“Tree of Knowledge” is an exhibition of rare works centered around Hilma af Klint’s series on paper, which bears the same name. Open at David Zwirner’s 34 East 69th Street gallery, the watercolors (dating between 1913 and 1915) were recently found to be one of Klint’s few bodies of works residing outside of the collection of the Hilma af Klint Foundation. Made following a four-year hiatus in the artist’s career, each of the eight works features a tree with a heart-shaped crown, reflecting Klint’s spiritual philosophies.