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. 2^b 3^b_.); 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. The absence or presence of a true cortical investment, or bark, is important, for a distinct bark is the characteristic of the _exogenous_ class:[42] a cortical integument, or rind, not separable from the enclosed structure, indicates the _monocotyledons_; and the entire absence of any rind, the _cryptogamia_. [42] 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_. The markings on the stems, occasioned by the scars or cicatrices left by the separation of the petioles or leaf-stalks (as on the stalk of a cabbage), afford important evidence, since they are commonly present, even when the cylindrical trunk is compressed into a flat thin layer of coal; as we shall often have occasion to remark. In this place it need only be stated, that by these scars may be detected the position of the leaves, and the form of the bases of the petioles or leaf-stalks; their probable direction, whether they were opposite, alternate, verticillate or spirally disposed, deciduous or persistent, imbricated or remote. Even when no traces of the leaves remain, the origin of the branches, and their bifurcation, may perhaps be determinable.