viewed through a railing; at right, a high-heeled foot, the vestige of a pedestrian striding off the paper; above, in a sea of faces, the startling aspect of a man staring at us, his tiny bow-tie echoing the umbrella’s great sweep. In terms of wash technique, this watercolor is unambitious, even clumsy, but as a composition it brims with energy and insights.

Cornelia Foss’s landscapes, too, radiate an enthusiasm for the observed. While Avery restlessly plies the territory between nature and abstraction, Ms. Foss sides conspicuously with nature, evoking the particularities of light in large landscapes of seashores and fields.

With rapid but controlled brushstrokes, Ms. Foss delineates rounding dunes or a wave’s diagonal beneath a vast sky. In “Gray Day” (2007), the artist’s sure grasp of color shows in the subtle beige of the beach, which, infinitesimally varied in warmer and cooler tones, neatly captures the effect of lightabsorbent sand on an overcast day. A handful of quick marks of midtoned blue perfectly describes a wave’s white foam, shadowed by its own crest. As in most of these landscapes, however, the sky is the most substantial of all, with subtle but decisive shifts of colors imparting a complex depth.

Among a number of smaller landscapes, portraits, and flower paintings in the gallery’s smaller room, “Tulips” (2007) stands out for its fiery reds and brilliant pinks. Where these vivacious petals turn toward shadow, colors poignantly express their constrained glow.

Though working far more naturalistically than Avery, Ms. Foss doesn’t always lend as much pictorial gravity to her forms. The twisting diagonal of a seashore, and even the horizon itself, sometimes seem to be swept up in the overall bowl of space rather than measuring out its dimensions. A canvas such as “Gulls” (2007), however, beautifully establishes the proximate and the distant, the large and the small. Here, a plane of silvery water slips beneath the thick luminosity of sky, while slivers of deep blue and light beige hold the horizon. In front rises a flutter of small, varied darks — a flock of birds. Against the fullness of air and of water, their dense notes powerfully elicit the abundance of nature.

Avery until August 10 (19 E. 70th St., between Fifth and Madison avenues, 212-794-0550);

Foss until June 9 (210 Eleventh Ave., between 24th and 25th streets, 212-334-3400).