ON LEPIDODENDRON AND CALAMITES. 339 
observations, together with the examination of some beautiful speci- 
mens in the collections of Robert Brown, now in the botanical depart- 
ment of the British Museum, and of Mr. Alexander Bryson, enable me 
to give a somewhat complete description of its singular structure, 
The axis of the stem cannot be considered as a true medulla or pith, 
inasmuch as it is composed not of simple cells, but of elongated 
utricles of various sizes, irregularly paeem and having thin walls 
marked with scalariform bars (t. 56, f. 2). These utricles, indeed, 
differ from the vascular tissue of the woody eylinder which sur- 
rounds them only in theirlength. The tissue of the woody cylinder 
consists of long scalariform vessels, which increase in size from the 
inner margin to the outer, this increase being sufficient to meet the re- 
quirements of the enlarged circumference, with the help of only a few 
additional series of vessels. As there is no true medullary cellular 
tissue in the axis, so there are no medullary rays passing through this 
cylinder. In radial sections an appearance is seen singularly resem- 
bling, to the naked eye, the “silver grain” produced in dicotyledonous 
woods by the medullary rays; but this arises from a very different 
cause. The diameter of the vessels is so great, that on a polished sur- 
face only the scalariform wall of the vessel, that lies on or near the 
surface, is exhibited ; and when the upper wall of a vessel is cut away, 
the lower wall is often so deeply buried in the opaque substance, that 
the peculiar structure is obscured. In the case of sections prepared 
for microscopic examination, both surfaces of some vessels are often 
removed, and the scalariform markings on the lateral walls, or on any 
horizontal walls which by chance occupy a medial position between the 
polished surfaces, only are seen. This absence of the scalariform bars 
gives at first sight the appearance produced by medullary rays. 
The continuous cylinder of scalariform vascular tissue appears to be 
penetrated by the vascular bundles which ultimately supply the leaves. 
These bundles apparently originate either in the scalariform tissue of 
the axis, or on the inner surface of the woody cylinder. They have 
been mistaken for, or misnamed, med rays. 
The woody cylinder is surrounded by a 1 great thickness of cellular 
tissue, which extends to the exterior of the stem, and is composed of 
three distinet and separable zones. The inner zone has never, as far 
as I know, been perfectly preserved in any specimen, yet traces of it 
sometimes may be seen; and it is rightly restored in Brongniart's 
Z 
