Maslen . — The Structure of Mesoxylon Sutclijfii (Scott). 391 
several of the sections two distinct bud-steles corresponding to a single leaf 
are seen embedded in the pericycle (see PI. XXXIV, Fig. 12) : each of these 
appears to be a complete closed stele, and they doubtless result from the 
division of the single larger stele which is seen in slides cut from the same 
specimen at a somewhat higher level. 
A transverse section of an axillary bud is shown on PI. XXXVI, 
Fig. 20, and it will be seen that the stele possesses a relatively large pith 
surrounded by five or six distinct bundles separated by broad medullary 
rays. Fig. 20 also shows some of the closely packed leaves of the bud, 
while on PI. XXXVI, Fig. 21, a better preserved bud-scale is shown with two 
very small bundles, b., b., close together near the centre of the leaf. 
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 out into a distinct branch. Presumably they were 
resting buds of some kind, though whether vegetative or reproductive has 
not been determined. 
Having described the general structure of Mesoxylon Sutcliffii, we 
proceed now to a more detailed description of the various tissues, taking 
them in order from within outwards. 
III. The Pith. 
The stem of Mesoxylon SiUcliffii possesses a relatively very large pith, 
as shown on PL XXXIII, Fig. 1, the average diameter in our specimens 
being about 1-4 cm., or nearly one-half that of the stem as a whole, including 
the leaf-bases. The large size of the pith appears to be characteristic of 
the new genus as a whole, since in four out of the five species yet discovered 
it has a diameter nearly half of that of the whole stem. There is a well- 
marked division of the pith into two regions : an outer narrow continuous 
zone of parenchymatous cells, p.p., and a much larger central region, c.p ., 
which has a discoid structure as seen in longitudinal sections (see PI. XXXIII, 
Fig. 2, c.p) resembling the well-known discoid pith found in most species of 
Cordaites . The discoid structure was no doubt due to the fact that the more 
central part of the pith was not able to follow the growth of the stem in 
length, and consequently split across at short intervals, leaving gaps between 
the persistent diaphragms ; at the outer edge, next the wood, the medullary 
tissue remained continuous. 
The central discoid pith has a diameter of about i*i cm., that of the 
whole pith being about 1-4 cm. In most of the sections the inner pith has 
contracted away from the persistent outer zone with which it was originally 
continuous. Thus, in the transverse section shown on PL XXXIII, Fig. 1, 
the outer edge of the discoid pith, c.p., has contracted away from the outer 
pith, p.p., at the sides of the section, leaving a space between, while at the top 
and bottom the two tissues are still continuous. Again, in the longitudinal 
D d 2 
