12 ANCIENT PLANTS 



Thus we may rattle the " stone " of a fossil fruit as 

 we do the dried nuts of to-day — the external resem- 

 blance between the living and the fossil is very 

 striking, but of the actual tissues of the fossil seed 

 nothing is left. 



Casts have been of great service to the fossil 

 botanists, for they often give clear indications of the 

 external appearance of the parts they represent; par- 

 ticularly of stems, leaf scars, and large seeds. But all 

 such fossils are very imperfect records of the past 

 plants, for none of the actual plant tissues, no minute 

 anatomy or cell structure, is preserved in that way. 



A type of fossil which often shows more detail, and 

 which usually retains something of the actual tissues of 

 the plant, is that known technically as the Impression. 

 These fossils are the most attractive of all the many 

 kinds we have scattered through the rocks, for they 

 often show with marvellous perfection the most delicate 

 and beautiful fern leaves, such as in fig. 5. Here the 

 plant shows up as a black silhouette against the grey 

 stone, and the very veins of the midrib and leaves are 

 quite visible. 



Fig. 6 shows another fernlike leaf in an impression, 

 not quite flat like that shown in fig. 5, but with a 

 slight natural curvature of the leaves similar to what 

 would have been their form in life. Though an im- 

 pression, this specimen is not of the "pressed plant" 

 type, it almost might be described as a bas-relief. 



Sometimes impressions of fern foliage are very large, 

 and show highly branched and complex leaves like those 

 of tree ferns, and they may cover large sheets of stone. 

 They are particularly common in the fine shales above 

 coal seams, and are best seen in the mines, for they 

 are often too big to bring to the surface complete. 



In most impressions the black colour is due to a 

 film of carbon which represents the partly decomposed 

 tissues of the plant. Sometimes this film is cohesive 

 enough to be detached from the stone without damage. 



