A large section of one of these nodules (12·5 cm. × 8·5 cm.) is shown in fig. 17. It illustrates the manner of occurrence of various fragments of different plants in which the structure has been more or less perfectly preserved. In this particular example we see sections of _Myeloxylon_ (I), _Calamites_ (II), Fern petioles (_Rachiopteris_) (III), Stigmarian appendages (IV), Lepidodendroid leaves (V), _Myeloxylon_ pinnules (VI), Gymnospermous seeds (VII), Twig of a _Lepidodendron_, showing the central xylem cylinder and large leaf-bases on the outer cortex, (VIII), Sporangia and spores of a strobilus (IX), Tangential section of a _Myeloxylon_ petiole (X), _Rachiopteris_ sp. (XI), _Rachiopteris_ sp. (XII), Band of sclerenchymatous tissue (XIII), _Rachiopteris_ sp. (XIV). The general appearance of a calcareous plant-nodule suggests a soft pulpy mass of decaying vegetable débris, through which roots were able to bore their way, as in a piece of peat or leafy mould. Overlying this accumulation of soft material there was spread out a bed of muddy sediment containing numerous calcareous shells, which supplied the percolating water with the material which was afterwards deposited in portions of the vegetable débris. According to this view the calcareous nodules of the coal seams represent local patches of a widespread mass of débris which were penetrated by a carbonated solution, and so preserved as samples of a decaying mass of vegetation, of which by far the greater portion became eventually converted into coal[120]. [Sidenote: FOSSIL NUCLEI.] In such nodules, we find that not only has the framework of the tissues been preserved, but frequently the remains of cell contents are clearly seen. In some cases the cells of a tissue may contain in each cavity a darker coloured spot, which is probably the mineralised cell nucleus. (Fig. 42, _A_, 1, p. 214.) The contents of secretory sacs, such as those containing gum or resin, are frequently found as black rods filling up the cavity of the cell or canal. The contents of cells in some cases closely simulate starch grains, and such may have been actually present in the tissues of a piece of a fossil dicotyledonous stem described by Thiselton-Dyer from the Lower Eocene Thanet beds[121], and in the rhizome of a fossil _Osmunda_ recorded by Carruthers[122]. (Fig. 42, _B_, p. 214.) Schultze in 1855[123] recorded the discovery of cellulose by microchemical tests applied to macerated tissue from Tertiary lignite and coal. With reference to the possibility of recognising cell contents in fossil tissue it is interesting to find that Dr Murray of Scarborough had attempted, and apparently with success, to apply chemical tests to the tissues of Jurassic leaves. In a letter written to Hutton in 1833 Murray speaks of his experiments as follows:—