In the majority of ferns, at a higher level, after the stele has increased greatly in diameter, a large-celled true pith or medulla, resembling the cortex in its characters, and quite distinct from conjunctive, from which it is separated by an internal endodernlis, appears in the centre.
The centre of the S~hooo- stele is however often occupied by a large-celled pith resembling the cortex in structure, the cortex and pith ~ together being classed as ground tissue.
When the pith is large celled, the xylems of the bundles are separated from it by a distinct layer of conjunctive tissue called the endocycle, and a similar layer, the pericycle, separates the phloem from the cortex.
The external conjunctive is usually a living comparatively small-celled tissue, whose cells are consider ably elongated in the direction of the stem-axis and frequently contain abundant starch.
The latter is produced (except in cases of complete astely where a cylinder is never formed) after a number of leaf-traces have appeared on different sides of the stem so as to form a circle as seen in transverse section, the spaces intervening between adjacent bundles becoming bridged by small-celled tissue closing the cylinder.