The Equisetopsida emerged in the Upper Devonian (>370 mya), and Equisetum (horsetails) may be the oldest surviving vascular plant genus. Horsetails and the Poales are the only plants possessing the hemicellulose (1->3, 1->4)-b-d-glucan (MLG). Xue and Fry show that variation has occurred in MLG structure during horsetail diversification, and that E. bogotense (the earliest-diverging species) has MLG composed almost solely of an ancestral tetrasaccharide repeat-unit, G4G4G3G, whereas other species in both subgenera (Equisetum and Hippochaete) have additional di- and trisaccharide repeats. Quantitatively, xyloglucan is down-played in monilophyte species rich in MLG or mannans. They conclude that plants have ‘experimented’ extensively with hemicellulose structures and proportions during evolution.
