This was a lecture writing day. At 9.00 this morning, I put a fresh cup of tea on my desk, pulled out relevant papers, Stewart and Rothwell’s paleobotany text, and Gifford and Foster’s morphology text, as well as my old notes to begin constructing tomorrow’s lecture on horsetails. I looked first at the old notes, and, tentatively, made a couple of Powerpoint slides, which was followed by indecision. I don’t like indecision when I start writing a lecture so I sat back and drank some tea. I began again. Horsetails—the genus Equisetum—fascinate me mostly because they were much more diverse in terms of numbers of species, varieties of forms, and importance in communities a couple of hundred million years ago than they are today. Horsetails have diminished from trees to spindly stalks. Something more than a dozen species remain, and they are common in moist environments—creek margins and roadside ditches—in much of the world. They just aren’t the trees they were 250 million years ago.
I began the lecture by placing Equisetum in the evolutionary tree of plant diversity, which is the way I set-up our exploration of each plant group discussed in the course. I wrote about the ecology, form, and reproduction of our extant horsetails. Then I looked back to ask about the earliest ancestors of this lineage that had horsetail-like traits. Those earliest relatives of the horsetail lineage are fossils known as Ibyka that date from the Devonian. Ibyka stood at a time when land plants consisted of little more than bifurcating stems, a time before leaves, when there was an evolutionary radiation of the simplest forms.
The fortunate early horsetails were well adapted for so-called coal swamps, the tropical wetland environment that may have been the most productive of its day. The coal swamp communities appear to have been relatively diverse, and, fortunately, it was an environment that fostered fossilization. From the coal swamps we have not only compression fossils that show shapes and surface details but also permineralized fossils in which three-dimensional forms and the internal structures have been retained, allowing intricate details of development and anatomy to be examined. Those coal swamps had tree-like relatives of Equisetum. I’ve seen some of the largest horsetails that are alive today—they are the species Equisetum giganteum. You can find them in northern Chile where the plain of the Atacama desert is broken by steep river valleys. The giant horsetails grow in the tangle of vegetation that holds tight to the riparian margin. Those giant horsetails are hardly thicker than a finger but are 12 to 15 feet tall. They seem slightly absurd—wobbly and too tall—and compared to the thick stumps of Carboniferous Calamites, the fossil tree-like horsetail of coal swamps, they seem to have taken a wrong turn.
Perhaps not, however. The huge arborescent Calamites are gone, now extinct for millions of years, but the thin, teetering contemporary horsetail remains. Indeed, while not rich in numbers of species, the extant horsetail seems to do well and can spread itself so well that it presents problems for farmers who work moist lowland fields where horsetails can be weedy.
I made Powerpoint slides over the course of the day. Typing text on slides and then scavenging the Internet and my slide collection for images that would help to demonstrate the ideas more fully. I took a break at lunch for salad and soup and read the food section of today’s New York Times, then returned to work on the lecture. The lecture, except for text on the last couple of slides, was finished by 5.00, when I wanted to walk home in the warm light and make a trip to the grocery store before dinner.
There was one image that I couldn’t fit into the lecture. I posted it above. In the photograph, there are tiny ball-like drops of water at the tips of the horsetail’s branches. The horsetail is guttating. Guttation is a water transport phenomenon in plants that happens usually on cool, moist days when evaporation from the plant surface is very limited. On those occasions water diffuses from the soil into the roots, where it enters the water conducting cells. Water pressure, on those cool, moist days, builds-up in the roots, forcing the water upward through the plant body—the effect of which is that water droplets are pushed outward through openings at the surfaces of leaves. Those emerging droplets are called guttation. Those moist, creek-side environments where we commonly find horsetails are good places for guttation.