[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. 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; particularly 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 impression, 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.