Leaves and stems:

Leaves are of two types: fertile (sporophylls) bearing spore sacs, and sterile (trophophylls) lacking spores. Both are evergreen, dark green to yellow-green, mostly spreading to reflexed but ascending at the tip, somewhat shiny, rarely with a few teeth on the tip half, and with stomates (pores) on both surfaces, more sparsely so on the upper surface (1 to 25 per half leaf). The largest leaves are 5 to 8 mm (to 1/3 inch) long, linear-oblong, the sides parallel for most of their length and tapering to a pointed tip. The shortest leaves are around the annual bud, 3 to 6 mm long, narrowly triangular, broadest at the base.

Leaves may appear whorled or nearly so but are spirally arranged with about 8 leaves in a cycle, more or less evenly spaced and may appear as columns when viewed from the side of the stem (8-ranked), though not always strongly so. Leaves form a wavy outline of long and short leaves along the stem, the short leaves marking the annual constriction. Stems are usually erect, single or multiple from the base, unbranched or with forked branches, the branches erect.

Spores:

Spore sacs (sporangia) develop on most of the current year's growth, one sac attached to the base of each sporophyll on the upper stem and branches, turning yellow as they mature and light brown when dry, splitting open to release the spores in late summer into fall. Old, dried sporangia from previous years persist on the stem

Leaf-like propagules (gemmae) may also be produced on claw-shaped branchlets in 1 to 3 whorls around the tip of the current year's growth. Gemmae are flattened fan-shaped, 4 to 5 mm long with 3 main leaves, the central leaf oblong and the 2 lateral leaves more elliptic, 1 to 1.5 mm wide, and all 3 pointed at the tip.

Notes:

Rock Firmoss, known as Lycopodium porophilum in older references, is the rarest of the 4 Huperzia species known to be in Minnesota with only 11 records in the Bell Herbarium. According to the DNR, it has something of a scattered distribution throughout its range as well as in Minnesota; most sites known to still exist here are concentrated in the northeast and southeast corners of the state. While regionally it is typically found on sandstone or rarely on shale (both sedimentary rock), the populations in Cook and Lake counties have been on diabase cliffs (igneous rock). Due to its rarity, it was listed as a Threatened species in 1984 and is currently Special Concern in Wisconsin.

Rock Firmoss is distinguished by its rock habitat, longest leaves to 8 mm long and linear-oblong with parallel sides, shortest leaves narrowly triangular, stomates on both leaf surfaces (magnification required to see), and 1 to 3 whorls of gemmae branchlets around the annual constriction. It is most similar to Shining Firmoss (Huperzia lucidula), which shares the wavy outline of long and short leaves along the stem, but the longest leaves are up to 11 mm long, broadest above the middle and irregularly toothed, lack stomates on the upper leaf surface, has only a single whorl of gemmae branchlets around the annual constriction, and is rarely found on rocks.

H. porophila hybridizes with H. lucidula; hybrids may have intermediate characteristics and scattered stomates on the upper leaf surface, but have aborted spores. Of the other two Huperzia species, Appalachian Firmoss (Huperzia appressa, a.k.a. H. appalachiana) and Northern Firmoss (Huperzia selago) both have leaves all narrowly triangular, widest at or near the base, and stomates on the upper leaf surface are usually more numerous (30+ per half leaf). In other respects, H. appressa leaves are smaller than H. porophila (upper leaves only to 3.5 mm long) and H. selago leaves are hollow and slightly inflated, at least at the base, where H. porophila leaves are flat.