86 THE PRESERVATION OF PLANTS AS FOSSILS. [CH. 



of a calcareous nodule, when seen under the microscope, presents 

 the appearance of a matrix of a crystalline calcareous substance 

 containing a heterogeneous mixture of all kinds of plant tissues, 

 usually in the form of broken pieces and in a confused mass. 



A large section of one of these nodules (12".") cm. x 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), Galamites (II), Fern 

 petioles (Rachiopteris) (III), Stigmarian appendages (IV), 

 Lepidodendroid leaves (V), Myeloxylon pinnules (VI), Gymno- 

 spermous 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 debris, 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 debris. According to this view the calcareous 

 nodules of the coal seams represent local patches of a wide- 

 spread mass of debris 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'. 



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 



1 Stur (85). 



