ROOTS AND ROOTLETS. 37 



not only the interior of most of the Stigmarian rootlets figured in this Memoir, 

 but also of almost every cell and vessel found in the deposits from which a large 

 majority of my specimens have been obtained. 



Plate XIV, fig. 08, is a specimen of a root, split vertically, drawn four-fifths 

 the size of the original, for which I am indebted to Professor Green of Leeds. Its 

 unseen exterior surface is furnished with the usual rootlet-scars. At a is the external 

 surface of the inorganic cast of the medullary cavity, exhibiting, though rather more 

 closely aggregated, longitudinal ridges like those seen in Plate XIII, fig. 64. The 

 well-defined longitudinal section of the vascular cylinder, b b, is transversely sub- 

 divided into small square areas by the primary medullary rays, b', which radiate 

 through the cylinder at right angles to its axis. The medullary cavity and all the 

 cortical zones are alike replaced by inorganic sandstone. 



Plate XIV, fig. 69, is a specimen from the Burntisland deposit, represented of 

 its natural size. This specimen would perplex an observer unfamiliar with the 

 internal structure of Stigmaria. It is the well-preserved external surface of a 

 large vascular cylinder, exhibiting very definitely the lenticular external termina- 

 tions of the primary medullary rays, b b, but amongst these are a few rootlet-scars, 

 as seen in the next specimen, fig. 70. 



Plate XII, fig. 70, is also a fragment of the exterior of a compressed vas- 

 cular cylinder, wholly composed of barred vessels or Tracheids ; but in it the 

 primary medullary rays are indistinctly shown. Forced rather deeply into 

 its substance are several rootlet-scars, g, arranged in their normal diagonal lines. 

 Such a specimen, seen apart from others, would inevitably indicate the direct 

 orientation of the rootlets from the vascular zone. What has occurred is obvious. 

 The whole of the cortical tissues have disappeared, but with so little disturbance 

 that, on both sides of the specimen, the bases of the several rootlets have become 

 impressed upon the exterior of the flattened vascular cylinder, without any 

 derangement of their normal relative positions. Such specimens teach caution 

 ere we conclude that, because two tissues are found in the closest possible 

 contact, they must once have been organically united. 



Plate XIII, fig. 71. An impression on shale from the Hutton Collection. It 

 is part of a dichotomising root, the surface of which exhibits, besides its rootlet- 

 scars, parallel longitudinal ridges, which either represent fissures in the original 

 bark, or elevations due to shrinkage ; between these ridges are fine undu- 

 lating lines, also running longitudinally, which also appear to have been caused 

 by a shrivelled state of the cortical surface. Another similar specimen, also from 

 the Hutton Collection, exhibits these latter lines ; but, in place of the coarse 

 longitudinal ridges of fig. 71, it has numerous strongly marked undulating ridges 

 and furrows running transversely across the fragment. Varied modifications of 

 the surface, especially in specimens of the larger roots from which all rootlet-scars 



6 



