The Primary Structure of the Stem of Calamites. 159 



all of which the pith, carinal canals, and cortex were 

 distinguishable. One of them was " not more than 0'033 

 inch in diameter," and the stage it had reached may be 

 inferred from the following description which Williamson 

 gives of it : — 



"The medullary cells are here unruptured, the medullary 

 fistular cavity having as yet no existence. Nine longitudinal 

 internodal canals are seen, and these form the only recognisable 

 line of demarcation between the pith and the bark [cortex]. There 

 is little difference between the cells of these two structures." 1 



Another, which was slightly more advanced, he describes 

 thus : — 



"We still discover the bark [cortex], the internodal canals, 

 again nine in number, and the medullary parenchyma; but the 

 bark [cortex] in this example is a thick layer of parenchyma of 

 coarser tissue than that composing the medulla, and the latter now 

 displays a central fissure, which obviously indicates the commence- 

 ment of the medullary fistular cavity. We have but still very slight 

 indications of the formation of woody wedges external to each 01 

 the internodal canals." 2 



From these descriptions and the figures which accom- 

 pany them, these two stems seem to have been so young 

 that even the primary structure had not received its full 

 development. Summarising the facts obtained from a study 

 of the whole of the specimens Williamson describes this 

 early condition and some of the subsequent changes in the 

 following terms : — 



" One thing is clear, viz., that the bark [cortex] as we see it in 

 Figs. 8, 10 and 13, is a primitive generalised parenchyma; but as 

 the stems become arborescent this generalised tissue developed 

 within its interior the thick layer of prosenchyma, which resembles 

 so closely the cork layer of living phanerogams." 3 



1 Loc. cit., p. 322. 2 Ibid., p. 322. 



3 Ibid., p. 324. The figures referred to are on Plates 19 and 20 of the 

 Memoir. \ 



