Isoetes echinospora Durieu (Isoetaceae)


Spring quillwort

Isoetes echinospora, which is growing in the Fern House of the Oxford Botanic Garden, is a quillwort that grows in the shallow water of nutrient-poor lakes. When viewed from above, these plants are star-shaped with grass-like leaves that emerge from a bulbous 'corm' embedded in the mud.

Isoetes echinospora is not a grass, nor indeed is it even a flowering plant like most of the other aquatics growing around it in its natural habitat. It is a lycophyte and its unit of dispersal is a spore not a seed. Other spore-producing plants include mosses, liverworts, hornworts, ferns and horsetails.

There are approximately 150 species of Isoetes living today. These few, diminutive species are the last relic of a once-dominant group that included the first ever trees. Some were over 30 metres high with complex crown and root systems. These trees formed the first extensive forests that spread around the globe during the Carboniferous Period between 300 and 350 million years ago.

These trees had extensive root systems. Their thick roots were modified stems and their fine roots were actually modified leaves. These rooting systems increased the weathering of silicate minerals from the soil, a chemical reaction that removes carbon dioxide from the atmosphere. This carbon was then washed into the sea where it is buried in limestone sediments.

The trees of these ancient forests also extracted large amounts of carbon dioxide from the atmosphere through photosynthesis. Some of this carbon was used to build the bodies of these Carboniferous giants while the rest was used for respiration and released back to the atmosphere as carbon dioxide.

When these trees died they fell over into the oxygen-poor waters of the swamp environment in which they grew. The bacteria and fungi that degrade carbon-rich bodies of dead plants need oxygen and cannot survive in these environments. Consequently, the dead trees did not rot, but instead their bodies (and carbon) remained relatively intact and were transformed into peat and coal over millions of years.

The combined activity of enhanced chemical weathering by these large root systems and plant burial by the first forests reduced the amount of atmospheric carbon dioxide. This caused a reverse green house effect which cooled the planet contributing to the onset of the Carboniferous ice age. While the plants in the Fern House are small today, their forebears had a major role sculpting the global climate in the past.

Further reading

Cleal CJ, Thomas BA 2009. Introduction to plant fossils. Cambridge University Press.

Freese B 2005. Coal: a human history. Arrow Books.

Liam Dolan