The ultrastructure of phellem cells of Quercus suber L. (cork oak) and Calotropis procera (Ait) R. Br. were analyzed using electron transmission microscopy to determine the presence or absence of plasmodesmata (PD). Different types of Q. suber cork samples were studied: one year shoots; virgin cork (first periderm), reproduction cork (traumatic periderm), and wet cork. The channel structures of PD were found in all the samples crossing adjacent cell walls through the suberin layer of the secondary wall. Calotropis phellem also showed PD crossing the cell walls of adjacent cells but in fewer numbers compared to Q. suber. In one year stems of cork oak, it was possible to follow the physiologically active PD with ribosomic accumulation next to the aperture of the channel seen in the phellogen cells to the completely obstructed channels in the dead cells that characterize the phellem tissue.

This light- and electron-microscope study of four species of Sphagnum reveals that stem elongation involves meristematic activities unique to the group and hitherto unrecognized. The internal tissue of the mature stem arises by the concerted activity of an apical (primary) and a subapical (secondary) meristem. The primary meristem comprises the immediate derivatives of the single apical cell. Following a small number of divisions, the primary derivatives differentiate into highly vacuolate parenchymatous cells with a storied arrangement. Subsequently, the large vacuoles are replaced by numerous small vacuoles and the cells then divide repeatedly, by transverse septa, producing files of about nine short cells. Finally, ninefold elongation of these secondary cells is responsible for extension growth of the main stem below the mature capitulum. An early step in primary differentiation is the confinement of pre-existing plasmodesmata to distinct pitted areas. Further enlargement of the cells during primary and secondary differentiation involves the thickening of non-pitted wall areas, followed by expansion and thinning out, while the pitted areas remain virtually unchanged. A cortical array of microtubules is regularly found in association with non-pitted wall areas, while the unexpanded pitted areas are associated with smooth endoplasmic reticulum showing continuity with desmotubules. Though sharing much the same cytology as the conducting cells in bryoid mosses, in terms of their development the central stem cells in Sphagnum are not homologous with those of other mosses. The unique mode of stem development may be an important factor in the ecological success of Sphagnum.

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.

Plasmodesmata in photosynthetic eukaryotes are found in all embryophytes, in many members of the Chlorophyta, and in the Phaeophyceae. The Phaeophyceae and the Chlorophyta clearly developed cell walls and multicellularity independently, so that (in the absence of lateral gene transfer) plasmodesmata evolved independently in these groups. The minimum number of independent origins of plasmodesmata in the Chlorophyta based on molecular phylogenies is two (Chlorophyceae sensu lato, Charophyceae sensu lato). Other intercellular connections in members of the Chlorophyta (Ctenocladus, Smithsoniella, Volvox) are structurally very different from true plasmodesmata. Recently published taxonomies of the Chlorophyta have five classes (Chlorophyceae, Oedegoniophyceae, Trentepohliophyceae, Klebsormidiophyceae and Charophyceae sensu stricto) with plasmodesmata out of a total of thirteen. However, it is by no means clear that these classes all acquired plasmodesmata independently.