Adelaide Tyrol
Horsetails and scouring rushes are among the commonest plants around, especially noticeable early in spring when the evergreen species among this group stand out against an otherwise mostly brown background. All summer, too, you can find these primitive plants in road ditches, pond and stream edges, wet meadows, and damp woods.
Yet they seem to be widely ignored. They aren’t flowering plants, they aren’t ferns, not mosses, and not lichens. They aren’t in the index of any of the natural history books that now are lying in a large heap around me.
There are nine different species of horsetails and scouring rushes in our region – all in the genus Equisetum – and, for some reason, none of them can attract attention from anyone.
Three hundred million years ago it would not have been possible to ignore this line of plants. Their ancestors include the well-known Calamites, a 30-foot horsetail that was abundant in Carboniferous times and was topped by huge, spore-bearing cones. This species and other horsetails are important among the plants that formed coal, and coal mining has revealed much about their size and structure.
The images of horsetails that have been preserved in coal deposits suggest that the equisetums of today may be not be significantly different from those of the Carboniferous period, making this genus one of the oldest of all present-day vascular plants.
The most conspicuous feature of all of the equisetums is that the cylindrical stem is divided by joints or nodes, making it look like a miniature bamboo. In between the nodes are internodes. Sheaths surround each node and the very tiny – and you might say useless – leaves are fused into the sheaths. The green stems and branches do the photosynthesizing.
The stems of all equisetums are loaded with silica – the element that helps makes rocks hard. It may serve to stiffen the stems, compensating for the fact that equisetum cell walls contain very little lignin, the component that strengthens most plants, especially woody ones. Curiously, equisetums are not eaten by insects nor do they suffer from fungal diseases. Perhaps these agents have an aversion to silica.
Like ferns, equisetums produce spores, which germinate and form gametophytes: tiny independent plants – that in turn produce sperm and eggs, which then join to create the next generation of equisetums. All of them reproduce vegetatively as well, from underground stems called rhizomes. Equisetum rhizomes go to remarkable depths for water. These plants are not large, but their rhizomes and roots may extend 12 feet into the ground. If they are found growing on what appears to be a dry site, such as a road cut, chances are that their burrowing rhizomes and roots have plunged as deeply as needed to find water. This allows them to survive fire, drought, and plowing, and when equisetums are weeds, they are a force to be reckoned with. A single rhizome-root system may cover hundreds of square feet, with thousands of aboveground stems.
Although all the equisetum species are in the same genus, most of the ones called horsetails have many slender branches in whorls, not a lot like a horse’s tail, but enough so that you can see why somebody made the connection. The scouring rushes, on the other hand, mostly have single erect stems whose high silica content makes them suitably abrasive for scrubbing pots, burnishing brass, and finishing violins. Scouring rush produces a smoother surface than sandpaper when used as a final polish on wood.
Common scouring rush (Equisetum hyemale) frequents forested floodplains, roadsides, swampy woods, and wet fields. In summer its long, dark green stems are often well over two feet high, but by the following spring the taller specimens have been knocked over and the whole patch is in disarray. The sheaths that surround the nodes are ash gray. Common scouring rush has both fertile and sterile stems, the only difference being that the fertile ones are topped by a sharp-tipped cone, just over a half inch long. Spores are shed from the mature cone in the summer. Branches are rare.
If you hold a common scouring rush stem over your kitchen stove gas burner, you will see that it burns reluctantly, and that an intact skeleton of silica remains, even after you get the stem red hot.
Another common species, field horsetail (Equisetum arvense), has two very different kinds of stems. The fertile ones have the same branchless form as a scouring rush, but these stems appear very early in the spring, are tan, and do not photosynthesize. They are quite beautiful, but by May that have shed their spores and withered. At about that time, green sterile stems emerge. Whorls of delicate branches emerge from each node. The two-foot-high plants look like miniature trees and can remind us of the time when they were as tall as trees – three hundred million years ago.