THE STEM OF CALAMITES 29 



species. It is obviously desirable to aim at establish- 

 ing species on anatomical lines, and if possible to cor- 

 relate them with the various forms of casts. A great 

 many attempts have been made in that direction, but 

 at present with imperfect success. We find, however, 

 the following Variations, whatever may be their taxo- 

 nomic value. In comparatively few cases we can trace 

 the ray in undiminished width throughout the whole 

 thickness of the wood. That seems to be the char- 

 acteristic of the type which the French authorities 

 call Arthropitys bistriata, a form of Calamites. Then 

 we may have the opposite extreme ; the ray may come 

 to an end almost immediately, and thus be shut off by 

 interfascicular wood ; or, again, it may gradually die out, 

 becoming encroached upon laterally by the wood, and cut 

 up by strands of intercalated tracheae. This last is the 

 commonest form among English Calamites {Calamites 

 communis, Binney). It is not very easy to determine 

 how this mode of enclosing a ray by the wood was 

 brought about, because the tracheae are very long and 

 the ray-cells which they replace very short, and yet the 

 radial arrangement of the elements is not disturbed. 

 The only explanation appears to be that the growth 

 must have taken place in the cambial cells themselves, 

 rather than in their products. 



I have hitherto spoken of tracheae, using the word 

 in its widest sense, so as to leave open the question as 

 to the true nature of the woody elements. First of all, 

 were they tracheides, derived from single cells, or vessels, 

 arising by cell-fusion ? We constantly find that they 

 have tapering ends, with no evidence of perforation ; 

 but there is this difficulty, that every now and then 



