Observations were revised for several diagnostic characters of Athyrium mesosorum, such as rhizome habit, scale, color and groove of leaf axes, marginal cells of frond, sorus and chromosome number, and these characters were compared with those of Athyrium and Diplazium. Among them following characters are important to elucidate the systematic position of it. The color of the leaf-axes changes from green to dark brown as the leaf develops, while that of Athyrium in general is reddish in the young stage. The transectional figure is of U shape with flat base but Athyrium has a V-shaped one (Figs. 1〜6). The marginal cells of frond are elliptic as compared with the elongated cells in most species of Athyrium (Figs. 7〜12). The sori are usually Asplenioid or Diplazioid, but extremely rarely Athyrioid; the latter form, on which base M_<AKINO> (1899) referred Asplenium mesosorum to Athyrium, seems to be aberrant. Chromosome number is n=41 in my materials (Fig. 13) although previously reported as n=40 typical for Athyrium. On the basis of these common characters with Diplazium, Athyrium mesosorum is considered to belong to Diplazium.
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