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 sun-ounding 

 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 acalarifonn 

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

 secondary wood presents the features characteristic of Cala- 

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

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

 1 WUliamson (78), p. 828, PI. xx. figs. 14 and 16. 



