VII. The Dictyoxylon cortex of lycopodiales as a constituent of coal

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Abstract
By the application of the metallographic method of polishing and etching, the characters of secondary xylem in transverse radial and tangential section can often be observed in certain bright bands. Nevertheless, the identification of periderm in coal can not be safely based upon the radial seriation of the cells and the absence of the characters of xylem, unless this negative evidence is supplemented by other features, such as the characteristic disposition of sclerotic bands in the periderm of certain Lycopodiales. A band of clarain in the Deep Soft seam of the Middle Coal Measures at Grassmoor Colliery, Derbyshire, has been investigated. The coal has a characteristic structure which is persistent over a considerable portion of the band, which represents a single plant fragment or assemblage of fragments of one kind. The band is shown to have the structure of a broken network of sclerotic fibres enclosing in its meshes a thin-walled tissue, and to possesss the megascopic and microscopic characters of a heterogeneous dictyoxylon periderm. The structure is compared with that of various dictyoxylon cortices. Those of pteridosperms are excluded by the fact that the structure under examination is that of a periderm and not of a primary cortex. It is shown by the examination of silicified structural material from Autun (France) that the band has the characters of the dictyoxylon cortex of certain Lycopodiales, and most closely resembles that of Sigillaria spinulosa (Germar) S. brardi (Brogn.), a plant which has been identified by impressions in the Middle Coal Measures at Longton, Staffordshire, not far from Grassmoor.

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