394 Maslen . — The Structure of Mesoxylon Sutcliffii (Scott). 
and in contact with the conjunctive tissue, while the elements outside this 
are radially arranged and presumably partly or entirely of secondary origin. 
Unfortunately the longitudinal sections do not suffice to decide the question 
as to the existence of some centrifugally developed primary xylem. An 
analogous case is found in Poroxylon 3 where, according to the observations 
of MM. Bertrand and Renault, there is no centrifugal primary xylem, but in 
this case also this is a point which might repay further investigation . 1 In 
the Cordaiteae of the type described by Renault, to which Mesoxylon is 
evidently in many respects closely related, the elements of the wood are 
radially arranged throughout, so that in transverse sections there is no dis- 
tinction between primary and secondary xylem. In the radial sections of 
Cordaites , however, we find a marked change, from the pith outwards, in the 
structure of the walls of the tracheides . 2 The narrow spiral elements of the 
protoxylem are succeeded by wider spiral tracheides, and these again by 
scalariform elements. It is not until many rows have been passed that we 
come to the pitted tracheides, which form the bulk of the wood. The 
transitional region between primary and secondary xylem in Cordaites is 
thus an extensive one, and it is impossible either in transverse or in longi- 
tudinal sections to draw a sharp line between the two tissues. The proto- 
xylem elements in Cordaites are localized in groups, often projecting some- 
what into the pith, and marking the position of the primary bundles. 
Comparing the structure of Cordaites with that of Mesoxylon Sutcliffii , it is 
seen that the radially-arranged xylem of a leaf-trace bundle of M. Sutcliffii 
agrees in a general way with the inner wood of Cordaites in consisting of 
spiral and scalariform tracheides, passing outwards, at the limit of the 
bundle, and by intermediate forms, into the pitted tracheides which form 
the bulk of the wood, but it has been impossible to identify any definite 
protoxylem elements in the radially-arranged portion of the bundle, either 
by their smaller size in transverse section or by their special thickening in 
longitudinal ones. In Mesoxylon Sutcliffii the smallest xylem elements 
always occur at the outer limit of the inner , centripetal wood. On 
analogy, then, with Cordaites , we may perhaps regard the radially- 
arranged xylem of the leaf-trace bundles of Mesoxylon Sutcliffii as repre- 
senting part of the primary wood and also the transition region between the 
primary and secondary xylem. Passing to the mass of irregularly (i. e. non- 
radially) arranged tracheides which constitutes the inner wood of the leaf- 
trace bundles, it is found, as already mentioned, that the smallest elements 
occur on its outer side, so that the structure of this part of the wood is 
apparently exarch, as is the case in Poroxylon , according to MM. Bertrand 
and Renault. 
1 Scott, Studies, 2nd Ed., p. 505. 
2 See figure of Cordaites ( Araucarioxylon ) Brandlingii, in Scott, Studies, 2nd Ed., Fig. 190, 
p. 528. 
