Maslen. — The S true hire of Mesoxylon Sutcliffii (Scott). 407 
PI. XXXIV, Fig. 11, more highly magnified, and it illustrates the general 
oval form (elongated tangentially) and the other general characters of the 
stele. The xylem portion of this stele measures about 0*70 mm. x 0-35 mm., 
and its flattened form is clearly natural and not due merely to the com- 
pression of the stem in which it is enclosed, since the latter shows but little 
compression, and that in a different plane from that of the bud-stele. The 
stele consists of a fairly large medulla, /., forming a continuous tissue and 
surrounded by a zone of radially arranged xylem elements, x. } separated by 
relatively broad medullary rays, m.r. The xylem elements of the bud-steles 
are much smaller than those of the leaf-trace bundles or of the continuous 
ring of secondary xylem of the main axis, and this difference in size of the 
tracheides appears to furnish a ready means of distinguishing the bud-steles 
from the leaf-trace bundles when they are seen in longitudinal sections. 
It has not been possible to distinguish any centripetally developed 
xylem elements in the bud-steles, although they are probably present, nor 
is any division into distinct bundles seen as long as the stele is embedded 
in the tissues of the main axis. 
As already mentioned, twin bud-steles are not infrequently seen em- 
bedded in the pericycle or the inner cortex of Mesoxylon Sutcliffii , as shown 
on PI. XXXIV, Fig. 12. Sections cut from the same specimen at a some- 
what higher level show only one larger stele, and the two steles in the 
lower section doubtless result from the division of this one as it passes 
slowly downwards and inwards. Division of a bud-stele is also shown in 
one of the longitudinal sections. 1 In all probability the oval tangentially 
elongated form of the bud-stele shown on PI. XXXIV, Fig. 11, is to be 
correlated with its approaching division into two steles. 
In many transverse sections, however, only a single bud-stele is shown, 
even when quite close to the secondary wood, which seems to indicate that 
the division of the bud-stele, if it took place, often occurred while it was 
traversing the secondary wood of the main axis. This division must have 
taken place if the connexion of the vascular tissues of the branch with those 
of the main axis was similar to that of Poroxylon, in which the vascular 
system of the branch was inserted on the two bundles of the main axis, 
between which the trace of the subtending leaf passed out. Unfortunately, 
none of the longitudinal sections afford clear evidence of the actual mode 
of insertion of the bud-steles on the bundles of the main axis. In connexion 
with the twin axillary steles of Mesoxylon Sutcliffii , it is interesting to note 
that in another species — M. platypodium — a form with very broad leaf- 
bases, there are always two axillary steles corresponding to a single leaf. 2 
Following the bud-stele outwards into the axis of the branch (bud), 
some interesting modifications of structure are seen, and it is particularly 
interesting to be able to compare the structure of the extremely young 
1 Slide No. 2674 (S). 2 Preliminary Note, p. 239. 
E e 2 
