Horsetails, the genus Equisetum
Horsetails, the genus Equisetum, are a very easily recognized group of plants that are commonly found throughout the world. They represent a very small remnant, only a single genus with less that 100 species, of a group that in the Paleozoic was a much more conspicuous component of the earth’s flora. From 100 million years, ending 250 million years ago, this group was a dominant in terrestrial habitats world wide and the group is responsible for much of our coal and oil deposits. They are common in most of the United States, sometimes as a roadside weed or in wet habitats.
Taxonomy and Phylogeny
The genus traditionally had been put in a phylum all by itself but more recent treatments lump the horsetails in the fern phylum (Pterophyta), usually putting them in an order (Equisetales) distinct from other fern groups. When treated this way, the orders of the Pterophyta diverged in the late Paleozoic era. Whether you consider horsetails to be a phylum by themselves or an order within the Pterophyta, they still are very easy to recognize because of their distinct structure.
Structure
Horsetails have a very distinctive form–they have jointed stems with small and inconspicuous leaves that appear as scales at the base of each section of stem. The stems are hollow and ribbed. The successive sections of stem get shorter and shorter, reminiscent of a logarithmic scale and there is a claim (I haven’t been able to verify this) that the plant was an inspiration to John Napier to invent logarithms. Napier invented logarithms in the 16th century to make calculations easier (many biology students don’t believe that logarithms make calculations easier!!). The stems sometimes have whorled branches (in which case they look a bit like a horse’s tail) but sometimes don’t (in which case they really do not look at all like a horse’s tail!). Horsetails have extensive underground horizontal stems (rhizomes) off of which emerge roots and vertical above ground stems. The commonly seen plant is a sporophyte that produces spores in a terminal cone or strobilis. Some species are dimorphic, with vegetative stems that are green and photosynthetic but produce no spore-producing strobili, and fertile stems that are brown (no chlorophyll, no photosynthesis) but do produce strobili and thus are only involved in reproduction. Fossil members of the group had secondary growth and grew up to 18 m in height, with stems a half a meter across. These plants were significant components in the deposits that yield coal and oil.
Reproduction
Like all plants, Equisetum exhibits alternation of generations. They have a easily visible sporophyte and hard-to-find gametophyte. Spores are produced in ‘cones’, terminal portions of stems where certain cells undergo meiosis to produce a ‘tetrad’ of four spores. The spores have extensions of the cell wall called elaters that move in response to humidity changes and provide some motility to the spores (see video below). The s