The leafy stems of Sphagnum (Bryophyta) contain highly differentiated polarized cells with axial arrays of endoplasmic microtubules

Contrary to the long-held belief that, internal to the cortical sterome, th e central region of Sphagnum stems comprises unspecialized parenchyma, the present light- and electron-microscope study has revealed that these cells in fact have a highly specialized cytoplasmic organization. Their key featu res are: (a) the absence of large central vacuoles; (b) a spindle-shaped nu cleus positioned internally; (c) a prominent axial system of endoplasmic mi crotubules associated with the nucleus, mitochondria, pleomorphic vacuoles, and membrane-bounded tubules and vesicles; (d) a distinct cytoplasmic pola rization, with the cellular region near the capitulum being richer in organ elles than the basal region; and (e) a high frequency of plasmodesmata in t he cross walls with an enlarged median region containing no discernible des motubule. Such a distinctive combination of cytological features has been h itherto only described for putative food-conducting cells in bryoid mosses. The results introduce a major new character common to Sphagnum and bryoid mosses and strongly suggest that this cytological organization underlines c ellular specialization in symplasmic transport.