344 ON LEPIDODENDRON AND CALAMITES. 
bundles which supply the leaves. The two stems are evidently built 
upon the same plan; and were we to substitute scalariform tissue for 
the gymnospermatous woody tissue, and scalariform utricles for true 
medullary parenchyma, and finally exclude the medullary rays, the de- 
scription of the Cycadean stem would apply to that of Lepidodendron. 
And it deserves special notice, that the surface of the Cycadean trunk 
is composed of the bases of the old leaves, together with the scales 
which in some species are interspersed among them, or alternate with 
them. The leaves do not disarticulate at the circumference of the 
stem, but at some distance from it, leaving a small portion of the base 
persistent. The scars of the outer surface of the stem give a different 
impression from those presented when the persistent bases of the leaf- 
stalks are removed. Whoever is even a little familiar with coal fossils 
is aware that there are two sets of scars on the stems of Lepidodendron 
—one superficial, the other internal. The fossils that present the first 
set are generally said to be “ corticated " stems, and those exhibiting 
the others “decorticated.” The “bark” is generally converted into 
a compact structureless coal, the outer surface of which has the one set 
of scars, and the inner surface the other. I believe this coal is pro- 
duced by the external of the two epidermal series, and that the outer 
scars were truly superficial, while the inner were produced by the 
vessels which passed to the bases of the leaves. The two sets of scars 
in Cycadean stems are analogous structures; but in Lepidodendron, 
the layer which bears the scars on its two surfaces is a compact cylin-. 
der; while in the Cycadee there is no connecting tissue uniting the 
bases of the leaves; they are closely packed together, but quite free 
from each other. It is evident that in many respects the fossil stem 
had a striking analogy in the arrangement of its parts to that of the 
recent Cycads, while it was, however, a true Cryptogam ; and if we 
now examine the slender stem of Lycopodium we shall find, I believe, 
that Lepidodendron, though more highly developed, does not differ 
essentially from it. 
Spring, in his * Monographie des Lycopodiacées’ (p. 293), describes 
the stem of this Order as composed of five parts :—1st. The woody 
axis; 2nd. A layer of delicate cells; 3rd. The liber; 4th. The her- 
baceous envelope ; and, 5th. The epidermis. 
is composed of bundles of scalariform vessels, scattered 
through a very delicate cellular tissue, in a regular figure, which varies 
