Sometimes impressions of fern foliage are very large, and show highly branched and complex leaves like those of tree ferns, and they may cover large sheets of stone. They are particularly common in the fine shales above coal seams, and are best seen in the mines, for they are often too big to bring to the surface complete.
In most impressions the black colour is due to a film of carbon which represents the partly decomposed tissues of the plant. Sometimes this film is cohesive enough to be detached from the stone without damage. Beautiful specimens of this kind are to be seen in the Royal Scottish Museum, Edinburgh where the coiled bud of a young fern leaf has been separated from the rock on which it was pressed, and mounted on glass. Such specimens might be called mummy plants, for they are the actual plant material, but so decayed and withered that the internal cells are no longer intact. In really well preserved ones it is sometimes possible to peel off the plant film, and then treat it with strong chemical agents to clear the black carbon atoms away, and mount it for microscopic examination, when the actual outline of the epidermis cells can be seen.
Fig. 6.—Impression of Neuropteris Leaf, showing details of veins, the leaves in partial relief. (Photo.)
Fig. 7.—Leaf Impression of Ginkgo, of which the film was strong enough to peel off complete
In [fig. 7], the impression is that of a Ginkgo leaf, and after treatment the cells of the epidermis were perfectly recognizable under the microscope, with the stomates (breathing pores) also well preserved. This is shown in [fig. 8], where the outline of the cells was drawn from the microscope. In such specimens, however, it is only the outer skin which is preserved, the inner soft tissue, the vital anatomy of the plant, is crushed and carbonized.
Leaves, stems, roots, even flowers (in the more recent rocks) and seeds may all be preserved as impressions; and very often those from the more recently formed rocks are so sharply defined and perfect that they seem to be actual dried leaves laid on the stone.
Fig. 8.—Outline of the Cells from Specimen of Leaf shown in [fig. 7]