The Structure of Mesoxylon Lomaxii and 
M. poroxyloides. 
BY 
D. H. SCOTT, F.R.S. 
With Plates LXXXVII-XC. 
HE genus Mesoxylon was established by Mr. Maslen and myself in 
jL 1910 for certain Palaeozoic stems, intermediate in structure between 
the genera Poroxylon and Cordaites. Mesoxylon Sutcliffii , one of the five 
species briefly characterized in our original Note, was very fully described 
by Mr. Maslen last year (Maslen, ’ll). His admirable account of this 
typical form, of which very ample material was available, enables me to 
deal somewhat more briefly with the two species now to be described, 
which are not so richly represented. The question of the specific value of 
the anatomical characters by which we distinguish the fragmentary remains 
of petrified Palaeozoic plants is a difficult one ; we have thought it 
advisable to give distinct names to well-characterized forms, even though 
we cannot in all cases be certain that the distinction may not be bridged, 
as more numerous specimens come under observation. 
It will be well to begin by recalling the characters assigned to the 
genus Mesoxylon : 
Pith relatively large, discoid. Wood dense, with narrow, usually uni- 
seriate medullary rays, and relatively narrow tracheides. Leaf-traces 
double where they leave the pith, the two strands uniting at a lower level, 
but undergoing further subdivision in the pericycle and cortex, before 
entering the leaf. 
Centripetal xylem present in the stem, where it forms part of the leaf- 
traces at the margin of the pith and throughout their course outwards into 
the leaves. 
Outer cortex strengthened by a system of sclerenchymatous bands of 
the Dictyoxylon or Sparganmn type. Wood of the kind usual in Cordaitales, 
the bulk of the secondary tracheides having multiseriate bordered pits on 
the radial walls. Tracheides of the leaf-traces, spiral or scalariform. 1 
1 The generic character is very slightly abridged from that given in our Note (Scott and 
Maslen, ’10, p. 237). 
[Annals of Botany, Vol. XXVI. No. CIV. October, 19x2.] 
