IGO PROF. W. C. WILLIAMSON ON THE STRUCTURE 



marking the junction of the woody zone with the pith (hg. 

 3 a) indicates, the portions c, c radiate from two of the lon- 

 gitudinal furrows characteristic of an ordinary Calamite, 

 whilst d corresponds with one of the intervening pro- 

 minent ridges. The portion c consists of the transversely 

 divided mouth of vessels, having a diameter of from -^^q 

 to y^^ of an inch ; they are arranged in from 20 to 25 

 regular rows, radiating from the pith [a) to the periphery 

 of the woody zone. At their medullary extremity these 

 rows of vessels combine to form a sharp woody wedge {c) , 

 which fits into one of the longitudinal grooves of an ordi- 

 nary Calamite. 



The appearance presented by these vessels when more 

 highly magnified is seen in fig. 4, where portions of two 

 tracts are represented. The walls of each tube do not 

 appear to have been very thick ; but it is difficult to de- 

 termine exactly how much of the substance represents the 

 original woody tissue, and how much is due to subsequent 

 mineral infiltration. There is now no hollow cavity within 

 each vessel. 



The prosenchymatous cells forming the intermediate 

 tracts (fig. 3 d) have a larger diameter than the cells, aver- 

 aging about ^1^ of an inch. They are also more symmetri- 

 cally arranged in linear series ; but in other respects their 

 distribution resembles that of the vessels just described. 

 They radiate from within outwards in from 30 to 35 re- 

 gular lines. When more highly magnified (fig. 5), each cell 

 appears to have thick walls, like those of recent woody 

 fibre, which I at first believed these tissues to be ; but I 

 think that the appearance in question is due to mineral 

 infiltration, and that the true walls of these cells were 

 thin. It will be observed that, in this section, their regular 

 radiating arrangement is that of the pleurenchyma or true 

 woody fibre of coniferous stems, rather than that of or- 

 dinary cellular tissue, or parenchyma. 



