406 Maslen. — The Structure of Mesoxylon Siitclijfti (Scott). 
portions of the bundles. On the other hand, the inner wood of the bundle 
has increased in relative size, and it now equals, or nearly equals, the radially 
arranged elements in amount. Moreover, evidence of mesarch structure in 
the inner wood is somewhat more clearly shown than is the case in other 
regions. Some of the elements which lie external to the thin-walled cells 
which mark the position of the protoxylem, px., probably belong to the 
inner wood. Unfortunately, we have no longitudinal sections passing through 
the bundles in the petioles, and so we have been unable to confirm our 
observations on the transverse sections. In Fig. 19 the disorganized phloem 
tissue is represented at and surrounding the bundle are some of the 
contents-filled cells of the leaf tissue. In the probable possession of 
mesarch leaf bundles our plant agrees with Poroxylon, some species of 
Cordaites, and Lyginodendron among fossil plants, as well as with the 
modern Cycads. Dr. M. C. Stopes has shown that in some species of Cor- 
daites (C. principalis , Germ.) the centrifugal part of the xylem is absent, so 
that the leaf bundles are exarch, and she suggests that f as a Cordaitean 
character possibly too much weight may have been attached to the presence 
of centrifugal xylem in the foliar strand \ 1 From an examination of the 
original figures of structure specimens ot Cordaitean leaves given by 
Renault, Grand’ Eury, and Felix, she concludes that ‘ the majority of 
known Cordaitean leaves appear to be without centrifugal xylem ’, and com- 
pares the leaf bundles of most species of Cordaites with the exarch petiolar 
bundles of Medullosa . 2 The bundles of Cordaites described by Dr. Stopes 
are far out in the flattened lamina of the leaf, while the leaf bundle of 
Mesoxylon Sutcliffii shown on PI. XXXV, Fig. 19, is quite near to the base 
of the petiole ; until sections of the lamina of Mesoxylon Sutcliffii have 
been identified, it will be impossible to determine to what extent the centri- 
fugal portion of the bundles persists out into the leaves. 
Exarch bundles are also found at the edges of the leaves of Poroxylon, 
while the more central main bundles are mesarch. 3 
VII. The Axillary Buds. 
The characteristic axillary buds of Mesoxylon Sutcliffii have already 
been briefly described. The transverse section represented on PI. XXXIII, 
Fig. 1, shows two buds, a.b., a.b., as well as bud-steles, a.b.s., passing out 
through the cortex to buds which arise in the axils of leaves at a level above 
that of the plane of section. PI. XXXIV, Fig. 10, shows part of a trans- 
verse section of a stem with a bud-stele, a.b.s., in the cortex cut nearly trans- 
versely, outside which is a row of six leaf-trace bundles and the subtending 
leaf-base into which the bundles will pass. The same bud-stele is shown on 
1 Dr. M. C. Stopes, On the Leal-structure ot Cordaites. New Phytologist, vol. ii, 1903, p. 97. 
2 loc. cit. 3 Bertrand and Renault, Sur les Poroxylons, loc. cit., p. 354. 
