Spinulum annotinum has erect, leafy stems arising at
intervals from horizontal (also
leafy) stems spreading along the surface, or beneath fallen
leaves, but not beneath the soil. The leaves spread from several
sides of the vertical stem, giving a round cross-section to the
branches (i.e. the branches not conspicuously flattened in appearance--see
the Diphasiastrum species for examples of flattened branches).
Spores are produced in sporangia located only in specialized "strobili"
or cone-like structures at the ends of the leafy vertical stems.
S. annotinum is separated from other species in this group
by having the strobilus sessile on the tip of branches (no narrow
elongated stalk between the leafy stem and the strobilus--see
Lycopodium clavatum for the alternative), and by the conspicuous
narrowings along the vertical stems, called annular (or annual)
The annular constrictions are the result of shorter leaves being
formed at the end of one year's growth and the beginning of the
next year's growth.
This species was long known as Lycopodium annotinum.