X] ARTHROPITYS. 311 



canals along the inner edge of the wood may still be readily 

 recognised. The appearance presented by a transverse section 

 of the secondary wood of a Calamite is that of regular 

 radial series of rather small rectangular tracheids, with oc- 

 casional secondary medullary rays consisting of narrow and 

 radially elongated parenchymatous cells. The principal rays^ 

 in the Arthropitys type of a Calamite stem are often found to 

 gradually decrease in breadth as they pass into the secondary 

 wood, until in the outer portion of the wood the primary 

 medullary rays are practically obliterated by the formation 

 of interfascicular xylem. 



In fig. 74, A, we have a portion of a single xylem group 

 of a thick woody stem. The stem from which the figure has 

 been drawn was originally described by Binney^ as Calamo- 

 dendron commune ; we now recognise it as a typical example of 

 the subgenus Arthropitys. The specific term communis was 

 used by Ettingshausen^ in 1855 in a comprehensive sense to 

 include more than twenty species of the genus Catamites, but 

 since Binney's use of the term it has come to be associated 

 with a definite type of Arthropitys stem, in which the primary 

 medullary rays decrease rapidly in breadth towards the peri- 

 phery of the wood. The wood of Binney's stem* measures 2'5 cm. 

 across, but the pith-cavity has been crushed to the limits of a 

 narrow band represented in the figure by the shaded portion. 

 The strand of cells, s, in the pith is a portion of a Stigmarian 

 appendage (" rootlet "), which penetrated into the hollow stem 

 of the Calamite and became petrified by the same agency to 

 which the preservation of the stem is due. These intruded 

 Stigmarian appendages are of constant occurrence in the cal- 

 careous nodules ; their intimate association with the tissues of 

 other plants is often a serious source of en-or in the identi- 

 fication of petrified tissues. The inner portion of one of the 



1 The term primary ray may be conveniently restricted to the truly primary 

 interfascicular tissue, and the term principal ray may be used for the outward 

 extension of the primary rays by the cambium [Williamson and Scott (*.)4), 

 p. 8781. 



2 Binney (68). ^ EttinRshausen (55). 



4 The sections of fossil plants described by Binney wore presented to the 

 Woodwardian Museum, Cambridge, by his sou (MrV. Binney). 



