THE CARBONIFEROUS FLORA. 



155 



In Sigillarim, Lepidodendra, &c, the following surfaces of the 

 stem may be presented to our inspection : 



1. The outer surface of the epidermis without its leaves, but 

 with the leaf-bases and leaf-scars more or less perfectly preserved. 

 On this surface we may recognise: (1) Cellular swellings or pro- 

 jections of the bark to which the leaves are attached. These may be 

 called leaf-bases, and they are sometimes very prominent. (2) The 

 actual mark of the attachment of the leaf situated in the most 

 prominent part of the leaf-base. This is the leaf-scar, (3) In the 

 leaf-scar when well preserved we can see one or more minute punct- 

 ures or prominences which are the points where the vascular bundles 

 passing to the leaf found exit. These are the vascular scars. 



When the leaves are attached, the leaf-scars and vascular scars 

 cannot be seen, but the leaf-bases can be made out. Hence it is 

 important, if possible, to secure specimens with and without the 

 leaves. In flattened specimens the leaf-bases are often distorted by 

 pressure and marked with furrows which must not be mistaken for 

 true structural characters. The leaf-bases, which are in relief on the 

 outer surface of the stem, of course appear as depressions on the 

 mould in the containing rock, in which the markings often appear 

 much more distinctly than on the plant itself. 



2. The outer surface of the epidermis may have been removed or 

 may be destroyed by the coarseness of the containing rock. In this 

 case the leaf-bases are usually preserved on the surface of the outer 

 or corky bark, but the leaf -scars and vascular scars have disappeared. 

 This gives that condition of Lepidodendroid trees to which the name 

 Knorria has been applied. When plants are in this state careful in- 

 spection may sometimes discover traces of the leaf-scars on portions 

 of the stem, and thus enable the Knorria to be connected with the 

 species to which it belongs. 



3. The outer or corky bark may be removed, exposing the sur- 

 face of the inner or fibrous and cellular bark, which in the plants in 

 question is usually of great thickness. In this case neither the leaf- 

 bases nor the scars are seen, but punctures or little furrows or ridges 

 appear where the vascular bundles entered the inner bark. Speci- 

 mens in this state are usually said to be decorticated, though only 

 the outer bark is removed. It is often difficult to determine plants 

 in this condition, unless some portion of the stem can be found still 

 retaining the bark ; but when care is taken in collecting, it will not 

 infrequently be found that the true outer surface can be recovered 

 from the containing rock, especially if a coaly layer representing the 

 outer bark intervenes between this and the inner impression. Sped- 



