It is often said that the surface of a Calamite stem was probably marked by regular ridges and grooves similar to those of the pith-cast, and that such external features are connected with the arrangement of the tissues in the vascular cylinder. The indication of grooves and ridges on the bark of fossil Calamites is no doubt the result of the existence in the hypoderm of firm strands alternating with strands of less resistant cells. It is very common to find Calamite pith-casts covered with a layer of coal presenting a ribbed surface, but this is simply due to the moulding of the coaly film on an internal pith-cast. The broad grooves on such a specimen as that of fig. 77 are, on the other hand, probably an indication of the existence of hypoderm bands similar to those in fig. 74 _B_, _h_. The specimen from which fig. 77 is drawn shows many interesting features. The figure given by Grand’Eury, of which fig. 77 is a copy, is somewhat idealised, but the various surfaces can be made out in the fossil. The surface of the coaly envelope surrounding the pith-cast, _a_, is distinctly grooved, but the depressions have nothing to do with the surface features of the wood or the pith-cast; they are no doubt due to the occurrence of alternating bands of thick- and thin-walled tissue in the hypodermal region of the cortex; the peripheral strands of bast cells would stand out as prominent ribs as the stem tissue contracted during fossilisation. At _b_ (fig. 77) we have a view of the wood in which the position of the principal rays is indicated by fine longitudinal lines at regular intervals; the oval projections just below the nodal line are probably the casts of infranodal canals (_cf._ p. 324). At _a_ the characteristic pith-cast is seen with a small branch-scar on the node. The scar on the middle node, _N_ 2, is probably that of a root, and a root _R_ is still attached to the node, _N_ 3. [Illustration: FIG. 77. Portion of a Calamite stem, showing the surface of the bark, _c_; the wood, _b_; the surface of the pith-cast, _a_. _N._1-_N._3. Nodes. _R._ Root. (After Grand’Eury. Partially restored from a specimen in the École des Mines, Paris.) ¾ nat. size.] [Sidenote: PERIDERM IN STEMS.]