Cellular Sheets Research Articles

Evidence of non-vascular land plants from the early Silurian (Llandoverian) of Virginia, U.S.A.

Evidence is presented for the earliest known occurrences of non-vascular land plants and of higher, septate fungi. Macerates of carbonaceous silstone lenses from the lower Massanutten Sandstone, early Silurian (Llandoverian) of Virginia, have yielded a diverse assemblage of microfossil elements. Parallel aligned, banded tubes with annular to spiral ribbing and rounded to papilliform ends, membranous cellular sheets, cuticles, trilete spores, small spore tetrads, and septate higher filamentous fungi were recovered from the macerates. The banded tubes are probably a significant analogue with supportive or conductive cell types, but are not considered tracheidal. The heterogeneous plant assemblage may represent a thalloid, non-vascular land plant, in part, with a tubular-filamentous (nematophytic) organization associated with a membranous cellular layer and cuticular covering. While no spores were established as nematophytic, the presence of trilete spores adds to the indirect evidence of multiple evolutionary convergence toward land-plant characters. This assemblage is interpreted as of land-plant origin, based on the inferred fluvial depositional model of the fossiliferous rocks. A glacio-eustatic sea-level drop in the late Ordovician is suggested as a stimulus to the advent of land plants in the early Silurian.

Three-dimensional studies of neurons in the lateral geniculate nucleus of the rat: II. Environment of perikarya and proximal parts of their branches

Lateral geniculate neurons were investigated with respect to the environment of perikarya and proximal branches using two- and three-dimensional analysis by electron microscopy. Two cells were completely analysed and reconstructed with regard to specialized neuronal contacts. No branches without α-cytomembranes could be found projecting from the perikaryon. A descriptive nomenclature was adopted for the different types of processes and contacts observed. Perikarya appeared to contact far less neuronal knobs than proximal branches. The neuronal knobs were preferentially grouped with particularly high concentrations on spines. Quantitative observations established that no clear ultrastructural differentiation of neuronal knobs into special groups could be made on this material. Specialized neuronal contacts of type V (vesicle-related) all appeared similar in ultrastructure. On the average, 30 % of the contact surface of the neuronal knobs was specialized, often in several patches. Specialized neuronal contacts V were observed between V processes and perikarya or other V processes, as were spines on perikarya and proximal neuron branches. No spine apparatuses could be found. Specialized neuronal contacts of type S (with symmetrical membrane specialization) were observed between no-V processes and perikarya, V processes or other no-V processes. The perikarya were otherwise invested by extensive cellular sheets, that were interpreted to be glial. This investment covered at least 80 % of the surface of the perikaryon, the rest mainly involved specialized neuronal contacts. A minute extracellular space was estimated for the perikaryon, but relatively more for the branches. It was primarily occuping “seams” between meeting cellular processes at the surface.

THE RELATION OF EXTRANEOUS COATS TO THE ORGANIZATION AND PERMEABILITY OF CELLULAR MEMBRANES

Comparatively little attention has been given in recent years to the existence of extraneous coatings on cells of multicellular tissues. Many of the earlier workers, in their investigations on the action of the normally environing salts, came to appreciate the presence of an intercellular cement. However, aside from the fact that the cement serves to hold cells together, little is known regarding its mode of origin, its significance in determining the permeability of cellular sheets, or its possible role in the problem of cellular organization.