Equisetum

Living Fossil Along the Connecticut River

The flora of the riverside contains species that trace their lineages back to the Carboniferous or the Age of Amphibians, over 300 million years ago. These plants, known as horsetails or scouring rushes, are members of the genus Equisetum. This genus consists of 25 species and is the only living representative of what was once a large and significant component of the coal age or Carboniferous swamp forests.

Both the living Equisetum species and the extinct coal age fossils are classified as members of the plant division Sphenophyta. Although contemporary Equisetum species are herbaceous, fossil relatives (genus Calamites) were medium size trees (20 meters or ca. 65 feet high).

A 300 MILLION-YEAR-OLD STEM CAST OF Calamites. THE ARROWS INDICATE THE NODES.

Equisetum hymale is a common horsetail in the Connecticut River flood plain, where large clones occur along the banks.

The individual stems are 2-3 feet in height and perhaps a third of an inch in diameter. The stem is green, and jointed, somewhat like a bamboo. The nodes (arrows mark the nodes) have a ring of very small leaves (without chlorophyll) and the internodes are leafless.

Small cones occur at the top of the stems and the plant reproduces sexually via single-celled spores rather than seeds.

Along the river, large populations of E. hymale often occur. These consist of thousands of green stems and may cover large areas. These populations are actually single individuals that have multiplied vegetatively from undergound rhizomes (stems) to form genetically homogenous clones. Such clones are very old, having been in existence for hundreds or even thousands of years. As the river bank erodes, pieces of the clones are broken free and moved to new locations where they may establish sister clonal populations. Thus, in contrast to animals (for example, humans) with their fixed life spans, Equisetum clones have almost limitless life spans. They are potentially IMMORTAL. Death of a clone is usually a consequence of environmental change.

As you canoe or boat on the Connecticut River, look along the bank for this interesting plant. Ponder why this unprepossessing organism is immortal. Think what it has witnessed during its plodding, dogged, clonal persistence; year after year, century after century. It will certainly be around when you are not.

In areas of the Connecticut River where the banks are eroding, you can often find another species of horsetail, Equisetum arvense. This species is easy to recognize: it has the same jointed stems as E. hymale, but the plant is smaller, being about a foot in height. The key distinguishing feature is the presence of a whorl of branches originating at each node.

E. arvense forms two types of stems: green stems with whorls of branches (A) and short-lived nongreen (whitish gray) stems in the spring that form the cones (B).

References:

de Kroon, H. and J. van Groenendael (eds.) 1997. The Ecology and Evolution of Clonal Plants.

Backhuys Publishers, Leiden.

Stewart, W. N. 1983. Paleobotany and the Evolution of Plants. Cambridge University

Press, Cambridge.

Gifford, E. M. and A. S. Foster. 1989. Morphology and Evolution of Vascualr Plants

3rd edition. W. H. Freeman and Company, New York.

Bold, H. C. 1973. Morphology of Plants. 3rd edition. Harper & Row, Publishers, New York.