Flower:

Elongating racemes at the tips of branching stems and arising from the leaf axils, with a small compact cluster of open flowers at the tip and fruit developing below it. Flowers are tiny, less than 1/8 inch (1.2 to 2 mm) long; petals are narrowly spatula-shaped, pale yellow to whitish, rudimentary, less than 1 mm long or more often absent altogether. In the center is a green, oblong-elliptic ovary topped with a short, stout style and surrounded by 4 stamens of equal size. The 4 sepals are pale yellow to green, paler along the edges, narrowly egg-shaped to somewhat boat-shaped, erect to ascending to spreading. Flower stalks are absent or very short, up to ~1 mm long at flowering time, and hairless.

Leaves and stems:

Leaves are basal and alternate, the lowest leaves largest, up to 4 inches (10 cm) long and ~1 inch (to 3 cm) wide, variable in shape, oblong to somewhat spatula-shaped in outline, often divided with 3 to 15 lobes, the terminal lobe largest, or sometimes barely lobed, the edges with large rounded teeth. Surfaces are hairless. Basal and lower leaves are stalked, the leaves becoming shorter stalked and less divided as they ascend the stem, the uppermost leaves often stalkless.

The base of the leaf stalk may or may not have a pair of small lobes (auricles). Stems are erect to spreading, usually branched, and hairless.

Fruit:

Flower stalks elongate some as fruit develops but rarely exceed 2 mm. Fruit is a cylindrical pod up to about 3/8 inch (6 to 10 mm) long, straight to slightly curved, bulging with 150 to 200+ seeds, held ascending to spreading.

Seeds are ~.5 mm long, yellow-brown, covered in minute pits.

Notes:

Stalkless Yellow-cress is a winter annual that blooms starting in late May into August, or a summer annual that blooms starting in late August into October. Its typical habitat is areas of seasonal flooding, usually emerging when water recedes. It is quite rare in Minnesota, where it reaches the northern tip of its range. According to the DNR, it was listed as a Special Concern species in 1996 but thought to be extirpated here since there was just a single report of it since the late 1800s. However, since 2012 it's been spotted in several locations along the Minnesota and Mississippi rivers. IMHO, it's a case of being in the right place at right time, plus being observant enough to pick it out from the other annuals that share the same habitat.

Overall, Stalkless Yellow-cress bears a resemblance to the common Bog Yellow-cress (Rorippa palustris), but in miniature. It does have some unique characteristics: flowers that commonly lack petals, fruiting stalks less than 2 mm long, and seeds are yellowish and covered in minute pits. Also similar is Bluntleaf Yellow-cress (Rorippa curvipes), which has darker brown seeds covered in bumps. Both R. palustris and R. curvipes have bright yellow-petaled flowers, usually have at least a few hairs on stems, and fruiting stalks are mostly more than 2 mm long. When not yet flowering, a clump of leaves could be mistaken for the invasive Narrow-leaf Bittercress (Cardamine impatiens), but its divided leaves look more compound with stalked leaflets.

A note about the seeds: when I first examined them under a microscope, the surface appeared to be covered in tiny bumps rather than pits, but there did seem to be depressions in some. Over time this became more pronounced so I've concluded the pitting is not necessarily immediately apparent. The yellowish color should be a good indicator, combined with the (nearly) petal-less flowers and short fruiting stalks.