This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Perennial plants with evergreen 1-nerved leaves arranged in 4-16 ranks. Sporanges coriaceous, flattened, reniform, 1-celled, situated in the axils of ordinary leaves or in those of the upper modified, bract-like ones, which are imbricated in sessile or peduncled spikes, opening transversely into 2 valves, usually by a line around the margin. Spores all of one kind, copious, sulphur-yellow, readily inflammable from the abundant oil they contain. [Greek, meaning wolf's-foot, perhaps in allusion to the branching roots of some species.]
About 100 species of wide geographic distribution, the largest occurring in the Andes of South America and in the Himalayas. Type species: Lycopodium clavatum L.
Sporophyls not closely associated in terminal spikes.
Stems rigidly erect; leaves ascending, nearly uniform.
Stems ascending; leaves spreading or deflexed, longer or shorter in alternating zones.
Leaves distinctly broadest above the middle, there usually erose-denticulate.
Leaves linear or nearly so, entire or minutely denticulate.
Sporophyls closely associated in terminal spikes.
Sporophyls similar to the foliar leaves in form and texture; sporanges subglobose.
Sporophyls linear-deltoid, mostly entire; plants small.
Sporophyls linear to lanceolate from a broader base; plants larger.
Peduncles slender, the leaves incurved and mostly appressed; spikes slender, the sporo-
phyls less than 3" long, abruptly subulate, incurved.
Peduncles very scout, the leaves more numerous and close, mostly ascending, not incurved; spikes stout, the sporophyls more than 4" long, attenuate, ascending, spreading
Sporophyls bract-like, very unlike the foliar leaves; sporanges reniform.
Stems with' numerous erect or assurgent leafy aerial branches, the spikes terminal upon some of these.
Leaves of the ultimate aerial branches in 5 or more rows.
Main stem creeping deep in the ground; aerial branches few, tree-like.
Main stem prostrate, or (in no. 10) a little below the surface; aerial branches, numerous, not tree-like.
Leaves of the ultimate aerial branches in 5 rows.
Leaves of the ultimate aerial branches in more than 5 rows.
Spikes solitary, sessile.
Spikes one or several, on elongate peduncles.
Leaves of the ultimate aerial branches in 4 rows.
Spikes sessile upon leafy branches.
Spikes borne upon bracteate peduncles, these terminal upon leafy branches.
Leaves of the ultimate aerial branches adnate considerably more than half their length.
Ultimate aerial branches conspicuously flattened; leaves of the under row
greatly reduced, minute, deltoid-cuspidate.
Ultimate aerial branches narrower and less flattened; leaves of the under row
scarcely reduced, acicular.
Leaves of the ultimate aerial branches adnate about half their length or less.
Stems without leafy aerial branches, the elongate peduncles arising directly from the