Yesterday’s trees, today’s horsetails

The person who first called these primitive plants horsetails must have had an overactive imagination or none at all. There is only a faint resemblance between a horse’s tail and a phase in the life cycle of this plant.

The name has endured, however, as the Latin name for horsetails is Equisetum (meaning horse bristle). They are closely related to ferns and a small group called club mosses. About 350 million years ago during the Carboniferous period of geological history, they formed a significant part of the vegetation on Earth. Many of the ancient horsetails were large and treelike, and, with the ferns, formed most of the coal and petroleum deposits we have today. Horsetails are found on all continents with the exception of Australia. Only about 30 living species are known worldwide, whereas there are about 900 species of ferns with us today.

Horsetails were known to our immediate ancestors as scouring rushes. The stems contain abrasive silica and were used to clean dirt and grease from cooking implements. The stems could be burned and the ashes used as a polishing powder. They have also been called scrub grass, polishing grass, and gun bright. Woodworkers used them to finish their creations before fine sandpaper was invented. Other elements in the stem cause a mild explosion when burned, and Indian medicine men employed them to produce supernatural effects during tribal ceremonies.

Horsetails, along with their relatives the ferns and club mosses, reproduce asexually by spores rather than by seeds. Though there is a small, inconspicuous sexual generation, the spore-producing phases of the life cycle are the forms with which we are most familiar.

The life cycle of a horsetail is complicated by the fact there are two different phases of the spore producing generation: fertile and sterile generations. The sterile phase is the one that faintly resembles a horse’s tail. The “horse hairs” of this stage are green branches that manufacture food by photosynthesis, a function of the leaves of a higher plant. The side branches are located at regular intervals along the stem in a whorl-like arrangement. The primary function of this stage is to manufacture carbohydrate and store it in the rootstock.

These sterile horsetails appear in late spring and last until fall. In early April in our area, the fertile or spore-producing plant grows from an underground rhizome and pokes its head above the soil. Lacking the bushy branches of the sterile plant, the fertile or spore-producing plant bears no resemblance at all to a horse’s tail. Rather, it looks like a long, cone-tipped spear. The spear head produces hundreds of spores. Each spore is equipped with a set of tightly bound, wing-like appendages that uncoil when it is released. This evolutionary adaptation greatly facilitates their dispersal. The spore producer withers away shortly after the reproductive bodies are released.

Each spore grows into a tiny, inconspicuous sexually reproducing plant called a prothallium. Male and female sex cells are produced, unite and develop into the asexual generations that produce the spores. Thus, as in many plants and some primitive animals, a sexually reproducing generation alternates with an asexual generation in a process known to biologists as metagenesis (many origins).

Horsetails in the Rock River Valley grow in a variety of situations. Sometimes they are found thriving along railroad tracks and embankments, but more commonly they grow adjacent to streams where their roots and rhizomes are useful in preventing soil erosion. On rare occasions, they may become numerous in fields used for agriculture and are considered undesirable weeds.

When afield, keep a sharp lookout for these interesting plants in whatever phase of their life cycle you may find them. They represent one of the few living fossils that are direct links to our distant past. Most of the plants and animals that lived in past geological eras are no longer with us.

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

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