XXXIII] MESOXYLON 275 



strands in the perimedullary zone; the centripetal xylem is 

 well developed, the medullary rays are deeper and the outer 

 cortex has shorter bands of mechanical tissue. 



In Mesoxylon poroxyloides Scott and Maslen 1 , the twin- 

 bundles of the traces unite soon after reaching the pith as in 

 M. Lomaxi', the secondary tracheids have only two rows, or 

 sometimes a single row, of bordered pits and the tracheids are 

 rather smaller than in M. Lomaxi (20 40/x as compared with 

 30 60/A) and the medullary rays are shallower. There is a 

 particularly broad zone of spiral and reticulate transitional 

 tracheids at the inner edge of the wood as in Cordaites and in 

 Dadoxylon Pedroi (fig. 476). The leaves of this species are believed 

 to be represented by the type described by Dr Benson as Cordaites 

 Felicis (fig. 465) 2 , but, as already suggested, it is very probable 

 that many or possibly nearly all the leaves from British Coal 

 Measures described as Cordaites may belong to Mesoxylon. 



The chief interest of the genus Mesoxylon is its close resemblance 

 in certain characters to Cordaites and Poroxylon: the presence 

 of strands of centripetal xylem in the perimedullary region is 

 an important feature in which Mesoxylon differs from stems 

 assigned (under the generic name Dadoxylon) to Cordaites. Meso- 

 xylon differs from Poroxylon in having a discoid pith like that of 

 Cordaites, but a more important difference is the absence in the 

 leaf-trace xylem of Mesoxylon of bordered pits of the Araucarian 

 type, whereas in Poroxylon Araucarian pits occur in both the 

 centripetal and centrifugal tracheids. In Poroxylon the secondary 

 xylem is manoxylic ; in Mesoxylon, as in Cordaites, it is pycnoxylic. 



The range of Cordaites and a consideration of other 

 imperfectly known genera. 



An increased precision in knowledge derived from anatomical 

 investigation often tends to demonstrate the untrustworthiness 

 of criteria based on external features previously employed with 

 confidence. This inevitable though, from the point of view of 

 the systematist, inconvenient result of intensive study is well 

 illustrated by the recent discovery of the stems named by 



1 Scott (12) p. 1017, Pis. LXXXVIH., xc. 2 Benson (12). 



182 



