X] PERIDERM IN STEMS. 317 



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 fos- 

 silisation. 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, iV 3. 



An interesting feature observed in some specimens of older 

 Calamite branches is the development of periderm or cork. This 

 is illustrated on a large scale by a unique specimen originally 

 described by Williamson in 1878\ Figs. 78 and 79 represent 

 transverse and longitudinal sections of this stem. This un- 

 usually large petrified stem was found in the Coal-Measures 

 of Oldham, in Lancashire. In the slightly reduced drawing, 

 fig. 78, the large and somewhat flattened pith, p, 4'2 cm. 

 in diameter, is shown towards the bottom of the figure. Sur- 

 rounding this we have 58 or 59 wedge-shaped projecting xylem 

 groups and broad medullary rays ; the latter soon become 

 indistinguishable as they are traced radially through the thick 

 mass of secondary wood, 5 cm. wide, composed of scalariform 

 tracheids and secondary medullary rays (fig. 78, 3). The 

 secondary wood presents the features characteristic of Gala- 

 mites (Arthropitys) communis (Binney). External to the wood 

 there is a broken-up mass, about 5-5 cm. wide composed of 

 1 Williamson (78), p. 323, PI. xx. figs. 14 and 15. 



