Plant of the Week Liverworts
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Plant of the Week LLiivveerrwwoorrttss It is hard to believe that a group of plants that includes so many beautiful species, has been saddled with such an awful name. The name liverwort comes from the Anglo- Saxon word “lifer” (liver) and “wyrt” (a plant), the inference being that plants that look like organs of the human body might 1 Telaranea centipes – a leafy provide medicinal benefits for that organ . liverwort Photo: Ron Oldfield Liverworts are close relatives of mosses. Traditionally, they were considered to be one class (Hepaticae) of the division Bryophyta of the Plant Kingdom. The other two classes were the mosses (musci) and the hornworts (Anthocerotae). Using modern molecular techniques, botanists have now elevated each of these classes to divisions of the plant kingdom, so now only mosses belong to the Division Bryophyta. Liverworts are placed in the Division Marchantiophyta and hornworts in the Division Anthocerophyta. All three divisions are now collectively referred to as “Embryophytes”, that is, land plants that do not have a vascular system. Embryophytes are believed to have evolved from a family of freshwater algae, the Charophyceae. In 1994, it was proposed that the three lineages of bryophytes, formed a gradient leading to the vascular plants. The most recent hypothesis is that hornworts share a common ancestor with vascular plants and liverworts are a sister lineage to all other extant embryophytes. Mosses bridge the gap between liverworts 2 and hornworts . Liverworts Mosses Hornworts Tracheophyte s Charophyceae Lunularia cruciata – a thallose liverwort Photo: Ron Oldfield There are two readily identifiable groups of liverworts: thallose liverworts appear to be relatively simple structures, and form flattened green plates that are not differentiated into stems and leaves. Leafy liverworts have clearly discernible stems and leaves. They are often identified as mosses. However, mosses usually have leaves spirally arranged around the stem whereas leafy liverworts usually have three rows of leaves, two flattened rows, one on each side of the stem, and a third row of underleaves growing, as the name suggests, on Zoopsis – a minute leafy liverwort the under surface of the stem. viewed through a powerful hand lens Photo: Ron Oldfield Zoopsis – viewed through a microscope. Photo: Ron Oldfield Liverworts produce spores together with helically coiled structures called elaters. When spores mature, the capsules burst open; the elaters twist hygroscopically and like tightly coiled springs suddenly released, explosively eject the spores. Spores and elaters. Photo: Ron Oldfield 1Livija Kent: http://www.bio.umass.edu/biology/conn.river/liverwts.html 2Vanderpoorten, A & Goffinet, B. 2009. Introduction to Bryophytes. Cambridge University Press. Alison Downing, Ron Oldfield and Kevin Downing Department of Biological Sciences, 28 May 2012 .