There is something didactic about a public museum, particularly one with as broad of a scope as the National Gallery of Art: The artists and styles on display function as a sweeping art history survey, cuing patrons in to what movements or talents merit their attention. But every piece selected to hang on those walls excludes another piece, which necessarily downplays or even omits the importance of certain eras of art.

Two new exhibitions at the National Gallery work to fill in the gaps, displaying underappreciated works that were formerly squirreled away in private collections. The first is a collection of 120 rare German drawings spanning several centuries. The second sheds light on the early years of American modernism.

Wolfgang Ratjen was born to a wealthy Berlin banking family in the middle of the 20th century, but he decided to forgo the family business and make a career as an art collector. Over the subsequent decades, he built an impressive collection of German drawings that spans three centuries and encompass numerous styles, including baroque, rococo and romanticism.

Andrew Robison, senior curator for prints and drawings for the National Gallery, said that the German art produced between the end of the Renaissance and the advent of expressionism received scant notice outside of German-speaking countries. Ratjen’s collection, then, offers a rare glimpse into an often-neglected body of work.

“I dare say there’s never been a collection of this quality of this period,” Robison said, adding that the collection is “one of the very greatest acquisitions the National Gallery has ever made.”

While the subject matter evolves through the years, the work is unified by an emphasis on precise depictions rendered with deliberate strokes of pen, pencil and brush. Earlier works deal largely with scenes from the Bible or Greek mythology, but pictures alluding to a specific historical event — for example, a drawing of an Indian rhinoceros which gained fame and the temporary nickname “Miss Clara” as she toured Germany during the 18th century — enliven the exhibit.

Landscapes were a favorite subject for German romanticists, a predilection that is reflected in much of the later work in the exhibition. Many of the early 18th- and late 19th-century drawings are outdoor scenes: churning seas, luminous trees and rearing mountains that contemplate the artist’s encounter with sublimity. Others reproduce with uncanny realism the minutiae of the natural world, including a series of pencil studies of leaves that capture every wrinkle, ridge and shadow. Similarly, the portraits scattered throughout are astonishingly lifelike.

The final room of the exhibition contains 60 of the National Gallery’s holdings that supplement Ratjen’s collection, bolstering certain styles or motifs that are introduced earlier in the exhibition.

“This is two celebrations combined,” Robison said. “One is an initiation, and one is a sort of culmination.”

The second exhibit, drawn from the private collection of Deborah and Ed Shein, jumps forward in time to focus on the incipient years of American modernism. The burgeoning movement had two loci. One was organized around American artists living in Paris; the other coalesced around photographer Alfred Stieglitz and his 291 gallery in New York.

As with the artists showcased in Ratjen’s collection, pioneers of American modernism are often eclipsed by their predecessors — luminaries such as Pablo Picasso, Henri Matisse and Marcel Duchamp — or by the later work of abstract expressionists such as Jackson Pollock, according to Charles Brock, associate curator of American and British paintings. Still, Brock said, their work, which came to incorporate new styles such as color synchronism and dada, reflected a transition from a Victorian mindset to a more modern culture.

Several of the earlier pieces in the Shein collection are clearly in conversation with the work being done by European modernists. Max Weber befriended Picasso while living in Paris, and when he returned in 1909 he brought with him the first Picasso painting to enter America. He also painted “The Fisherman,” a piece in the Shein collection whose use of intersecting planes and pastiches of bird and fish images pays clear homage to Picasso’s groundbreaking work in defining cubism.

Other pieces, such as Arthur B. Davies’ “Mountaineers,” Marsden Hartley’s “Pre-War Pageant” and Stanton MacDonald-Wright’s “Still-Life Synchromy” plumb the possibilities of abstraction with gorgeous patterns of shifting, vibrant colors. Others are more grounded in representations of early 20th-century life: Works by John Storrs and Charles Demuth seek the aesthetic principles governing industrial America, focusing on the contours of an increasingly mechanized world.

Some of the artists represented, such as Georgia O’Keeffe and Duchamp (who relocated to America after declaring Europe to be “finished”), won acclaim in their lifetimes. Others were not so fortunate — Patrick Henry Bruce, whose “Painting (Still Life)” explores cubism with a series of interlocking shapes, was forced to destroy much of his work and ultimately committed suicide during the Great Depression.

“A lot of these artists were really risking their livelihood by embarking on this very experimental work,” Brock said.

“German Master Drawings From the Wolfgang Ratjen Collection, 1580-1900” is on display May 16 to Nov. 18; “American Modernism: The Shein Collection” is on display May 16 to Jan. 2.