VARIOUS KINDS OF FOSSIL PLANTS 9 



a rock it will split so as to show the perfect form 

 of the surface of a stem, while its reverse is left on 

 the stone as is shown in fig. 2. Had we only the 

 reverse we should still have been able to see the form 

 of the leaf bases by taking a wax impression from it; 

 although there is nothing of the actual tissue of the 

 plant in such a fossil. Sometimes casts of leaf bases 



Fig. 2. A, Cast of the Surface showing the Shape of Leaf Bases of Siglllaria; B, the 

 reverse of the impression left on the adjacent layer of rock. (Photo.) 



show the detail preserved with wonderful sharpness, as 

 in fig. 3. This is an illustration of the leaf scars of 

 Lepidodendron, which often form particularly good casts. 

 In other instances the cast may simply represent the 

 internal hollows of the plant. This happens most com- 

 monly in the case of stems which contained soft pith 

 cells which quickly decayed, or with naturally hollow 

 stems like the Horse-tails \Equisetum) of to-day. Fine 

 mud or sand silted into such hollows completely filling 

 them up, and then, whether the rest of the plant 

 were preserved or not, the shape of the inside of the 



(C122) 2 



