Since the microscopical examination of fossil plant-anatomy was rendered possible, a more correct interpretation of decorticated and incomplete specimens has been considerably facilitated. The examination of tangential sections taken at different levels in the cortex of such a plant as _Lepidodendron_ brings out the distribution of thin and thick-walled tissue. Regularly placed prominences on such a stem as the _Knorria_ shown in fig. 23 are due to the existence in the original stem of spirally disposed areas of thin-walled and less resistant tissue; as decay proceeded, the thinner cells would be the first to disappear, and depressions would thus be formed in the surrounding thicker walled and stronger tissue. If the stem became embedded in mud or sand before the more resistant tissue had time to decay, but after the removal of the thin-walled cells, the surrounding sediment would fill up the depressions and finally, after the complete decay of the stem, the impression on the mould or on the cast, formed by the filling up of the space left by the stem, would have the form of regularly disposed projections marking the position of the more delicate tissues. The specimen represented in the figure is an exceedingly interesting and well preserved example of a Coal-Measure stem combining in itself representatives of what were formerly spoken of as distinct genera. [Illustration: FIG. 23. A dichotomously branched Lepidodendroid stem (_Knorria mirabilis_ Ren. and Zeill.). After Renault and Zeiller[147]. (¼ nat. size.) The original specimen is in the Natural History Museum, Paris.] The surface of the fossil as seen at _e_ affords a typical example of the Knorria type of stem; the spirally disposed peg-like projections are the casts of cavities formed by the decay of the delicate cells surrounding each leaf-trace bundle on its way through the cortex of the stem. The surface _g_ exhibits a somewhat different appearance, owing to the fact that we have the cast of the stem taken at a slightly different level. The surface of the thick layer of coal at a shows very clearly the outlines of the leaf-cushions; on the somewhat deeper surfaces _b_, _c_ and _d_ the leaf-cushions are but faintly indicated, and the long narrow lines on the coal at _c_ represent the leaf-traces in the immediate neighbourhood of the leaf-cushions. [Sidenote: IMPERFECT CASTS.]