Flower:

Loose branching cluster of stalked flowers on the upper stem. Flowers are ¾ to 1 inch across, bright to sometimes a dullish yellow, with 5 spreading, broadly oval to wedge-shaped petals, widest above the middle and tips rounded to nearly straight across and sometimes a bit ruffled along the edge. The petal base is often streaked with orange or red. The central style is 5 parted near its tip, the 5 yellow stamens broadly spreading from the exposed center. The 5 sepals are narrowly lance-shaped or awned with glandular toothed edges, and less than half the length of the petals. Flower stalks are about ¼ to 1/3 inch at flowering, up to ½ inch in fruit, occasionally with 1 or 2 small, leaf-like bracts.

Leaves and stems:

Leaves are erect, alternate, narrowly lance-shaped, 2/3 to 1¼ inch long, less than 1/8 inch wide, stalkless, mostly toothless and hairless, becoming smaller as they ascend the stem. The lowest leaves wither away by flowering time. On the flowering branches leaves are small and bract-like, sometimes with a few glandular teeth or hairs around the edge. Stems are slender, typically single from the base, erect with multiple, ascending branches towards the top, round to obscurely angled in cross section, mostly smooth or with minute hairs towards base. 

Fruit:

Fruit is an egg shaped capsule, 1/8 to ¼ inch long, the sepals shedding as fruit matures. The capsule splits from the tip into 5 pairs of sharply tipped, wedge shaped sections, each with 1 or 2 brown seeds.

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. Stiffstem Flax flowers are much larger, ¾ to 1 inch across, its 5 petals are broadly oval to wedge shaped, broadest towards the tip, the central style and 5 spreading yellow stamens are openly exposed and there is often a red to orange eye in the center. Grooved Yellow Flax (Linum sulcatum) has smaller flowers, ½ inch or less, deep yellow with oval-oblong petals, the stamens and styles tightly clustered in the center. 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. There are 2 recognized varieties, though some older references list more, which are now generally considered separate species; var. rigidum is found in Minnesota and while some national distributions show var. similans also present in Minnesota there are no herbarium records under that name. Distinguishing characteristics between the vars are poorly documented and unknown to us.