246 



CORDAITEAE 



[CH. 



STEMS, i. Pith-casts. 

 Artisia. Sternberg. 



A character to which authors tend to attach excessive import- 

 ance as a diagnostic feature is the almost invariable tendency of 

 the parenchymatous pith of Cordaites to break up on contraction 

 into transverse diaphragms, thus producing what is known as 

 a discoid pith. In the stem shown in fig. 473 the pith is repre- 

 sented by a more or less cylindrical cast characterised by fairly 

 regular transverse ribs and narrow grooves; in the upper part 



FIG. 473. Cordaites (or Mesoxylont) stem showing the discoid pith 

 partially enclosed by wood. ( nat. size.) M. S. 



of the fossil the peripheral tissue of the pith is preserved in the 

 form of narrow plates projecting from the inner face of the wood. 

 As Renault 1 pointed out, this type of pith is the expression of 

 certain conditions of growth and is not a satisfactory distinguishing 

 feature of any particular genus or family. The same tendency 

 to form a discoid pith is characteristic of Mesoxylon, and it occurs 

 also in some other Palaeozoic genera. Corda long ago figured 

 a stem attributed by him to Lomatofloyos with a typical discoid 

 pith, and a similar pith is recorded in a stem of Dicranophyllum 2 . 

 Among recent plants Juglans regia affords perhaps the most 

 familiar instance of an identical form of pith : the same type 



1 Renault (79) B. p. 287. 



2 Renault and Zeiller (88) A. PI LXXI. 



