206 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
may represent the disorganized cell-fibres of barred cells. The exterior of the medullary 
axis is occupied by the usual ring of barred vessels, but it is much narrower than in 
the previously described types, being not more than "035 in breadth. Yet more remark- 
able is the almost complete absence, from the transverse section, of the woody zone. 
In figs. 12 & 13 it is scarcely visible, but it may be represented by the minute barred 
vessels of the vertical section (fig. 14, d). The inner bark ( g ) consists of parenchyma, 
the cells of which are very minute, being rarely more than '0012 in diameter. More 
externally we have a very thick middle bark, consisting of a coarser parenchyma ( h ) 
with larger cells, and about half an inch in thickness. External to this are the radiating 
lines of a thin subepidermal layer of prosenchyma (i), about *06 to Y2 in thickness, the 
transition from the coarse parenchyma of the middle bark to the elongated prosenchyma 
being very abrupt. The outermost layer of epiderm is wanting in all the specimens 
which I have seen of this type. I have not had the opportunity of examining the ori- 
ginal specimen in Mr. Aitken’s cabinet ; but Mr. Butterworth’s example of the same 
plant, from South Owram, exhibits the subepidermal surface of the outer bark, which is 
covered, not with oblong projections, as in the case of Lepidodendron selaginoides , but 
with hexagonal areola about a quarter of an inch in breadth, as represented in fig. 15. 
This distinction shows that, though in their internal organization the two plants 
approach very nearly to one another, they are nevertheless different. The aspect of the 
well-marked vascular bundles proceeding to the leaves (m, figs. 13, 14) is also different. 
They leave the thin vascular zone, and plunge into the parenchymatous bark with little 
or none of the perishable investment derived from the delicate cells of the inner bark 
seen in Lepidodendron selaginoides ; hence they appear in all the sections as dark radii 
of well-defined vessels without any open space between them and the bark itself. 
Before leaving these examples of the genus Lepidodendron, I would call attention to 
some sections which illustrate yet more clearly the nature of the apparently persistent 
petioles that adhere to the bark of some examples, and also throw light upon the scars 
that characterize the Lepidodendroid stem. Plate XXY. fig. 16 represents a section 
kindly lent to me by Mr. Dawkins, who also obliged me further by placing in my hands 
half of the specimen from which the section was obtained. It is a radial longitudinal 
section of the epidermal layer of a Lepidodendron with its attached petioles; 16 k is the 
layer of tubular prosenchyma constituting the subepidermal tissue in all these plants ; i is 
a fragment of the outer bark, consisting of the ordinary forms of short prosenchymatous 
cells; whilst at l we have a series of petioles, which, in this section, appear to be 
turgid and succulent at their bases, but become more shrivelled and thin as they ascend 
outwards from the bark. The specimen supplied to me by Mr. Dawkins enabled me to 
make a series of sections of these petioles. Plate XXVI. fig. 17 represents a tangential 
one made through the bases of the petioles external to the subepidermal prosenchyma, 
16 k. The petioles are here in close contiguity, and of more or less regular rhomboidal 
forms. The position of the vascular bundle going to each leaf is indicated faintly in a 
few of the petioles by a rather darker spot (in). Plate XXVI. fig. 18 is a section made 
