VARIOUS KINDS OF FOSSIL PLANTS ii 



it was then known that the plant had a hollow pith, 

 with transverse bands of tissue across it at intervals 

 which caused the curious constrictions in the cast. 



Another form of cast which is common in some 

 rocks is that of seeds. As a rule these casts are not 

 connected with any actually preserved tissue, but they 



Fig. 5. — Leaf Impressions of "Fern" Sfhenopteris on Shale. (Photo.) 



show the lexjernal form, or the form of the stony part 

 of the seed! Well-known seeds of this type are those 

 of Trigonocarpon, which has three characteristic ridges 

 down the stone. Sometimes in the fine sandstone in 

 which they occur embedded, the internal cast lies em- 

 bedded in the external cast, and between them there 

 is a slight space, now empty, but which once con- 

 tained the actual shell of the seed, now decayed: 



