Phylum Sphenophyta: Horsetails
Only one herbaceous genus— Equisetum—of 15 species remains of this once large group of woody trees of Carboniferous Age forests. Equisetum is one of the easiest plants to recognize: It has jointed, ribbed and hollow stems impregnated with so much silica that a rasping noise is heard when stems are rubbed together. Another of its common names, “scouring rush”, indicates one of the early settlers' uses of the plants. At each stem node there is a ring of small leaves fused in a sheath. Some species additionally have a whorl of branches at each node, which gives rise to the “horsetail” common name. The aerial shoots arise from an extensive rhizome system. Equisetum sperm—like those of the rest of the ferns and fern allies—require an external film of water in which to reach the eggs; Equisetum is most often found in sites that are moist for at least part of the growing season. Sexual reproduction is not necessary to propagate horsetails, however, which often spread vegetatively by means of rhizomes.
Equisetum grows worldwide except for Australia, New Zealand, and Antarctica. Most species grow in the Northern Hemisphere between about 40° and 60° N latitude. The modern plants resemble their arborescent ancestors that grew 250 million years ago—which might make Equisetum the oldest living vascular plant genus and the one least changed over time. The horsetails have no commercial role and are of minor importance in natural ecological systems.
The life cycle of Equisetum is basically that of the ferns (and the psilotophytes and some lycophytes) with only morphological differences. Sporangia are clustered in cones (strobili) at the tips of ordinary-appearing vegetative shoots in some Equisetumspecies, while in others the sporophyte is a simple non-green, short-lived, unbranched shoot with terminal strobili. The sporangia hang in groups of five to ten from umbrella-like sporangiophores that compose the strobili. Each spore is wrapped in four thickened, hygroscopic bands called elaters (but different in structure from the moss elaters). As the elaters dry, they twist and turn giving buoyancy to the spores. The spores germinate, producing small, green, thalloid gametophytes, which anchor with rhizoids to a moist surface rich in nutrients. Multiflagellate sperm swim in water to the archegonium, one fertilizes the single egg at its base, the zygote forms an embryo in the archegonium and the young sporophyte is nourished by the gameto phyte to which it is attached until the organs are sufficiently developed to sustain the sporophyte independently.