284 
PKOEESSOK W. C. WILLIAMSON ON THE ORGANIZATION 
elusion that the Lepidostrobi , with their peculiar macrospores and microspores, belong 
to the same plant. I will examine each of these forms in detail. 
Plate XLI. tig. 1 represents a section, rather less than a quarter of an inch in length, 
of a compressed twig of a Lepidodendron. Nearly all the stems found in these beds are 
thus compressed, the peat and its contents having apparently been heavily weighted by 
the superimposed volcanic masses. In the case of the larger Diploxylon - stem the central 
woody cylinders have been strong enough to resist the pressure, their thick cortical layers 
alone having yielded to it. The smaller Diploxylons are somewhat more compressed, 
as is also the case with the Lepidodendroid twigs. 
In the section under consideration we have a central vascular cylinder (fig. 1, c), in 
the middle of which is a small vacant space. In other and similar sections this vacant 
area is occupied by a very delicate form of cellular tissue. The vascular cylinder consists 
of an aggregation of barred vessels not arranged in radiating lines ; it is surrounded by 
a mass of parenchyma, the innermost portion of which (g) is somewhat different from 
the rest. This parenchyma is continuous with that of the bases of the leaves (Z), whilst 
at l' we have sections of the free extremities of several of the leaflets. Such are the broad 
features of the majority of these sections; but a closer study of a large number of 
specimens reveals differences which it may be well to study in the order of their deve- 
lopment. 
Plate XLI. fig. 2 represents the extreme tip of a very slender twig, not more than 
one twelfth of an inch in diameter. In its centre is a small bundle of barred vessels ; 
the rest of the section is composed of cells whose maximum diameter is rarely •0012, 
those of the epidermal surface being smaller and more dense than those of the interior 
of the section. The bases of the leaves (Z), with the exception of that marked l', exhibit 
none of the peculiar form which characterizes them when perfect, as seen in fig. 1, Z. 
Fig. 3 represents the central vascular bundle of a specimen in all respects similar to 
fig. 2. It consists of an irregular cylinder of barred vessels of various sizes ; but the 
transverse section of the largest is not more than ’0025 at its greatest diameter, whilst 
others are even less than ’0005*. In the centre of the bundle is a very small area (a) 
of irregular shape, in which there are no vessels, but which exhibits faint traces of 
cellular tissue. The entire compressed vascular cylinder has a maximum diameter of 
about •015. No traces of vascular bundles appear in the young leaves. The external 
aspect of these young leaves is represented in Plate XLV. fig, 31. Longitudinal 
sections of them show that the basal half of each one is turgid and thick, whilst at 
about half its length it suddenly contracts into a thin and semimembranous form. 
Advancing from this example we pass through intermediate forms to Plate XLI. fig. 1, 
where, as we have already seen, the leaves are fully formed, and where there is a 
slight tendency to differentiation between an inner bark (g) and an outer one (i). This 
difference in the transverse sections is scarcely capable of being described, though the 
eye sees at a glance that the two tissues are not exactly alike. It partly consists in a 
& All these dimensions refer, it is scarcely necessary to say, to the standard of an inch. 
