Leaves and stems:

Leaves may appear whorled or nearly so but are spirally arranged, 4 leaves in a cycle on branches and 6 on main stems, more or less evenly spaced and appear as 4 or 6 columns when viewed from the side of the stem (4 or 6-ranked). Leaves are tightly appressed to slightly ascending, less than ¼ inch long (2.5 to 4 mm), to .6 mm wide, toothless, hairless to finely hairy on the surface, prominently keeled, lance-linear tapering to a pointed tip with a white, spine-like bristle at the apex and sparse long, spreading hairs along the edges. The bristle tips may persist or fall off with age. Leaves are usually evergreen, sometimes turning bronzy or red-tinged.

Stems are horizontal, running above ground, irregularly forked, rooting at the forks and the roots developing on the upper side of the stem. At fairly regular intervals, erect shoots emerge that are 1 to 2 inches long with 1 to 3 forked branches. Plants typically form long or spreading mats.

Spores:

Spores develop in spike-like structures called strobili. Strobili are single at branch tips, ¼ to 1¼ inches long, stalkless, and at a glance appearing similar to the sterile branches but more squarish in cross-section. Each tiny spore sac is attached to a scale (sporophyll) that is similar to the leaves but 2 or 3 times as broad and may be more finely fringed around the edges. Scales are initially appressed, become more spreading to release the spores in late summer into fall. Two different kinds of spores are produced, though the spore sacs look alike and magnification is required to differentiate them.

Notes:

Rock Spikemoss is occasional to common in open, dry, sandy or rocky places throughout much of Minnesota and is a tiny, mat-forming plant easily overlooked, mistaken for a moss or miniature clubmoss. It is among the species once in its own family, Selaginellaceae, then lumped into Lycopodium (L. rupestris), then separated again. The Selaginellaceae family is distinguished from Lycopodiaceae by having 2 different kinds of spores and being much smaller. It has the single genus, Selaginella, and over 700 species world-wide. Attempts to break this down further are on-going. Good luck to them, I say.

S. rupestris has the widest distribution range of the entire genus and is one of only two Selaginella species known to be in Minnesota. While there can be a lot of variability in hairiness and sporophyll shape, this is likely the only Selaginella you'll encounter in Minnesota. The second species, Northern Spikemoss (Selaginella selaginoides) is extremely rare in the state, shoots are unbranched, leaves lack the bristle tips, and it's found in wet habitats or wet crevices on the rocky north shore of Lake Superior.