Flower:

Loose, elongating cluster of white flowers on slender stalks at the tip of the stem. Flowers are about ½ inch across, have 4 white petals, occasionally pinkish or lavender, broadest above the middle, rounded and sometimes slightly notched at the tip. In the center are 6 creamy white to pale yellow stamens and a pale green style.

The 4 sepals surrounding the base of the flower are 3 to 5 mm long, less than half as long as the petals, oblong to narrowly egg-shaped, green to yellowish.

Leaves and stems:

There are both basal and alternate stem leaves compound with 5 to 13 leaflets. Basal and lower stem leaves are up to ~3 inches (2 to 7+ cm) long, the leaflets round or kidney shaped, the tip leaflet being the largest, ~¾ (2 cm) long, often with shallow, scalloped teeth or lobes. Leaflet surfaces are hairless though edges may be sparsely hairy.

Upper stem leaves are widely spaced, shorter stalked than the basal leaves, the leaflets narrowly elliptic to linear with smooth edges. Leaf stalks are mostly smooth or with short, sparse hairs. Stems are unbranched, slender and erect but very weak, often flopping over unless supported by surrounding vegetation. 

Fruit:

Fruit is a slender pod, straight, erect to ascending, 1 to 1¾ inch (2.5 to 4.5 cm) long with a short beak at the tip.

Notes:

Cuckoo Flower is native to temperate regions of both North America, Europe and western Asia with old and new world populations separate as varieties, though not all authorities accept this separation. Some consider all populations introduced from Europe until further study is done, though it seems rather unlikely the Minnesota populations are anything but native and any revised taxonomy should eventually reflect that. Worldwide there are several varieties or subspecies mentioned in various references, at least 3 in Europe and 2 others in North America, though again they are not universally recognized.

What is considered the native North American var. palustris is restricted to cooler latitudes; in Minnesota it's found only in cold and wet, northern forested bogs, cedar swamps, and streams. According to the DNR, only 10 known populations exist in Minnesota, even after extensive biological surveys, and it was listed as a Threatened species in 2013. Its greatest risks are from land use changes, mining in particular, and forest management changes that alter the hydrology and canopy cover in its often shaded, wet habitat. In these habitats it is typically lax and spindly and is characterized by smaller basal leaves with smooth, toothless edges and by upper stem leaves that are stalked. In Europe, var. pratensis is a more robust, widely adapted species found in warmer and drier upland forests and meadows, is typically pink in color, with larger and toothed basal leaflets and upper stem leaves that are stalkless. This variety was introduced into North America, moved around by gardeners and has become naturalized and weedy in places like roadsides and lawns, from New England to Indiana.

Our Minnesota Cuckoo Flower is somewhat similar to our native Spring Cress, (Cardamine bulbosa) that can also be found in heavily shaded, wet woodlands where it to grows weak and spindly. However both its round to kidney shaped basal leaves and upper stem leaves are simple, not compound divided. Pennsylvania Bittercress (C. pensylvanica) does have divided leaves like Cuckoo Flower, but its flowers are much smaller, 1/8 to ¼ inch across, and is a much more robust plant.