The whole cylinder is enclosed by the peculiarly differentiated innermost cell-layer of the cortex, known as the endodermis.
A striking feature in the roots of several genera, excluding the Abietineae, is the occurrence of thick and somewhat irregular bands ofthickening on the cell-walls of the cortical layer next to the endodermis.
The primary vascular bundles in a young conifer stem are collateral, and, like those of a Dicotyledon, they are arranged in a circle round a central pith and enclosed by a common endodermis.
Rarely in the leaf, frequently in the stem (particularly in Pteridophytes), and universally in the root, the phloeoterma is developed as an endodermis (see below).
Outside this are three arcs of large cells showing characters typical of the endodermis in a vascular plan.t; these are interrupted by strands ofnarrow, elongated, thick-walled cells, which send branches into the little brown scales borne by the rhizome.