[Illustration: Fig. 2.--A, Cast of the Surface showing the Shape of Leaf Bases of _Sigillaria_; B, the reverse of the impression left on the adjacent layer of rock. (Photo.)]
In other instances the cast may simply represent the internal hollows of the plant. This happens most commonly 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 stem remains as a solid stone. Where this has happened, and the outer part of the plant has decayed so as to leave no trace, the solid plug of stone from the centre may look very much like an actual stem itself, as it is cylindrical and may have surface markings like those on the outsides of stems. Some of the casts of this type were for long a puzzle to the older fossil botanists, particularly that illustrated in fig. 4, where the whole looks like a pile of discs.
[Illustration: Fig. 3.--Cast of the Leaf Bases of _Lepidodendron_, showing finely marked detail. (Photo.)]
[Illustration: Fig. 4.--"_Sternbergia._" Internal cast of the stem of _Cordaites_.]
The true nature of this fossil was recognized when casts of the plan were found with some of the wood preserved outside the castings; and 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.
[Illustration: Fig. 5.--Leaf Impressions of "Fern" _Sphenopteris_ on Shale. (Photo.)]
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 show the external 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 embedded _in the external_ cast, and between them there is a slight space, now empty, but which once contained the actual shell of the seed, now decayed. Thus we may rattle the "stone" of a fossil fruit as we do the dried nuts of to-day--the external resemblance between the living and the fossil is very striking, but of the actual tissues of the fossil seed nothing is left.