THE FOSSIL OSMUNDACE^}. 223 



of the young tracheae only persist at the angles where the tracheae meet and in the 

 areas separating the vertical series of pits (fig. 6). The opposing bars of secondary 

 thickening that separate the several pits of each vertical series are quite free from one 

 another in the median vertical plane in the long axis of the wall. If there is only 

 one series of pits, the opposing bars of the two contiguous tracheae run freely across 

 from one corner to the other without coming into contact with one another. If there 

 are several vertical series of pits, the bars are cemented together by an intervening 

 middle substance at the intervals between each series of pits. 



In the transverse section of such a wall an empty space will be seen in the middle 

 of its substance corresponding with the position of each vertical series of pits, whereas 

 the angles and the intervals between the series of pits will be solid throughout (fig. 6). 

 In applying these facts to the fossil Osmundacew, no difficulty is met with in the cases 

 of Osmundites Dunlopi, 0. Gibbiana, and 0. skidegatensis. In these the walls are 

 represented by their dark carbonaceous remains, and wherever detail is preserved 

 their appearance fits in accurately with the description of the recent Osmundacese 

 given above. This is best shown in Osmundites skidegatensis (fig. 7), where the 

 transverse section of the tracheal wall shows two black bars separated by white spaces 

 in the pitted regions, but connected up by a black mass in the unpitted regions and 

 at the angles where the trachea? meet. 



We feel satisfied that in the living Zalesskya gracilis the tracheal wall also possessed 

 the same structure, although it is somewhat masked in the fossil owing to some peculiarity 

 in its method of fossilisation. Indeed, in some of the sections a structure similar to that 

 of Osmundites skidegatensis is clearly present in the root-steles and leaf-traces (fig. 20). 

 Elsewhere it appears that the disintegrated carbonaceous substance of the wall, during 

 the processes of partial decay and fossilisation, has become redeposited on the surfaces 

 of the wall, both on the internal surfaces of the lumens of the tracheae and on the 

 limiting surfaces of the empty spaces in the substance of the walls, producing the thin 

 black lines referred to above. This still leaves the black lines at the angles and the 

 short lines opposite unpitted areas in the wall unexplained. It does not seem probable 

 that these represent cavities existing in the living material, but they may indicate the 

 position of tracts of some less resistant substance than lignin in the tracheal wall, 

 which, by its early decay after the death of the plant, formed cavities in the wall 

 before actual fossilisation took place. 



Structure of Phloem and Cortex. 



The xylem is surrounded by a continuous ring of phloem (figs. 3 and 9, ph.), separ- 

 ated from the tracheae by a broad zone of some five to six layers of vertically elongated 

 parenchymatous cells forming the xylem sheath (figs. 4 and 9, xy. sh.). Some of these 

 cells are filled with a dark brown substance which sometimes appears granular, and the 

 innermost often have their angles marked by dark lines similar to those that occur at 

 the angles of the tracheae. Probably the walls at these points were more or less 



