Another symbiotic associate with roots plays an important role in many forested ecosystems worldwide. It is the actinorhiza, an actino-mycete (filamentous, branching, gram-positive bacteria) that forms nodules and fixes dinitrogen in a fashion analogous to that used by rhizobia. A majority of actinorhizal species are pioneers on nitrogen-poor, open sites (Baker and Schwintzer, 1990). The dominant acti-norhizal genus is Frankia, occurring on roots of eight plant families, encompassing 24 genera and some 230 species of dicotyledons. Prominent actinorhizal plant families and genera include Betulaceae, on 47 Alnus species; Casuarinaceae, on 16 Casuarina species and 54 Alloca-suarina species; Myricaceae, on 28 Myrica species; and Rhamnaceae, on 31 Ceanothus species. These genera are widespread in ecosystems on all continents except Australia (Baker and Schwintzer, 1990).
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