Contrary to the long-held belief that, internal to the cortical
sterome, the 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 features are: (a) the absence of large
central vacuoles; (b) a spindle-shaped nucleus positioned internally;
(c) a prominent axial system of endoplasmic
microtubules associated with the nucleus, mitochondria, pleomorphic vacuoles,
and membrane-bounded tubules
and vesicles; (d) a distinct cytoplasmic polarization, with the
cellular region near the capitulum being richer in
organelles than the basal region; and (e) a high frequency of
plasmodesmata in the cross walls with an enlarged
median region containing no discernible desmotubule. Such a distinctive
combination of cytological features has
been hitherto 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 cellular specialization in symplasmic transport.