Horsetails are one of the most ancient lineages of plants and are relatively unchanged from the fossil record. If you look closely at the nodes of a green vegetative shoot, you will see that branches and leaves have not only switched roles, they have also switched places, with the photosynthetic branches emerging below the papery, non-photosynthetic leaves. The stems of horsetails are covered in silica, giving them the common name scouring rush, as they were formerly used to clean pots due to the abrasive nature of silicate granules. This is what gives the epidermis of the shoot its rough texture.
Gametophyte Morphology
Horsetail gametophytes are reduced and thalloid. Bisexual gametophytes grow from homospores and produce both antheridia and archegonia.
Figure \(\PageIndex{1}\): Whole mount of an Equisetum gametophyte. A indicates a long, thin rhizoid. B indicates one of several antheridia, each positioned on an extending lobe from the long, linear thallus. Scale=0.6mm. Jon Houseman, CC BY-SA, via Wikimedia Commons.Figure \(\PageIndex{2}\): An Equisetum gametophyte with an antheridium indicated by the red box. This partial view of the thallus shows lobes on the edge where antheridia appear as darker, round areas. Within these, many sperm cells are produced. Photo by Bruce Kirchoff, CC BY 2.0, via Wikimedia Commons.
Sporophyte Morphology
Vegetative Shoots
On the sporophyte, the leaves are dark, papery and non-photosynthetic. Branches are photosynthetic and produced in whorls on the vegetative shoot. Shoots contain silica, which has an abrasive texture.
Figure \(\PageIndex{3}\): A vegetative shoot of Equisetum. There are rings of long, thin, green branches emerging from each node. Above each whorl of branches, a ring of pointy brown leaves are pressed up against the main stem. Photo by Maria Morrow, CC-BY 4.0Figure \(\PageIndex{4}\): A close up of the vegetative shoot of Equisetum. In this image, you get a better view of the leaves. They start out as a united tube surrounding the main stem, then turn dry and brown, separating into individual, wispy points. The whorl of leaves looks a bit like a grassy crown encircling the stem. Photo by Maria Morrow, CC-BY 4.0
Reproductive Shoots
Sporangia are produced in a terminal strobilus on the reproductive shoot. In some species, this reproductive shoot lacks chlorophyll and is instead fed through the rhizome of connected vegetative shoots. Spores are photosynthetic and have four hygroscopic arms called elaters.
Figure \(\PageIndex{5}\): Two reproductive shoots of Equisetum. These shoots are a very pale yellow, lacking chlorophyll and unable to do photosynthesis. The leaves on the reproductive shoots still occur in whorls and are larger than on the vegetative shoots shown above. At the top of each shoot, there is an elongated structure that comes to a point and looks like it is covered in polka dots. These are the strobili. Each polka dot is the top of a sporangiophore, each of which bear many sporangia, perhaps visible to you as little white frills surrounding the polka dots. Photo by Maria Morrow, CC-BY 4.0Figure \(\PageIndex{6}\): These images show a preserved Equisetum strobilus. In the first image, we are looking straight at the tops of the sporangiophores, but some sporangia are visible underneath them. There is a section where you can see the cone axis (labeled A). The second image is a cross section through the strobilus, viewing the sporangia dangling from the sporangiophores. In both images, the labels represent the same structures: A) cone axis, B) a sporangium, C) sporangiophore. Photos by Curtis Clark, CC BY-SA 2.5, via Wikimedia Commons with labels added by Maria Morrow.Figure \(\PageIndex{7}\): These images show a micrograph of a strobilus longitudinal section. The labels are as follows: A) cone axis, B) sporangium, C) sporangiophore, D) spore. Note that the spores are all about the same size (homospores) and are wrapped in thread-like structures. These thread-like structures are the elaters. Photo by Maria Morrow, CC-BY 4.0Figure \(\PageIndex{7}\): Equisetum spores under the microscope. The spores are green and photosynthetic. Each spore has four long, transparent arms extending out from it. Some of these arms (elaters) are wrapped around the spore. The elaters react to moisture, curling in around the spore when wet and extending outward when dry, to aid in spore dispersal. Photo by Keisotyo, CC BY-SA, via Wikimedia Commons.
Video \(\PageIndex{1}\): This video shows how the elaters of Equisetum spores respond to changes in humidity. Retrieved from YouTube.
