Equisetum species (Equisetaceae)



Equisetum is an ancient genus of plants whose close relatives are fossilized in sediments up to 350 million years old. Today, this once diverse group of plants is restricted to fewer than twenty species. Special characteristics of horsetail shoots made their ancestors evolutionarily and ecologically successful. Today, these resilient shoot systems often give gardeners severe headaches as they try to eradicate horsetails from their gardens.

Horsetails produce massive systems of underground, branching shoots called rhizomes - the parts of the plant that you see above ground are only the tips of those branches - and it is virtually impossible to remove every piece of rhizome from infected soil.

While horsetail aerial shoots are generally less than one metre tall today - exceptionally they may be as much as three metres in height - they are dwarfs compared to some of their ancient relatives. During the Carboniferous Period, between 299 and 359 million years ago, close relatives of modern day horsetails formed a considerable proportion of the 'trees' in the world's first forests.

The giant horsetails, called the Calamitales, formed trees between 10 and 30 metres in height and represent one of the first evolutionary 'experiments' in being tall (or, to put it another way, 'being a tree'). A radical innovation in stem strengthening meant horsetails could to grow upwards without their stems buckling.

The ability of a plant or an animal to support its own weight is determined by the mechanical strength of its component tissues. Herbs are made of soft tissues and cannot support much weight - try to balance a pound coin on a dandelion head; the head will collapse. Wood on the other hand is mechanically stronger and can support the considerable weight of a large tree.

In giant horsetails another way evolved; impregnation of the stems with silica (silicon dioxide, SiO2) made them mechanically strong. Silica, chemically identical to the tough quartz mineral that makes granite rock so hard, reinforced the stems and allowed them to grow taller than plants with silica-less stems.

Structural reinforcement of stems with minerals also allowed another group of plants, bamboos, to grow upwards without collapsing under their own weight. The hard, flinty canes of bamboo canes are enriched in silica.

The bamboos of today are a reminder of the giant horsetails of the past and demonstrate how plants have co-opted the mechanical qualities of a tough rocky mineral during their evolution to hold them upright.

Further reading

Kenrick P, Davis PG 2004. Plant fossils. Natural History Museum.

Liam Dolan