Lyginopterid seed ferns in basal Upper Carboniferous marine black shale deposits of midcontinent North America

Permineralized plant remains occur individually and in small mats within anoxic and dysoxic marine black shales deposited seaward of the Wedington delta (Fayetteville Formation, Middle Chester) in northwestern Arkansas. Plant debris and associated cephalopods occur within 20 km of offshore bars that developed at the mouths of delta distributary channels. The bars were produced by longshore paleocurrents flowing eastward among the southwestern margin of the Ozark Dome. The flora includes several genera of arborescent lycopods, zygopteridalean ferns, filicalean ferns, calamites, medullosan seed ferns and seed fern remains with features that are characteristic of the Lyginopteridaceae. Among the lyginopteridalean stems are specimens that have been described as Megaloxylon wheelerae, an arborescent form, as well as several undescribed species.Two stems with prominent leaf scars represent one species of a new genus. The stems measure 35 cm and 34 cm long, and are approximately 2.5 cm in diameter. Leaf arrangement is helical, approaching a 3/8 phyllotaxis, with internodal lengths of about 3 cm. Stems are eustelic with a polygonal primary body 3-4 mm in diameter. Each leaf trace diverges from the cauline bundle as a single strand. The strand trifurcates in the cortex to produce a distinctive configuration of two c-shaped lateral bundles separated by a tangentially elongated central strand.Pith cells are large with prominent walls. Some show internal contents suggestive of secretory substances. The primary body is surrounded by a distinctive zone of wood 3-4 cm wide. Large rays 2-5 cells wide and more than 8 mm high separate wedges of tracheids that typically are 2-4 cells wide. Secondary tracheids display multiseriate oval bordered pits on the radial walls. Vascular cambium and secondary phloem are present, but incompletely preserved. The cortex is differentiated into an inner parenchymatous zone with horizontally elongated “sclerotic” nests, and an outer zone of alternating bands of vertically elongated sclerenchyma and parenchyma.This material significantly increases our knowledge of Namurian floras inhabiting non peat-forming environments, and emphasizes the systematic diversity among hydrasperman gymnosperms at the base of the Upper Carboniferous.

The morphology and relationships of Equisetum fluviatoides sp.nov. from the Paleocene Ravenscrag Formation of Saskatchewan, Canada

Vegetative and associated fertile remains of Equisetum have been recovered from early Tertiary sediments of the Ravenscrag Formation, Saskatchewan, Canada. The morphology of both reproductive and vegetative organs of this fossil species is remarkably similar to that of extant Equisetum fluviatile, the swamp horsetail. Aerial axes of the fossil are 3.0–19.0 mm in diameter, with internodal lengths of up to 30.0 mm. The stems are hollow and the central cavity is large. Branches, apparently simple, are borne in whorls at the nodes. Leaf collars at the nodes are up to 23.0 mm long, longer than broad, with leaves fused in the lower four-fifths of the collar. The leaf apices are long attenuate. Cones are up to 14.0 mm long, bearing peltate, six-sided sporangiophores in whorls of five. The fossil record suggests stasigenesis in the evolutionary history of some members of the genus Equisetum since the beginning of the Tertiary, and perhaps longer.

The nodes of Ranvier

At a node of Ranvier the axon is reduced in diameter; on the largest mammalian fibres the reduction is to less than half of the internodal diameter. The axoplasm at the node has optical properties different from the internodal axoplasm, but there is no visible transverse boundary across it. The length of the portion of axon left uncovered at the node is less than 0·5 μ in the largest mammalian fibres; it is somewhat greater on the smaller fibres. The area of the cylinder of axon membrane exposed at the node increases only slightly with increasing fibre diameter. This area is 4 μ 2 , and the area of the surface of the narrow portion of the axon is about 90 μ 2 , in the largest mammalian fibres. The exposed portion of the axon is probably not covered by Schwann cell protoplasm, but is surrounded by a 'cementing disk’ composed of scleroprotein material, continuous with the neurilemma (inner endoneurium). This disk has a radial thickness of 5 μ in the largest mammalian fibres. Outside this disk is a perinodal space with no stainable contents, bounded externally by the collagenous outer endoneurium (sheath of Key & Retzius). Methylene blue and silver nitrate entering the fibres at the node stain the whole narrow region, suggesting that this, rather than the myelin-free area, may constitute the effective nodal membrane. Periodic interruptions of the myelin occur along fibres of the spinal cord of rabbits, probably on all fibres. On central fibres of 3 to 15 μ in diameter the internodal lengths vary from 300 to 1700 μ . As in peripheral nerves internodal length increases with growth; in smaller rabbits this length is less for any given fibre diameter than in larger animals. In new -born rabbits the smallest fibres have internodes as short as 200 μ .