36 STIGMARIA FICOIDES. 



Plate XIII, fig. 65, is part of a fragment of a Stigmaria from a sandstone bed 

 in the Mountain Limestone district of Weardale. When broken longitudinally 

 the specimen, fig. 64, was found loose in the cavity, 65, b. Both these specimens 

 were figured in Lindley and Hutton's ' Fossil Flora of Great Britain,' vol. i, pi. 35. 

 The concave surface, 65, b, was supposed by the above authors to be merely a cast 

 of the exterior of fig. 64, which latter was regarded as " a woody core communi- 

 cating by means of woody elongations with the tubercles on the outside ; this core 

 has evidently contracted, since the plant was embedded and now lies loose in the 

 cavity of the stem " (loc. cit., p. 106). This sentence affords a fair example of the 

 errors to which observers are liable when interpreting specimens of the histology of 

 which they are ignorant. As we have just seen the supposed " woody core " is but an 

 inorganic cast of the hollow interior of the true woody cylinder, the external surface 

 of which cylinder is represented by the concavity, b. This cast of the latter surface 

 displays the peripheral ends of the primary medullary rays where the rootlet bundles 

 escaped from the cylinder to enter the bark. All the tissues between that cylinder 

 and the outermost surface of the cortex have disappeared, being replaced by the 

 inorganic material, d, which has moulded itself upon the cylinder. The exterior 

 of this specimen shows the characteristic rootlet-scars. 



Fig. 66 is a second specimen resembling fig 65, also from the Hutton 

 Collection but which shows much more distinctly than the last does, 

 the casts of the oblique lines of large external orifices of the primary medul- 

 lary rays. As in fig. 65, all the cortical tissues have disappeared, and were 

 replaced by soft sedimentary mud before the tissues of the vascular cylinder 

 were decayed. This latter member also disappeared ultimately, both in figs. 65 

 and 66. 



Plate XIII, fig. 67, is a transverse section of a Stigmarian root from which all 

 the original organic elements have disappeared, the woody wedges of the vascular 

 cylinder, b, having been the last to do so. The inorganic sediment has here 

 occupied not only the whole of the cortical area but also the central medullary 

 cavity and the primary medullary rays, b', radiating from that cavity. The woody 

 wedges are now only represented by the dark, vacant spaces, b — b. Specimens 

 like this and the three just described are instructive. They demonstrate how 

 superimposed layers of tissue may have disappeared, not simultaneously, but in 

 succession, and their places have been occupied by inorganic materials in a similarly 

 successive manner. Some of these materials have been introduced in a plastic state, 

 like those filling the areas a and d of fig. 67 ; but, had cavities like those left by the 

 decayed wedges, b, of fig. 67, instead of being left empty, been filled subsequently 

 to the replacement of the other structures, a and d, by plastic sediment, this could 

 only have been done by mineral matter in solution and capable of filtering through 

 the clay. It has been such a deposition from infiltrated solutions that has occupied 



