408 Maslen.— The Structure of Mesoxylon Sutcliffii (Scott). 
stem of the bud with that of the ordinary mature stem. PI. XXXVI, 
Fig. 20, shows a transverse section of an axillary bud magnified about 
40 diameters. Surrounding a relatively large continuous pith, p., is a ring of 
some five or six minute but distinct bundles, b ., separated by broad medullary 
rays, m.r. The xylem of the bundles consists mainly of rows of very small 
tracheides, and within these there is some evidence of minute patches of 
irregularly arranged elements, evidently corresponding to the centripetally 
developed wood of the bundles of the main axis. Outside the wood some 
rows of thin-walled phloem elements are visible. 
Between the larger bundles, and in the medullary rays, are seen small 
bundles (PI. XXXVI, Fig. 20), which are evidently leaf-trace bundles, 
passing out to the bud-scales, and similar ones are also seen traversing the 
pericycle and cortex (Fig. 20, /./., &c.). Each of these minute leaf-trace 
bundles appears to consist of about half a dozen very small tracheides 
without radial arrangement, and to be composed entirely of the centri- 
petally developed xylem sometimes seen at the margin of the pith. 
No arrangement of the leaf-trace bundles in pairs is visible, although 
opposite to each ‘ gap ’ between the main bundles two leaf-trace bundles 
are usually seen, one of these, however, being much further out than the 
other. The wood at this early stage in the development of the ‘ branch ’ is 
of a much less dense character, with broader medullary rays, than is charac- 
teristic of the ordinary secondary xylem of the mature stem of Mesoxylon 
and other Cordaiteae, while it more resembles the less dense centrifugal 
wood forming part of the leaf-trace bundles, which may be partly of 
primary rather than of secondary origin (ante, p. 394). Indeed, it seems 
probable that the whole of the xylem of the bundles in the section shown 
on PI. XXXVI, Fig. 20, is best regarded as primary wood. 
Outside the stele occurs a relatively broad band (PI. XXXVI, Fig. 20, 
pet) consisting of small cells, some of which possess dark contents. This 
tissue, which closely resembles the pith in appearance, probably corresponds 
with the tissue which has been distinguished as the pericycle in the main 
axis. Beyond this tissue appears a larger-celled cortex, c. PL XXXVI, 
Fig. 20, also shows some of the closely-packed leaves or scales, s., ^.,of the 
bud. Another somewhat better preserved section of a bud-scale is shown 
on PL XXXVI, Fig. 21. It shows a small-celled epidermal layer, e.\ a 
distinct hypodermal tissue near the outer surface, consisting of small 
thick- walled cells, h . ; a relatively large-celled mesophyll tissue, m., and two 
very minute bundles, b., b ., close together near the centre of the leaf, each 
consisting of about half a dozen or fewer exceedingly small tracheides about 
0*007 mm - i n diameter. 
In all the sections of Mesoxylon Sutcliffii which have been examined, 
the buds appear to be in almost exactly the same stage in development ; in 
no case has a bud grown into a distinct branch. Presumably they were 
