INVESTIGATION OF FOSSIL STEMS. 63 
tigation, the process described at the conclusion of this 
section should be employed. 
The following data may be thus obtained. If the struc- 
ture be entirely cellular, and it can be satisfactorily ascer- 
tained that it never possessed vascular tissue, the original 
belonged to the Cryptogamia; %.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 man- 
ner (see Plate V. fig. 4), it belonged to some other tribe of 
exogenous plants. 
If the walls of the tubes be studded with glands (ign. 1, 
fig. 1, ¢; Plate V. figs. 2. 3°-), the fossil belongs to the 
Coniferee. . 
If any vestige of a central pith be discovered, the exoge- 
nous nature of the original is undoubted, for no other class, 
as we previously stated, possesses a central cellular column. 
The absence or presence of a true cortical investment, or 
bark, is important, for a distinct bark is the characteristic of 
the exogenous class :* a cortical integument, or rind, not 
separable from the enclosed structure, indicates the monoco- 
* An apparent exception to this rule is found in the fossil genus 
Clathraria, described hereafter, which has a distinct hollow cortical 
cylinder, that separates from the internal axis: this is not true 
bark, but is formed by the consolidation of the bases of the petioles 
or leaf-stalks; see Lign. 54. 
