74 
Seward . — ^4 Contribution to our 
developed secondary xylem agrees in the main very closely 
with that of the stem described above. On the inner side of 
the centrifugal wood there occur in Lyginodendron Oldhamium 
groups of primary xylem ; each group represents a leaf-trace- 
bundle and possesses a well-defined mesarch structure, the 
position of the primary xylem-elements being recognized by 
the presence of the narrow spirally-thickened protoxylem- 
tracheids, and the larger scalariform tracheids, in addition to 
the reticulately marked elements 1 . In some examples of 
Lyginodendron Oldhamium an inner zone of centripetal xylem 
has been described, probably identical with that in Nield’s 
specimen. This inner xylem is described by Williamson and 
Scott as ‘ anomalous wood , 5 and compared with similar 
structures in certain recent Dicotyledons 2 . In attempting to 
establish the precise affinities of the stem under discussion it 
is extremely important to decide, as far as possible, whether 
or not there are any traces of primary xylem like that of 
Lyginodendron Oldhamium . Owing to the imperfect state of 
preservation in that part of the stem where the primary xylem 
should occur, it is very difficult to give a decided and 
satisfactory opinion as to the original existence of such 
primary groups as occur in the previously recorded examples 
of the genus. 
It is, in the first place, conceivable that the primary xylem- 
elements have become disorganized during the growth and 
increase in thickness of the stem. This possibility necessitates 
considerable caution in relying on negative evidence. On the 
other hand there are certain appearances presented by sections 
of Nield’s stem which, without absolutely proving the existence 
of primary xylem-tracheids, render it probable that traces of 
such groups have been partially preserved. There are often 
found groups of tracheids, or occasionally single isolated 
tracheids, between the radially-disposed bands of centrifugal 
and centripetal wood. These, it may be, are portions of 
primary xylem-strands. The unfortunate imperfection in the 
1 See the figures and descriptions in Williamson and Scott’s paper. 
2 Williamson and Scott, loc. cit, pp. 722, 723. 
