ON LEPIDODENDRON AND CALAMITES, 345 
in the differgnt species. This axis is surrounded by a layer of lax, 
delicate, cellular tissue, which Spring considers to be the channel 
through which the sap circulates, and the seat of growth in the stem, 
—the inner portion being developed into wood vessels, and the outer 
into “liber.” The “liber " is composed of elongated cells, with thick- 
ened walls. Spring gives to it this name because of its analogy to 
the liber in dicotyledons. This layer is often so thin that it is diffi- 
cult to detect. It is surrounded by a thick greenish layer, composed 
of large elongated cells, with thin walls; and this is covered with an 
epidermis, consisting of small cells with thick walls. The vascular 
bundles pass through the various layers of cellular tissue from the axis 
to the circumference. 
The great difference between the stem of Lepidodendron and Lycopo- 
dium is the existence of a pseudo-medulla, and the arrangement of the 
vascular tissue as a solid cylinder in the fossil genus, compared with 
the central position and loose structure of the vascular tissue in the re- 
cent plant. In both the recent and fossil stems, the vascular tissues 
are surrounded by a zone of thin-walled cells, which has disappeared 
in all the dried specimens of Lycopodium I have examined, leaving the 
axis free, and which, as we have seen, is very rarely preserved in Lepi- 
dodendron. : 
Calamites.—Few fossils have been more misunderstood than the set 
of plants to which the name Calamites is given. One of the least 
errors regarding them was that which placed the stem upside down, 
and made the cylindrical roots its leaves. Calamites is rarely pre- 
served so as to exhibit structure, being almost always converted into 
amorphous coal, and exhibiting an apparently furrowed and jointed 
stem, somewhat resembling the recent Eguisetacee. The few speci- 
mens that have been found with the internal organization of the stems 
preserved, show a structure different from what had been assumed to 
be that of Calamites, and have been constituted into the genus Calamo- 
dendron. Like Lepidodendron, Calamites must have been a very brittle 
plant, as its remains occur in such a fragmentary condition, that great 
difficulty has been experienced in determining the different parts of the 
plant. The branchlets and foliage have been referred to the genus ds- 
terophyllites, supposed to be independent aquatic plants, and the fruits, 
form the genus Volkmannia. 
The stem of Calamites was formed on a different plan from that of 
