Axillary hairs in diverse moss lineages

New observations and an overview of the axillary hair structure in the groups of mosses sister to peristomate ones are provided. Three moss lineages, Takakiopsida, Sphagnopsida, and Andreaeobryopsida are characterized by mucilage release through the apical pore or otherwise apical structures. Axillary hairs of Andreaea however, differ from these lineages and are similar to all other mosses, where mucilage exudes through small temporary breakings along the entire cell wall of its upper cells. Fine structure of axillary hairs is illustrated for Sphagnum, Takakia, Andreaeobryum, and partly for Andreaea and Oedipodium, and general characteristics are given for most orders. A unique arrangement of axillary hairs is found in Sphagnum, where they occasionally appear not only in the leaf axils but also along the leaf base abaxially. Another rare feature of Sphagnum is that its axillary hairs are penetrable for fungi through the apical pore, which was previously known in mosses only for Takakia. A highly specialized structure regulating mucilage release in Takakia is described. Actin filaments and especially tubulin microtubules were found to be outstandingly abundant in axillary hairs of Physcomitrium but not in paraphyses, as seen in its GFP-labelled plants, and this feature may be common in other mosses.