Lign. 2. Sections of Fern-Stems.
Transverse sections (half the diameter) of two stems of recent Arborescent Ferns, to show the zone of woody fibre disposed in arcs. This structure is seen in the silicified trunks from Chemnitz.
INVESTIGATION OF FOSSIL STEMS.
If the stem be in a state of preservation that will admit of the slicing or chipping off a piece for microscopical investigation, the process described at the conclusion of this section should be employed.
The following data may be thus obtained. If the structure be entirely cellular, and it can be satisfactorily ascertained that it never possessed vascular tissue, the original belonged to the Cryptogamia; i.e. to fuci, mosses, and the like.
If it consist of parallel tubes, and has neither pith, nor rays passing from the centre to the circumference, the tree or plant was endogenous, like the Palm. If any trace be present of tissue crossing the longitudinal tubes at right angles, and radiating from the centre to the circumference, this will prove the existence of medullary rays, and the original must have been exogenous, as the Oak, Elm, &c.: and if in a transverse section the tubes appear of equal size, the tree was probably coniferous, or cycadeous (i.e. related to the plants called Cycas and Zamia); but if larger tubes appear among the smaller ones, disposed in a definite manner (see [Plate V. fig. 4]), it belonged to some other tribe of exogenous plants.
If the walls of the tubes be studded with glands ([Lign. 1, fig. 1, c]; [Plate V. figs. 2b 3b].); the fossil belongs to the Coniferæ.
If any vestige of a central pith be discovered, the exogenous nature of the original is undoubted, for no other class, as we previously stated, possesses a central cellular column.