1. Equisetum L. (horsetail, scouring
(Hauke, 1963, 1979)
Plants perennial, homosporous, with rhizomes. Aerial stems erect or less
commonly ascending, branched or unbranched, green except for the fertile stems
of some species, hollow and with longitudinal canals in the tissue, jointed at
the nodes, with a ring of longitudinal ridges. Leaves whorled, reduced, fused
for part of their length to form a collarlike sheath around the stem, the lobes
appearing as small, tan to black teeth or scales along the tip of the sheath.
Sporangia saclike, in whorls on the underside of highly modified leaves
(sporangiophores), these peltate, hexagonal in surface view, and aggregated in
dense whorls into conelike strobili occurring at the stem or branch tips. Spores
35–70 mm in diameter, green,
globose, each with 4 spirally curled, white filaments (elaters) that uncurl
upon drying. Gametophytes green, growing at or near the soil surface, disk- to
cushion-shaped, irregularly lobed. Fifteen species, worldwide.
The strobili of Equisetum species are conelike, with dense, hexagonal
plates in surface view. At maturity, the internodes between the whorls of
sporangiophores elongate, allowing the spores to be shed. The green spores are
relatively short-lived and germinate quickly under proper conditions. The four
elaters associated with each spore spread when the spores dry and apparently
aid in spore dispersal. Vegetative reproduction of the plants is accomplished
by water, as portions of rhizomes are washed downstream during floods and later
form new roots and aerial stems.