Flower:

Flowers are rarely produced. When present, a single flower with a single style and a single stamen is in a cavity in the center of a leaf.

Leaves and stems:

Plants have 1 or 2 leaves (known as fronds), usually unequal in size, each oval to round in cross-section and in outline, rounded across the top, .5 to 1.4 mm long, 1 to 1.3 times as long as wide, with a pouch at the base from which young plants arise. Color is transparent green with 1 to 10 stomates (pores) across the surface, and a jewel-like sparkle below the surface. Plants have no stems and no roots and float on the water's surface.

Fruit:

Fruit is rarely produced. When present, a flower may produce a single smooth, globular seed.

Notes:

From the distribution map you can see that Wolffia is not well-documented in Minnesota, no doubt due at least in part to it's tiny size: it is considered the smallest flowering plant in the world and it's difficult to prepare herbarium specimens. The two Wolffia species in Minnesota are very similar but not that difficult to distinguish once you know the secret: Wolffia borealis leaves are elliptic to egg-shaped in outline, flat across the top, bright to dark green covered in tiny whitish stomates (pores), where Wolffia columbiana leaves are rounder, rounded across the top, and transparent green with only 1 to 10 stomates and a jewel-like sparkle under the surface. One observer thought W. borealis resembled a click beetle where W. columbiana was more like a jellybean. Maybe that helps (or not). Both species commonly grow together and with other duckweeds.