. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. Fig. 11. Archaeosigillaria plumsteadiae sp. nov. Compression of leaf and leaf cushion resulting in the production, in the latex mould, of a leaf cushion showing only the 'false leaf scar'. A. Original shape of leaf and cushion (as seen in profile at edge of stem in Figs 12C, 13B), surrounded by uncompressed matrix (M) which also fills the endocortical cavity; the leaf lies in the matrix as on the lower surface of a horizontal stent. PT—plant tissue. B. Compression distorts the shape of both leaf base
. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. Fig. 11. Archaeosigillaria plumsteadiae sp. nov. Compression of leaf and leaf cushion resulting in the production, in the latex mould, of a leaf cushion showing only the 'false leaf scar'. A. Original shape of leaf and cushion (as seen in profile at edge of stem in Figs 12C, 13B), surrounded by uncompressed matrix (M) which also fills the endocortical cavity; the leaf lies in the matrix as on the lower surface of a horizontal stent. PT—plant tissue. B. Compression distorts the shape of both leaf base and leaf. The leaf cushion becomes less protruding, the thickness of the leaf lamina is reduced and its angle of emergence is decreased. Co—coaly matter. C. Removal of coaly matter (by subsequent diagenesis or weathering) gives a negative impression of the leaf cushion with a narrow mould of the leaf lamina going down into the matrix. D. Latex (La) applied to this negative mould fails to penetrate the narrow mould of the leaf lamina, leaving a 'false leaf scar' on the (positive) cast of the leaf cushion (Fig. 13 A-B, in Fig. 12D). leaf itself. As a result, the leaf cushion, seen (as a protruding 'positive' feature) on the latex cast prepared from this mould, does not show the leaf but merely a transverse marking—a kind of 'false leaf scar'—in the middle of the cushion ( in Fig. 12D) where the latex failed to flow into the narrow space repre- senting the leaf. In such a specimen the leaf may be seen in profile at one or both margins of the compressed stem (Fig. 12C; see left side of Fig. 13B). In one specimen of Archaeosigillaria cf. picosensis the narrow mould of the leaf itself was wide enough to allow latex to enter, so producing a somewhat flattened replica of the original leaves attached to the leaf cushion surface (Figs 12A-B, 14A, F). Although, in what is said above, the distinction is made between an endo- cortical cast and the external mould, the f
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectnaturalhistory, booky
