160 



THE GEOLOGICAL HISTORY OF PLANTS. 



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 disorganised 

 before it was fossilised. The outer surface of the axis is striated 

 longitudinally, and in some places marked with impressions of tort- 

 uous fibres, apparently those of the inner bark. In the cross-sec- 

 tion, 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. This 

 tree has an erect, ribbed trunk, twelve feet in height and fifteen 

 inches in diameter, swelling to about two feet at the base. 



2. Favularia Type. — This has been well described by Brongniart 

 and by Renault,* and differs from the above chiefly in the fact that 

 the outer exogenous woody zone is composed of reticulated instead 

 of scalariform tissue, and the inner zone is of the peculiar form 

 which I have characterised as pseudo-scalariform. 



3. Sigillaria Proper. — This I have illustrated in my paper in 

 the " Journal of the Geological Society " for May, 1871, and it ap- 

 pears to represent the highest and most perfect type of the larger 

 ribbed Sigillaria. This structure I have described as follows, bas- 

 ing my description on a very fine axis found in an erect stem, and 

 on the fragments of the woody axis found in the bases of other erect 

 stems : 



a. A dense cellular outer bark, usually in the state of compact 

 coal — but when its structure is preserved, showing a tissue of thick- 

 ened parenchymatous cells. 



b. A very thick inner bark, which has usually in great part 

 perished, or been converted into coal, but which, in old trunks, con- 

 tained a large quantity of prosenchymatous tissue, very tough and 

 of great durability. This " bast-tissue " is comparable with that of 

 the inner bark of modern conifers, and constitutes much of the min- 

 eral charcoal of the coal-seams. 



c. An outer ligneous cylinder, composed of wood-cells, either 

 with a single row of large bordered pores,f in the manner of pines 



* "Botanique Fossile," Paris, 1881. 



f These are the same with the wood-cells elsewhere called discigerous 

 tissue, and to which I have applied the terms uniporous and multiporous. 

 The markings on the walls are caused by an unlined portion of the cell- 

 wall placed in a disk or depression, and this often surrounded by an 



