404 Maslen. — The Structure of Mesoxylon Sutcliffii (Scott). 
bilobed bundle (i.e. a double leaf-trace) at the base of the petiole, and 
division of this did not take place until after the bundles had entered the 
leaf. It thus appears that in the early division of the leaf-trace bundles, 
as in many other respects, Mesoxylon is much nearer to Cordaites than to 
any other form, while to a certain extent it combines characters found in 
Poroxylon , Lyginodendron , and Calamopitys. 
VI. The Cortex and Leaf-bases. 
The primary cortex and leaf-bases of Mesoxylon Sutcliffii consist of 
parenchyma traversed in the outer part by strengthening bands similar to 
those of many other Palaeozoic plants. 
The thickness of the cortex between the leaf-bases can hardly be 
determined, as the surface of the stem is practically covered with bases of 
leaves. 
The junction between the pericycle and the cortex is not a very sharp 
one, although it is as a rule fairly well defined by the difference between the 
contents-filled cells of the pericycle and the empty-looking ones of the 
cortex which are shown on PI. XXXIII, Fig. 7, i.e., and PL XXXV, Fig. 17, 
i.c. The cells of the inner part of the cortex are also usually somewhat 
smaller than those of the pericycle. Longitudinal sections show that the 
cells of the inner cortex are arranged in more or less vertical rows. The 
outer portion of the primary cortex and the leaf-bases consist of much larger 
cells reaching a diameter of 0-2 mm., and a rough arrangement in vertical 
rows is sometimes visible in this portion also. At the outer limit of the 
cortex there is some trace of a small-celled epidermal layer. 
The Dictyoxylon outer cortex contains a number of strengthening bands 
arranged as more or less radial plates as seen in transverse sections 
(PI. XXXIII, Fig. t, d. , and PI. XXXIV, Fig. 10, l.b). Each plate or band 
consists of much smaller but thicker- walled elements than those forming the 
rest of the outer cortex. In longitudinal sections the plates are seen to be 
composed of much elongated fibres with oblique ends, very different from 
the empty, often radially elongated, cells in which the bands are enclosed. 
Secondary cortical tissues were formed in abundance in Mesoxylon 
Sutcliffii, and. a number of successive periderms are seen which ultimately cut 
right down to the pericycle, or even deeper still. Numerous wavy bands of 
periderm are seen on PI. XXXIII, Fig. 1, pm., and PI. XXXIV, Fig. 9, p r .,p' 
Fig. 10, p' . Secondary cortical tissue was formed on both sides of the 
phellogen and often in about equal quantity. In some places there is 
a marked difference between the elements formed on the outer side and 
those developed on the inner side of the phellogen ; the former being radially 
flattened cells with thicker walls, and the latter often radially elongated cells 
with thinner walls. 
In the deep-seated origin of the periderm Mesoxylon Sutcliffii resembles 
