FROM THE COAL-FORMATION OF NOVA SCOTIA. 



839 



readily separates from that of the outer cylinder, is striated longi- 

 tudinally. The outer cylinder, which constitutes by much the 

 largest part of the whole, is also composed of scalariform tissue; but 

 this is radially arranged, with the individual cells quadrangular in 

 cross section. The cross bars are similar on all the sides and usually 

 simple and straight, but sometimes branching or slightly reticulated. 

 The wall intervening between the bars has extremely delicate lon- 

 gitudinal waving lines of ligneous lining, in the manner first de- 

 scribed by Williamson*, as occurring in the scalariform tissue of 

 certain Lepidodendra (fig. 4). A few small radiating spaces, par- 



Fig. 3. — Aotis of Diploxylon, as seen 

 on weathered surface. (Natural 

 size.) 



Fig. 4. — Portion of 

 Scalariform Tissue. 

 (Magnified.) 



a. Medullary cylinder, filled with sandstone. 



b. Medullary sheath of scalariform tissue. 



c. Exogenous cylinder of scalariform tissue, radi- 



ally arranged and with concentric lines. 



tially occupied with pyrites, obscurely represent the medullary rays, 

 which must have been very feebly developed. The radiating bundles 

 passing to the leaves run nearly horizontally; but their structure is 

 very imperfectly preserved. The stem being old and probably long 

 deprived of its leaves, they may have been partially disorganized 

 before it was fossilized. The outer surface of the axis is striated lon- 

 gitudinally, and in some places marked with impressions of tortuous 

 fibres, apparently those of the inner bark. In the cross section, where 

 weathered, it shows concentric rings; but under the microscope these 

 appear rather as bands of compressed tissue than as proper lines of 

 growth. They are about twenty in number. Though apparently 

 of very lax tissue, the wood of the outer cylinder may, in con- 

 sequence of the strength of the vertical rods and transverse bars of 



* Monthly Microscopical Journal, August 1869. 



3i2 



