498 STUDIES IN FOSSIL BOTANY 



and in the organisation of the petiole, as well as in the 

 arrangement of the steles, appear to be of considerable 

 weight. The two families probably represent parallel 

 lines of development. 



A comparison has been suggested between the 

 complex system of steles in Cladoxylon and the deeply 

 lobed and forked stele of Asterochlaena (see Vol. I. 

 p. 324), and.it is possible that there may be a real 

 affinity between the two groups. 



There are other types of Cycadofilices still unde- 

 scribed ; one of these may be shortly referred to, though 

 its affinities cannot yet be determined. This stem 

 comes from the Lower Carboniferous of the Tweed ; 

 the first specimen was discovered by Mr. Mathieson in 

 1 87 1 ; within the last few years additional material has 

 been collected by Mr. Kidston, who has the fossil under 

 investigation ; it has been named Stenomyelon tweedi- 

 anum, Kidston in MS. The primary wood consists 

 of a mass of tracheides, continuous except where 

 it is traversed by bands of parenchyma, which may 

 be regarded as forming a narrow and irregular pith. 

 The position of the protoxylem appears to have been 

 external. The secondary wood is of considerable 

 thickness, and has a rather dense structure, with narrow 

 medullary rays. The tracheides are mostly of the 

 usual multiseriate -pitted type, though transitional scalari- 

 form elements occur near the protoxylem. The outer 

 cortex has the common strengthening-bands of scleren- 

 chyma ; numerous leaf-trace bundles lie between wood 

 and cortex, and there is some evidence that they arose 

 by the subdivision of a smaller number of main strands. 



