ORGANIZATION OF VOLKMANNIA DAWSONI. 31 



we see that this central axis consists of vessels ; but I am 

 unable to discover in them any trace either of trans- 

 verse bars or of reticulations. This/ however, is in all 

 probability due to the peculiar state of mineralization which 

 the entire specimen exhibits. The general outlines are 

 everywhere exquisitely preserved ; but the minute texture, 

 both of cells and vessels, is so altered that no reliance can 

 be placed upon its indications. That the vessels have been 

 thickened by internal deposits of some kind is clear enough. 

 The greater diameter of the vascular axis between its most 

 distant angles is '065 . That of the largest of its component 

 vessels is about '0037 of an inch. Of the tissues originally 

 occupying the space (c) between the axis just described 

 and the outer prosenchymatous cylinder (b), we have little 

 or no indication, a few shreds of what may have been 

 cellular tissue being the only remains of them. 



In describing the prosenchymatous cylinder (b) , we have 

 to distinguish between its several parts. It gives off the 

 bractigerous disks ; or perhaps it may be more correctly 

 defined as consisting of the central portion of each disk, 

 prolonged downward to the node below, as shown in 

 figures 1 and 2. It of course follows that its prosenchy- 

 matous walls are thinnest in a transverse section made 

 immediately above each disk (fig. 4 b) , and thickest at the 

 points where the disk springs from it (fig. 4 b ! ) . The pros- 

 enchymatous structure of its walls is shown at fig. 2 b l \ 

 where a tangential section has intersected the cells in 

 the plane of their longer axes. The mean length of these 

 cells is very variable, their diameter about '0024 of an 

 inch. The diameter of the exterior of the cylinder at 

 its narrowest part, just above a node, is '13. At each 

 node the cylinder just described gives off a thick bracti- 

 gerous disk (d), which extends ' upwards and outwards, 

 at an angle of about 45 , curving slightly upwards and 

 inwards as it ascends. It extends about the eighth of 



