Maslen . — The Structure of Mesoxylon Sutcliffii (Scott). 395 
In connexion with the subject of the inner wood of the leaf-traces of 
Mesoxylon Sutcliffii , mention may also be made of Pitys antiqua and Dado - 
xylon Spenceri , two plants which are probably closely related to the Cor- 
daiteae, and of Calamopitys fascicularis , which probably also belongs to the 
same plexus of intermediate forms between the Pteridosperms on the one 
hand and the Cordaiteae on the other. Pitys antiqua 1 is a tree found in 
the Lower Carboniferous of Southern Scotland, and its wood agrees with 
that of the Cordaiteae and Mesoxylon , except for the greater width of the 
medullary rays. Around the large pith as many as forty or fifty xylem 
strands are found, most of which are embedded in the pith at some little 
distance from the inner edge of the woody zone, with which they only come 
into contact when about to make their exit as leaf-traces. Here the leaf- 
trace as a whole consists of but one bundle, and this appears to correspond 
with the inner wood only of the trace bundle of Mesoxylon Stitcliffii , and 
from this it differs in being definitely mesarch in structure and in being 
usually separated from the main zone of wood. In Dadoxylon Spenceri , 1 2 
a Coal-Measure form, the leaf-traces are given off in pairs as in Mesoxylon , 
and here they are also in contact with the woody zone. Traced downwards, 
the strands of each pair fuse as in Mesoxylo7i ; they are mesarch in the 
upper part of their course, but at a lower level the centripetal wood appears 
to die out, a change which also occurs, as we shall see, in Mesoxylon Sutcliffii. 
Dadoxylo?i Spenceri possesses dense secondary wood with narrow tracheides 
and uniseriate medullary rays, i. e. wood of the Cordaitean type, and in all 
probability close relationship will ultimately be found to exist between this 
form and the genus Mesoxylon. 
In the stem of Calamopitys fascicularisp again, which stands probably 
nearer to the Pteridosperms than Pitys and Dadoxylon , we find the pith 
surrounded by a circle of primary xylem strands, eight or nine in number. 
Each of the traces consists of a single very large bundle in which no 
radially arranged elements are found, and in which the structure is distinctly 
mesarch. In the large strands near their exit the protoxylem is central ; 
lower down, as the strand diminishes in diameter, the centripetal wood 
becomes relatively reduced in amount. 
Returning to Mesoxylon Sutcliffii , there may be just a suggestion in 
some of our slides that the inner wood is not entirely centripetal in its 
development, i. e. that there may be a few non-radially arranged elements 
outside the mass of thin-walled tissue which marks the position of the 
protoxylem. Although it has been impossible to prove that this is the 
case, the analogy of Dadoxylon Spe7iceri and the other forms just described 
appears to make it not improbable that the inner wood of the perimedullary 
1 Scott, Primary Structure of certain Paleozoic Stems, &c. Trans. Roy. Soc. Edinburgh, 
vol. xl, pt. ii, 1902. 
2 Scott, loc. cit., p. 357. 
Scott, loc. cit., p. 332. 
