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''Ptilidium'' is a genus of liverwort, and is the only genus in family Ptilidiaceae. It includes only three species: ''Ptilidium californicum'', ''Ptilidium ciliare'', and ''Ptilidium pulcherrimum''. The genus is distributed throughout the arctic and subarctic, with disjunct populations in New Zealand and Tierra del Fuego. Molecular analysis suggests that the genus has few close relatives and diverged from other leafy liverworts early in their evolution. ==Description== The name of the genus comes from the Greek word ''ptilidion'' for "small feather", in reference to the multiply deeply divided leaves with fringed edges, which give the plant a "feathery" appearance. Unlike other leafy liverworts, the underleaves are not significantly smaller than the lateral leaves. The "flossy" appearance from the leaf edges, together with the characteristic yellowish-brown or reddish-brown color make the genus easy to recognize. Like ''Ptilidium'', ''Blepharostoma'' and ''Trichocolea'' have deeply divided leaves with marginal cilia, however ''Ptilidium'' differs from these other two genera in that its leaf cells have bulging trigones (thickenings at the corners between cell walls). The plants grow in dense mats, with stems growing either prostrate or ascending. Individual stems are once or twice pinnate, rarely with branches and only a few short rhizoids. The leaves are incubous and divided deeply into three to five portions, and edges of the leaf divisions are fringed with cilia. The underleaves are similar to the lateral leaves, but are slightly smaller. All species are dioicous, producing antheridia and archegonia on separate plants. The archegonia are terminal on a main stem. Mature sporophytes develop from within a large perianth with three distal folds. The three species in the genus may be distinguished by the density of cilia along the leaf margin, depth of lobing in the leaf, width of the leaf base, and the substrate on which it is found growing. ''P. californicum'' has few cilia along the edge of its leaves and has underleaves lobed to about seven-eighths of their length. The other two species have many marginal cilia and underleaves lobed to no more than half their length. ''P. ciliare'' usually grows on soil and has leaf lobes that are 15–20 cells wide at their base. ''P. pulcherrimum'' usually grows on wood or rock, and has leaf lobes normally 6–10 cells wide at their base.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ptilidium」の詳細全文を読む スポンサード リンク
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