A few months ago, Dr. Chad Husby visited RBGE (the Royal Botanic Garden in Edinburgh). Chad is a botanical horticulturalist at Fairchild Tropical Botanic Garden, Coral Gables, Florida, with a particular interest in ferns and especially Equisetum. He was particularly interested to see the Equisetum collection at RBGE, and was able to identify an unusual horsetail that Gunnar Ovstebo had collected in California, that turned out to be Equisetum x ferrissii Clute, the hybrid between E. hyemale x E. laevigatum.
More recently, I have been reading Chad’s paper ‘Biology and Functional Ecology of Equisetum with Emphasis on the Giant Horsetails’, Botanical Review (2013) 79:147-177. It includes a discussion of the ancient history of the Equisetum and their now extinct relatives (Sphenopsida) which were the principle vegetation of terrestrial habitats during the Upper Devonian and Carboniferous periods, when the continents were very warm and wet. These plants became the primary components of the coal layers. A remarkable feature of modern horsetails is the extent to which they have retained characteristics of their ancient ancestors.
Something that surprised me was that, even though these ancient horsetails grew to enormous size – up to 20 m tall and 60 cm in diameter – they were not made primarily of lignin. I had always understood that the invention of lignin was one of the major factors that allowed plants to live on land by providing support and allowing for water transport. Incorporation of high concentrations of silicon within cells walls may have contributed to the mechanical strength of the stems, consistent with the high requirement of silicon among Equisetum. Quite possibly, the horsetails were eventually outcompeted by other plants that had acquired lignin.
Chad’s work has focussed on the giant horsetails in Mexico and South America (see photo of Chad with the E. giganteum Lluta Valley from the Atacama Desert in Chile), of which there are two species E. giganteum and E. myriochaetum (and their hybrid E. x schaffneri). E. giganteum reaches 5 m tall and has the broadest range, north to south and east to west, of any Equisetum in Latin America. These giant horsetails are mainly found at higher altitudes and in recently disturbed sites, apparently because they cannot compete well against a dense vegetation of angiosperms. Part of their secret to success is the massive rhizome system underground that can survive surface disturbance and can regenerate.
Well, there is a lot more fascinating information in the paper. I am hoping that Chad will visit again and have a chance to meet more BPS members!