Connor has put together this entry:
Today’s Botany Photo of the Day exhibits a member of one of the lesser known groups of the vascular plants, the monilophytes. This particular species, Equisetum pratense, is commonly called meadow horsetail and has, like many horsetails, a very wide distribution.
The monilophytes and the seed plants (gymnosperms and angiosperms), make up a group known as the euphyllophytes. In A classification for extant ferns (PDF), Smith et al. depict the euphyllophytes as being a sister group to the other major vascular plant group, the lycophytes. This means that an early dichotomy gave rise to the lycophytes on the one hand and the seed plants and ferns on the other.
Within the monilophytes aka the ferns, Smith et al. have described four classes, taxonomy above which is still unclear and has been avoided. Included in the four classes is Equisetopsida with a single order and family, Equisetales and Equisetaceae. The lone genus is Equisetum with 15 species.
Despite of having only one of almost everything, the equisetophytes are numerous in the fossil record. Spenophyta: Fossil Record describes these unfortunate organisms, some of which reached 30 metres in height and possessed secondary growth.
Unlike the seed plants, Equisetum is homosporous, meaning they produce only one type of spore. The seed plants (PDF) produce two types of spores which develop into two types of unisexual gametophytes, pollen and (what later become seeds) ovules. Compared to the life cycle (PDF) of a horsetail which includes a free-living, bisexual (sometimes just male), and photosynthetic gametophyte, it is clear that a trend to gametophytically reduce has been present (courtesy of BIOL 210!).