Flower:

Cluster of stalked flowers (raceme) at the top of the stem, elongating to about 4 inches in fruit, sometimes with 1 to a few lateral racemes from the base of the cluster. Open flowers are all grouped at the tip of the cluster with fruit developing  on the elongating stem below. Flowers are about 1/8 inch across, have 2 white petals each about 2 mm long, notched to about half their length, alternating with 2 whitish, egg-shaped, sepals about the same length that are spreading to reflexed and can make the flower appear 4-petaled. In the center are 2 white stamens slightly longer than the petals. Individual flower stalks are hairless while the cluster stalk (axis) is sparsely short-hairy.

Leaves and stems:

Leaves are thin, opposite with each pair at right angles to the pair below, ½ to 3 inches (1.5 to 7.5 cm) long and 1/3 to 2¼ inches (to 5.5 cm) wide, hairless, generally egg to heart-shaped with a pointed tip, and coarsely toothed around the edges, the teeth more or less perpendicular to the blade edge (dentate). The top-most pair of leaves is smallest and short stalked to nearly stalkless, those below have narrowly winged stalks up to 1½ inches (4 cm) long. Stems are single, erect and hairless below the flower clusters. Colonies cam form from underground, horizontal stems (rhizomes).

Fruit:

Fruit is a capsule 1.6 to 2.6 mm (less than 1/8 inch) long and to 1.2 mm wide, rounded at the tip and tapering to the stalk, containing a single seed. The surface lacks any ribs or grooves and is covered in hooked hairs that attach themselves to anything that passes by.

Notes:

Alpine Enchanter's Nightshade is a delicate little plant typically found in rich, cool, moist woods and swampy places, often growing on mossy rocks and logs. A personal observation: while not documented in any reference that I could find, all plants that I have come across (and other images, as well) have the second pair of leaves from the base the largest, the pair at the top quite small, and frequently have exactly 4 pairs of leaves. This may be due to the particular degree of maturity, but it is a curious thing nonetheless.

Related species Common Enchanter's Nightshade (Circaea canadensis a.k.a. C. lutetiana) is a much larger plant with open flowers spread out along the raceme rather than clustered at the top, leaves up to 5 inches long that are more finely toothed, mature capsules are broader and grooved/ribbed, and is common in more degraded woodlands. C. alpina is rarely taller than about 10 inches while its larger cousin is rarely less than a foot tall. There are several subspecies worldwide and 2 recognized in North America, with subsp. pacifica found in the western third of North America, and subsp. alpina a circumboreal subspecies found in Minnesota, the northern half of the US and throughout Canada. The two can hybridize, the hybrid (C. ×sterilis) having intermediate characteristics.