Most leafy liverworts have stems with 2 lateral rows of leaves; many species have a third row of reduced leaves on the underside of the stem. Liverwort sporophytes have a foot, embedded into the gametophyte tissues, and a spore capsule, normally raised above the gametophyte by a fragile stalk and persisting for only a day or so. Spores are released when the capsule wall ruptures, generally into 4 sections. Inside the capsules are hygroscopic cells (elaters), which help to disperse the spores. Many liverworts reproduce asexually by gemmae, small groups of cells produced on the thallus or leafy stem. Since they lack any conducting tissues (xylem, phloem), liverworts mainly absorb water directly through leaves, stems or thallus.
The 2 theories of liverwort evolution state that they developed from a primitive group of vascular plants (ie, those having true conducting tissues) or from some green algal ancestor. They belong to a group that lost much evolutionary potential by having the gametophyte generation dominant and by lacking conducting tissues, so that their size is greatly limited (most range from a few millimetres to 20 cm).
Liverworts grow mostly in moist, shady places on rocks, trees, rotten wood, humus or soil. In Canada, they are rarely abundant in ground vegetation except in the temperate rain forests of coastal BC, where they can dominate ground cover and densely clothe branches and tree trunks. Over 7000 species occur worldwide, about 85% having leafy gametophytes.
Author GUY R. BRASSARD
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