Flower:

Loose, upright branching cluster of stalked flowers on the upper stem. Flowers are 1/3 to ½ inch across, bright to pale yellow, with 5 spreading, oval to oblong petals. The 5 yellow stamens and 5-parted style are tightly clustered together in a tuft in the center. The 5 narrowly lance shaped sepals have 3 prominent winged veins on outer surface and sharp gland-tipped teeth around the edges. Flower stalks are about 1/8 inch or less with 1 or 2 small, leaf-like bracts.

Leaves and stems:

Leaves near the base are opposite, short and lance-elliptic, these withered by flowering time. Leaves above are alternate, stalkless, lance-linear, 2/3 to 1 inch long, less than 1/8 inch wide, 3-nerved, becoming smaller as they ascend the stem. On the flowering branches leaves are small and bract-like, often with a glandular teeth or hairs around the edge. At the base of the leaf is a pair of tiny brown to black glands. Stems are typically single from the base, erect with multiple, ascending branches towards the top, round in cross section but with prominent wings or angles. 

Fruit:

Fruit is a round to egg shaped capsule, about 1/8 inch long with an abruptly pointed tip, shorter than the persistent sepals, splitting from the tip into 5 pairs of sharply tipped, wedge shaped sections.

Seeds are oval-elliptic, shiny brown, about 2 mm long and 1 mm wide.

Notes:

There are two native yellow flax species in Minnesota, both inhabitants of dry, sandy prairie and can be encountered in close association but are easy to distinguish from each other. Grooved Yellow Flax has smaller flowers, ½ inch or less, deep yellow with oval-oblong petals, the stamens and styles tightly clustered in the center. Stiffstem Flax (Linum rigidum) flowers are much larger, ¾ to 1 inch across, its five petals are broadly oval to wedge shaped, broadest towards the tip, the central style and 5 spreading yellow stamens are openly exposed, and lacks the pair of glands at the leaf base. Like many dry prairie species, the yellow flaxes are best observed in the morning, both tending to shed their petals before the heat of the noon-day sun, after which time they can be very difficult to spot amongst the prairie grasses.