8 STUDIES IN FOSSIL BOTANY 



which may reproduce with the greatest beauty an 

 external or internal surface, but which tell us nothing 

 of organised structure. The beautiful Fern-like fronds, 

 so frequent in the Coal-measures, are perhaps more 

 familiar to the ordinary observer than any other speci- 

 mens of fossil plants. In them we have essentially 

 the print of the frond on the shale lying above and 

 below it. The finer the mineral material was when 

 deposited, the more perfect will be the impression. 

 The substance of the frond is reduced to a layer of coal 

 in which there is little or no structure to be traced, 

 though occasionally very resistant parts, such as spores 

 or cuticle, will have left recognisable remains. 



Other comparatively familiar fossils are the stems of 

 the Lepidodendra and Sigillariae. In many of these 

 specimens the markings of the surface, as characterised 

 by the bases and scars of the leaves, are preserved 

 exactly as in nature (see Figs. 54 and 86, pp. 130 and 

 207). Where this is the case the whole substance of the 

 plant has been replaced by mineral matter. In other 

 instances we have a mould of the external surface, eleva- 

 tions being represented by depressions, and vice versa. 

 The markings, in all these cases, will necessarily depend 

 on the state of preservation of the specimen when 

 incrustation took place. Thus in Lepidodendron we 

 have quite different superficial markings according to 

 the condition of the specimen, whether all its tissues 

 were intact, or the epidermis had been stripped off, or 

 a thin or thick layer of the cortex had also been lost. 

 At least four different " genera " have been founded on 

 these different states of preservation of one and the 1 

 same plant (see Chapter V. p. 131). 



